††thanks: Corresponding Author††thanks: Corresponding Author

On behalf of the e-ASTROGAM Collaboration

# The e-ASTROGAM mission
(exploring the extreme Universe with gamma rays in the MeV – GeV range)

Alessandro De Angelis  Istituto Nazionale di Fisica Nucleare, Sezione di
Padova, I-35131 Padova, Italy  Istituto Nazionale di Astrofisica, Padova,
Italy  Dipartimento di Matematica, Informatica e Fisica, Università di Udine,
I-33100 Udine, Italy  Laboratorio de Instrumentaçao e Particulas and Instituto
Superior Tecnico, Lisboa, Portugal  Vincent Tatischeff  CSNSM, CNRS and
University of Paris Sud, F-91405, Orsay, France  Marco Tavani  INAF/IAPS, via
del Fosso del Cavaliere 100, I-00133, Roma, Italy  University of Roma “Tor
Vergata”, I-00133, Roma, Italy  Istituto Nazionale di Fisica Nucleare, Sezione
di Roma “Tor Vergata”, I-00133 Roma, Italy  Uwe Oberlack  Institute of Physics
and PRISMA Excellence Cluster, Johannes Gutenberg University Mainz, D-55099
Mainz, Germany  Isabelle A. Grenier  AIM Paris-Saclay, CEA/IRFU, CNRS, Univ
Paris Diderot, F-91191 Gif-sur-Yvette, France  Lorraine Hanlon  School of
Physics, University College Dublin, Ireland  Roland Walter  University of
Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland  Andrea Argan  INAF
Headquarters, Viale del Parco Mellini, 84, I-00136, Roma, Italy  Peter von
Ballmoos  IRAP Toulouse, 9 av. du Colonel-Roche - BP 44 346, F-31028 Toulouse
Cedex 4, France  Andrea Bulgarelli  INAF/IASF Bologna, Via Gobetti 101,
I-40129 Bologna, Italy  Immacolata Donnarumma  INAF/IAPS, via del Fosso del
Cavaliere 100, I-00133, Roma, Italy  Now at Agenzia Spaziale Italiana, Roma,
Italy  Margarita Hernanz  ICE (CSIC-IEEC), Campus UAB, Carrer Can Magrans s/n,
E-08193 Cerdanyola del Valles, Barcelona, Spain  Irfan Kuvvetli  DTU Space,
National Space Institute, Technical University of Denmark, Kgs. Lyngby,
Denmark  Mark Pearce  KTH Royal Institute of Technology, Dept. of Physics,
10691 Stockholm, Sweden  Andrzej Zdziarski  Nicolaus Copernicus Astronomical
Center, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland
Alessio Aboudan  Dept. of Physics and Astronomy University of Padova and INAF,
via Marzolo 8, I-35131 Padova, Italy  Marco Ajello  Department of Physics and
Astronomy, Clemson University, Clemson, SC 29634, USA  Giovanni Ambrosi  INFN
Perugia, Perugia, Italy  Denis Bernard  Institute of Physics and PRISMA
Excellence Cluster, Johannes Gutenberg University Mainz, 55099 Mainz, Germany
Elisa Bernardini  DESY, Platanen Allee 6, D-15738 Zeuthen, Germany  Valter
Bonvicini  INFN Trieste, via A. Valerio, I-34127 Trieste, Italy  Andrea Brogna
Institute of Physics and PRISMA Excellence Cluster, Johannes Gutenberg
University Mainz, 55099 Mainz, Germany  Marica Branchesi  Università degli
Studi di Urbino, DiSPeA, I-61029 Urbino, Italy  Istituto Nazionale di Fisica
Nucleare, Sezione di Firenze, Italy  Carl Budtz-Jørgensen  DTU Space, National
Space Institute, Technical University of Denmark, Kgs. Lyngby, Denmark  Andrei
Bykov  Ioffe Institute, St.Petersburg 194021, Russia  Riccardo Campana
INAF/IASF Bologna, Via Gobetti 101, I-40129 Bologna, Italy  Martina Cardillo
INAF/IAPS, via del Fosso del Cavaliere 100, I-00133, Roma, Italy  Paolo Coppi
Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT
06520-8101, USA  Domitilla De Martino  NAF - Osservatorio Astronomico di
Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy  Roland Diehl  Max
Planck Institut fuer extraterrestrische Physik, Giessenbachstr.1, D-85748
Garching, Germany; Excellence Cluster Universe, Germany  Michele Doro
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova,
Italy  Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova,
I-35131 Padova, Italy  Valentina Fioretti  INAF/IASF Bologna, Via Gobetti 101,
I-40129 Bologna, Italy  Stefan Funk  Erlangen Centre for Astroparticle
Physics, D-91058 Erlangen, Germany  Gabriele Ghisellini  INAF - Osservatorio
di Brera, via E. Bianchi 46, I-23807 Merate, Italy  J. Eric Grove  U.S. Naval
Research Laboratory, 4555 Overlook Ave SW, Washington, DC 20375, USA  Clarisse
Hamadache  CSNSM, F-91405 Orsay Campus, France  IN2P3-CNRS/Univ. Paris-Sud,
F-91405 Orsay Campus, France  Université Paris-Saclay, F-91405 Orsay Campus,
France  Dieter H. Hartmann  Department of Physics and Astronomy, Clemson
University, Clemson, SC 29634, USA  Masaaki Hayashida  Institute for Cosmic
Ray Research, the University of Tokyo, Kashiwa, Chiba, 277-8582, Japan  Jordi
Isern  ICE (CSIC-IEEC), Campus UAB, Carrer Can Magrans s/n, E-08193 Cerdanyola
del Valles, Barcelona, Spain  Gottfried Kanbach  Max-Planck-Institut fur
Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany  Jürgen
Kiener  CSNSM, F-91405 Orsay Campus, France  IN2P3-CNRS/Univ. Paris-Sud,
F-91405 Orsay Campus, France  Université Paris-Saclay, F-91405 Orsay Campus,
France  Jürgen Knödlseder  IRAP Toulouse, 9 av. du Colonel-Roche - BP 44 346,
31028 Toulouse Cedex 4, France  Claudio Labanti  INAF/IASF Bologna, Via
Gobetti 101, I-40129 Bologna, Italy  Philippe Laurent  PC, Univ Paris Diderot,
CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, 10 rue Alice Domont et Léonie
Duquet, F-75205 Paris Cedex 13, France  Olivier Limousin  CEA/Saclay
IRFU/Department of Astrophysics, Bat. 709, F-91191, Gif-Sur-Yvette, France
Francesco Longo  Istituto Nazionale di Fisica Nucleare, Sezione di Trieste,
I-34127 Trieste, Italy  Dipartimento di Fisica, Università di Trieste, I-34127
Trieste, Italy  Karl Mannheim  Universitaet Wuerzburg, Campus Hubland Nord,
Lehrstuhl fuer Astronomie, Emil-Fischer-Strasse 31, D-97074 Wuerzburg, Germany
Martino Marisaldi  University of Bergen, Norway  INAF/IASF Bologna, Via
Gobetti 101, I-40129 Bologna, Italy  Manel Martinez  IFAE-BIST, Edifici Cn.
Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spain  Mario N.
Mazziotta  Istituto Nazionale di Fisica Nucleare, Sezione di Bari, I-70126
Bari, Italy  Julie McEnery  NASA Goddard Space Flight Center, Greenbelt, MD
20771, USA  Sandro Mereghetti  INAF/IASF, Via Bassini 15, I-20133 Milano,
Italy  Gabriele Minervini  INAF/IAPS, via del Fosso del Cavaliere 100,
I-00133, Roma, Italy  Alexander Moiseev  CRESST/NASA/GSFC and University of
Maryland, College Park, USA  Aldo Morselli  Istituto Nazionale di Fisica
Nucleare, Sezione di Roma “Tor Vergata”, I-00133 Roma, Italy  Kazuhiro
Nakazawa  Department of Physics, the University of Tokyo, 7-3-1 Hongo, Bunkyo-
ku, Tokyo 113-0033  Piotr Orleanski  Space Research Center of Polish Academy
of Sciences, Bartycka 18a, PL-00-716 Warszawa, Poland  Josep M. Paredes
Departament de Fisica Quantica i Astrofisica, ICCUB, Universitat de Barcelona,
IEEC-UB, Marti i Franques 1, E-08028 Barcelona, Spain  Barbara Patricelli
Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy
Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy  Jean
Peyré  CSNSM, F-91405 Orsay Campus, France  IN2P3-CNRS/Univ. Paris-Sud,
F-91405 Orsay Campus, France  Université Paris-Saclay, F-91405 Orsay Campus,
France  Giovanni Piano  INAF/IAPS, via del Fosso del Cavaliere 100, I-00133,
Roma, Italy  Martin Pohl  Institute of Physics and Astronomy, University of
Potsdam, 14476 Potsdam, Germany  Harald Ramarijaona  CSNSM, F-91405 Orsay
Campus, France  IN2P3-CNRS/Univ. Paris-Sud, F-91405 Orsay Campus, France
Université Paris-Saclay, F-91405 Orsay Campus, France  Riccardo Rando
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova,
Italy  Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova,
I-35131 Padova, Italy  Ignasi Reichardt  Universitat Rovira i Virgili, Carrer
de l‘Escorxador, E-43003 Tarragona, Spain  Marco Roncadelli  INFN Pavia, via
A. Bassi 6, I-27100 Pavia, Italy  INAF Milano, Milano, Italy  Rui Curado da
Silva  LIP, Departamento de F sica Universidade de Coimbra, P-3004-516
Coimbra, Portugal  Fabrizio Tavecchio  INAF - Osservatorio di Brera, via E.
Bianchi 46, I-23807 Merate, Italy  David J. Thompson  NASA Goddard Space
Flight Center, Greenbelt, MD, USA  Roberto Turolla  Dept. of Physics and
Astronomy University of Padova, via Marzolo 8, I-35131 Padova, Italy
University College London, United Kingdom  Alexei Ulyanov  School of Physics,
University College Dublin, Belfield, Dublin 4, Ireland  Andrea Vacchi
University of Udine and INFN GC di Udine, via delle Scienze, I-33100 Udine,
Italy  Xin Wu  University of Geneva, Department of Nuclear and Particle
Physics, 24 quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland  Andreas
Zoglauer  University of California at Berkeley, Space Sciences Laboratory, 7
Gauss Way, Berkeley, CA 94720, USA

###### Abstract

e-ASTROGAM (‘enhanced ASTROGAM’) is a breakthrough Observatory space mission,
with a detector composed by a Silicon tracker, a calorimeter, and an
anticoincidence system, dedicated to the study of the non-thermal Universe in
the photon energy range from 0.3 MeV to 3 GeV – the lower energy limit can be
pushed to energies as low as 150 keV, albeit with rapidly degrading angular
resolution, for the tracker, and to 30 keV for calorimetric detection. The
mission is based on an advanced space-proven detector technology, with
unprecedented sensitivity, angular and energy resolution, combined with
polarimetric capability. Thanks to its performance in the MeV-GeV domain,
substantially improving its predecessors, e-ASTROGAM will open a new window on
the non-thermal Universe, making pioneering observations of the most powerful
Galactic and extragalactic sources, elucidating the nature of their
relativistic outflows and their effects on the surroundings. With a line
sensitivity in the MeV energy range one to two orders of magnitude better than
previous generation instruments, e-ASTROGAM will determine the origin of key
isotopes fundamental for the understanding of supernova explosion and the
chemical evolution of our Galaxy. The mission will provide unique data of
significant interest to a broad astronomical community, complementary to
powerful observatories such as LIGO-Virgo-GEO600-KAGRA, SKA, ALMA, E-ELT, TMT,
LSST, JWST, Athena, CTA, IceCube, KM3NeT, and the promise of eLISA.

Keywords: High-Energy Gamma-Ray Astronomy, High-Energy Astrophysics, Nuclear
Astrophysics, Compton and Pair Creation Telescope, Gamma-Ray Bursts, Active
Galactic Nuclei, Jets, Outflows, Multiwavelength Observations of the Universe,
Counterparts of gravitational waves, Fermi, Dark Matter, Nucleosynthesis,
Early Universe, Supernovae, Cosmic Rays, Cosmic Antimatter

High-Energy Gamma-Ray Astronomy, High-Energy Astrophysics, Nuclear
Astrophysics, Compton and Pair Creation Telescope, Gamma-Ray Bursts, Active
Galactic Nuclei, Jets, Outflows, Multiwavelength Observations of the Universe,
Counterparts of gravitational waves, Fermi, Dark Matter, Nucleosynthesis,
Early Universe, Supernovae, Cosmic Rays, Cosmic Antimatter

###### pacs:

PACS 95.55 Ka, PACS 98.70 Rz, 26.30.-k

###### Contents

  1. I Introduction
  2. II Science Case
    1. II.1 Processes at the heart of the extreme Universe: prospects for the Astronomy of the 2030s
      1. II.1.1 Gamma-Ray Bursts
      2. II.1.2  e-ASTROGAM and the new Astronomy
    2. II.2 The origin and impact of high-energy particles on Galaxy evolution, from cosmic rays to antimatter
      1. II.2.1 What are the CR energy distributions produced inside SNRs and injected into the surrounding ISM?
      2. II.2.2 How do CR fluxes vary with Galactic environments, from passive interstellar clouds to active starburst regions and near the Galactic Center?
      3. II.2.3 Where are the low-energy CRs and how do they penetrate dense clouds?
      4. II.2.4 The origin and energy content of Galactic wind and Fermi bubbles
      5. II.2.5 Antimatter and WIMP Dark Matter
    3. II.3 Nucleosynthesis and the chemical evolution of our Galaxy
      1. II.3.1 What are the progenitor system(s) and explosion mechanism(s) of thermonuclear SNe? Can we use SN Ia for precision cosmology?
      2. II.3.2 How do core-collapse supernovae (CCSNe) explode? What is the recent history of CCSNe in the Milky Way?
      3. II.3.3 Nova explosions
      4. II.3.4 How are cosmic isotopes created in stars and distributed in the interstellar medium?
    4. II.4 Observatory science in the MeV - GeV domain
  3. III Scientific Requirements
  4. IV The Scientific Instrument
    1. IV.1 Measurement principle and payload overview
      1. IV.1.1 Silicon Tracker
        1. Silicon detectors
        2. Mechanical structure
        3. Front-End Electronics
      2. IV.1.2 Calorimeter
        1. Scintillation crystals and housing
        2. Photosensors: silicon drift detectors
        3. Mechanical structure
        4. Front-End Electronics
      3. IV.1.3 Anticoincidence System
      4. IV.1.4 Data Handling and Power Supply
      5. IV.1.5 Trigger logic and data flow architecture
    2. IV.2 Performance assessment
      1. IV.2.1 Background model
      2. IV.2.2 Angular and spectral resolution
      3. IV.2.3 Field of View
      4. IV.2.4 Effective area and continuum sensitivity
      5. IV.2.5 Line sensitivity
      6. IV.2.6 Polarization response
    3. IV.3 Technology readiness
  5. V Mission Configuration and Profile
    1. V.1 Orbit and launcher
    2. V.2 Spacecraft and system requirements
      1. V.2.1 Attitude and Orbital Control Systems
      2. V.2.2 Thermal control system
  6. VI Summary

##  I Introduction

e-ASTROGAM is a gamma-ray mission concept proposed as a response to the
European Space Agency (ESA) Call for the fifth Medium-size mission (M5) of the
Cosmic Vision Science Programme. The planned launch date is 2029.

The main constituents of the e-ASTROGAM payload will be:

  * •

A Tracker in which the cosmic $\gamma$-rays can undergo a Compton scattering
or a pair conversion, based on 56 planes of double-sided Si strip detectors,
each plane with total area of $\sim$1 m2;

  * •

A Calorimeter to measure the energy of the secondary particles, made of an
array of CsI (Tl) bars of 5$\times$5$\times$80 mm3 each, with relative energy
resolution of 4.5% at 662 keV;

  * •

An Anticoincidence system (AC), composed of a standard plastic scintillator AC
shielding and a Time of Flight, to veto the charged particle background.

If selected, e-ASTROGAM will operate in a maturing gravitational wave and
multimessenger epoch, opening up entirely new and exciting synergies. The
mission will provide unique and complementary data of significant interest to
a broad astronomical community, in a decade of powerful observatories such as
LIGO-Virgo-GEO600-KAGRA, SKA, ALMA, E-ELT, LSST, JWST, Athena, CTA and the
promise of eLISA.

The core mission science of e-ASTROGAM addresses three major topics of modern
astrophysics.

  * •

Processes at the heart of the extreme Universe: prospects for the Astronomy of
the 2030s

Observations of relativistic jet and outflow sources (both in our Galaxy and
in active galactic nuclei, AGNs) in the X-ray and GeV–TeV energy ranges have
shown that the MeV–GeV band holds the key to understanding the transition from
the low energy continuum to a spectral range shaped by very poorly understood
particle acceleration processes. e-ASTROGAM will: (1) determine the
composition (hadronic or leptonic) of the outflows and jets, which strongly
influences the environment – breakthrough polarimetric capability and
spectroscopy providing the keys to unlocking this long-standing question; (2)
identify the physical acceleration processes in these outflows and jets (e.g.
diffusive shocks, magnetic field reconnection, plasma effects), that may lead
to dramatically different particle energy distributions; (3) clarify the role
of the magnetic field in powering ultrarelativistic jets in gamma-ray bursts
(GRBs), through time-resolved polarimetry and spectroscopy. In addition,
measurements in the e-ASTROGAM energy band will have a big impact on
multimessenger astronomy in the 2030s. Joint detection of gravitational waves
and gamma-ray transients would be ground-breaking.

  * •

The origin and impact of high-energy particles on galaxy evolution, from
cosmic rays to antimatter

e-ASTROGAM will resolve the outstanding issue of the origin and propagation of
low-energy cosmic rays affecting star formation. It will measure cosmic-ray
diffusion in interstellar clouds and their impact on gas dynamics and state;
it will provide crucial diagnostics about the wind outflows and their feedback
on the Galactic environment (e.g., Fermi bubbles, Cygnus cocoon). e-ASTROGAM
will have optimal sensitivity and energy resolution to detect line emissions
from 511 keV up to 10 MeV, and a variety of issues will be resolved, in
particular: (1) origin of the gamma-ray and positron excesses toward the
Galactic inner regions; (2) determination of the astrophysical sources of the
local positron population from a very sensitive observation of pulsars and
supernova remnants (SNRs). As a consequence e-ASTROGAM will be able to
discriminate the backgrounds to dark matter (DM) signals.

  * •

Nucleosynthesis and the chemical enrichment of our Galaxy

The e-ASTROGAM line sensitivity is more than an order of magnitude better than
previous instruments. The deep exposure of the Galactic plane region will
determine how different isotopes are created in stars and distributed in the
interstellar medium; it will also unveil the recent history of supernova
explosions in the Milky Way. Furthermore, e-ASTROGAM will detect a significant
number of Galactic novae and supernovae in nearby galaxies, thus addressing
fundamental issues in the explosion mechanisms of both core-collapse and
thermonuclear supernovae. The $\gamma$-ray data will provide a much better
understanding of Type Ia supernovae and their evolution with look-back time
and metallicity, which is a pre-requisite for their use as standard candles
for precision cosmology.

Figure 1: Point source continuum differential sensitivity of different X- and
$\gamma$-ray instruments. The curves for INTEGRAL/JEM-X, IBIS (ISGRI and
PICsIT), and SPI are for an effective observation time $T_{\rm obs}$ = 1 Ms.
The COMPTEL and EGRET sensitivities are given for the typical observation time
accumulated during the $\sim$9 years of the CGRO mission (see Fig. 1 in tak13
). The Fermi/LAT sensitivity is for a high Galactic latitude source in 10
years of observation in survey mode. For MAGIC, VERITAS (sensitivity of
H.E.S.S. is similar), and CTA, the sensitivities are given for $T_{\rm obs}$ =
50 hours. For HAWC $T_{\rm obs}$ = 5 yr, for LHAASO $T_{\rm obs}$ = 1 yr, and
for HiSCORE $T_{\rm obs}$ = 1000 h. The e-ASTROGAM sensitivity is calculated
at $3\sigma$ for an effective exposure of 1 year and for a source at high
Galactic latitude.

In addition to addressing its core scientific goals, e-ASTROGAM will achieve
many serendipitous discoveries (the unknown unknowns) through its combination
of wide field of view (FoV) and improved sensitivity, measuring in 3 years the
spectral energy distributions of thousands of Galactic and extragalactic
sources, and providing new information on solar flares and terrestrial gamma-
ray flashes (TGF). e-ASTROGAM will become a key contributor to multiwavelength
time-domain astronomy. The mission has outstanding discovery potential as an
Observatory facility that is open to a wide astronomical community.

e-ASTROGAM is designed to achieve:

  * •

Broad energy coverage (0.3 MeV to 3 GeV), with one-two orders of magnitude
improvement in continuum sensitivity in the range 0.3 MeV – 100 MeV compared
to previous instruments (the lower energy limit can be pushed to energies as
low as 150 keV, albeit with rapidly degrading angular resolution, for the
tracker, and to 30 keV for calorimetric detection);

  * •

Unprecedented performance for $\gamma$-ray lines, with, for example, a
sensitivity for the 847 keV line from Type Ia SNe 70 times better than that of
INTEGRAL/SPI;

  * •

Large FoV ($>$2.5 sr), ideal to detect transient sources and hundreds of GRBs;

  * •

Pioneering polarimetric capability for both steady and transient sources;

  * •

Optimized source identification capability afforded by the best angular
resolution achievable by state-of-the-art detectors in this energy range
(about 0.15 degrees at 1 GeV);

  * •

Sub-millisecond trigger and alert capability for GRBs and other cosmic and
terrestrial transients;

  * •

Combination of Compton and pair-production detection techniques allowing
model-independent control on the detector systematic uncertainties.

##  II Science Case

e-ASTROGAM will open the MeV region for exploration, with an improvement of
one-two orders of magnitude in sensitivity (Fig. 1) compared to the current
state of the art, much of which was derived from the COMPTEL instrument more
than two decades ago. It will also achieve a spectacular improvement in terms
of source localization accuracy (Fig. 2) and energy resolution, and will allow
to measure the contribution to the radiation of the Universe in an unknown
range (Fig. 3). The sensitivity of e-ASTROGAM will reveal the transition from
nuclear processes to those involving electro- and hydro-dynamical, magnetic
and gravitational interactions.

Figure 2: An example of the capability of e-ASTROGAM to transform our
knowledge of the MeV-GeV sky. Upper panel: The upper left figure shows the
1-30 MeV sky as observed by COMPTEL in the 1990s; the lower right figure shows
the simulated Cygnus region in the 1-30 MeV energy region from e-ASTROGAM.
Lower panel: comparison between the view of the Cygnus region by Fermi in 8
years (left) and that by e-ASTROGAM in one year of effective exposure (right)
between 400 MeV and 800 MeV.

An important characteristic of e-ASTROGAM is its ability to measure
polarization in the MeV range, which is afforded by Compton interactions in
the detector. Polarization encodes information about the geometry of magnetic
fields and adds a new observational pillar, in addition to the temporal and
spectral, through which fundamental processes governing the MeV emission can
be determined. The addition of polarimetric information will be crucial for a
variety of investigations, including accreting black-hole (BH) systems,
magnetic field structures in jets, and the emission mechanisms of GRBs.
Polarization will provide definitive insight into the presence of hadrons in
extragalactic jets and the origin of ultra-high-energy cosmic rays (CR).

Figure 3: Compilation of the measurements of the total extragalactic gamma-ray
intensity between 1 keV and 820 GeV FermiEGB , with different components from
current models; the contribution from MeV blazars is largely unknown. The
semi-transparent band indicates the energy region in which e-ASTROGAM will
dramatically improve on present knowledge.

In the following sections, the core science questions physcase  to be
addressed by e-ASTROGAM are presented. The requirements coming from the
scientific objectives, and driving the instrument design, are presented in
Sect. III.

###  II.1 Processes at the heart of the extreme Universe: prospects for the
Astronomy of the 2030s

The Universe accessible to e-ASTROGAM is dominated by strong particle
acceleration. Ejection of plasma (jets or uncollimated outflows), ubiquitous
in accreting systems, drives the transition from the keV energy range, typical
of the accretion regime, to the GeV-TeV range, through reprocessing of
synchrotron radiation (e.g, inverse Compton, IC) or hadronic mechanisms. For
some sources the MeV band naturally separates the acceleration and
reprocessing energy ranges. Other systems, instead, radiate the bulk of their
output in the MeV band. This is the most frequent case for AGNs at
cosmological distances.

e-ASTROGAM will also study extreme acceleration mechanisms from compact
objects such as neutron stars and (supermassive) black holes. Its polarimetric
capabilities and its continuum sensitivity will solve the problem of the
nature of the highest energy radiation.

The transition to non-thermal processes involves, in particular, the emission
of relativistic jets and winds. In our Galaxy, this is relevant for compact
binaries and microquasars. The interplay between accretion processes and jet
emission can best be studied in the MeV region, where disk Comptonization is
expected to fade and other non-thermal components can originate from jet
particles. e-ASTROGAM observations of Galactic compact objects and in
particular of accreting BH systems (such as Cygnus X-1 Zdziarski2014 , Cygnus
X-3 (tavani2009 ; Abdocygx3 ), V404 Cygni roques15 ) will determine the nature
of the steady-state emission due to Comptonization and the transitions to
highly non-thermal radiation (Fig. 4). The main processes behind this emission
are Compton scattering by accelerated non-thermal electrons and its
attenuation/reprocessing by electron-positron pair production. The magnetic
field in the BH vicinity can be quite strong, and have both random and ordered
components; synchrotron emission by the electrons in the accretion flow may
give rise to polarized MeV emission (e.g. Romero2014 ), which can be measured
by e-ASTROGAM, together with spectral transitions. Signatures of $e^{+}e^{-}$
production and annihilation (e.g. Siegert2016 ) can be detected by e-ASTROGAM.

Figure 4: Left panel: Different states of Cyg X-3 in the soft X-ray/hard X-ray
ranges (adapted from Koljonen ). Right panel: spectral energy distribution
(SED) of Cyg X-3 during the $\gamma$-ray flaring activity in 2011 (adapted
from piano12 ). The purple curve is the e-ASTROGAM 3$\sigma$ sensitivity for
an 8-day observation, from a simulation taking into account the expected
background; the time matches the integration time of the AGILE $\gamma$-ray
spectrum (red points). The green curve refers to a leptonic model and the
black curve to a hadronic model.

e-ASTROGAM offers a unique way to study accelerated jets in blazars on short
and long timescales. Among the unsolved questions are the origin of the
photons undergoing Comptonization, the location of the acceleration region in
jets (near to, or far from, the central black hole), and the presence of
additional components of accelerated electrons i.e. whether or not there is a
mildly relativistic population of electrons. The latter question has important
implications for the understanding of acceleration processes such as shock or
magnetic field reconnection.

Figure 5: SED from a collection of different spectral states of the FSRQ 3C
279 showing a dramatic gamma-ray flaring activity, including the minute-
timescale episode detected by Fermi in June 2015 Fermi3c279 . The purple solid
line is the 3$\sigma$ e-ASTROGAM sensitivity calculated for a 50 ks exposure.

Very fast variations of the gamma-ray emission have been recently detected and
are challenging current models; an example is provided by the flat spectrum
radio quasar (FSRQ) 3C 279 Fermi3c279 . During a flare in 2015 Fermi3c279 ,
variations of 2-3 minutes were detected in gamma-rays, i.e. well below
standard light-travel times usually assumed in theoretical models. These very
rapid variability phenomena are the most compelling evidence of the occurrence
of out-of-equilibrium particle acceleration most likely produced by magnetic
field reconnection with remarkably high efficiency. On a smaller scale the
magnetic field merging and the acceleration of relativistic electrons and ions
have been typically observed in solar flares, where sub-second timescales and
gamma-ray emissions into the 100 MeV range have been frequently detected. This
process, called “super-acceleration” (see tavani2013 ), has recently been
invoked to explain the unexpected gamma-ray flare of the Crab Nebula
(tavani2011 ; Abdocrab ), and leads to a very efficient mechanism for magnetic
energy dissipation. It may apply to very rapid gamma-ray emission from compact
objects with timescales and intensities incompatible with the current
paradigms. e-ASTROGAM will provide crucial information in the strongly
variable spectral range 1-100 MeV (Fig. 5), constraining the electron
population spectrum and the responsible particle acceleration mechanisms. An
outstanding unsolved issue is the existence of gamma-rays produced by hadronic
processes. The origin of photons can be effectively probed both by much
improved spectral measurements in the MeV-GeV band (detecting the “pion
bump”), and by polarimetric observations. Polarimetry is indeed a powerful
tool to establish the nature of the emitters (hadrons vs. leptons) and, in
case of leptonic (i.e., inverse Compton) emission, the nature of the soft
photon target radiation ZhangBoettcher2013 .

An example is the very bright (about 50 times our sensitivity at 1 MeV) AGN
3C279 ZhangBoettcher2013 . In the 100 keV-10 MeV range, the polarization in
leptonic emission models is low and rapidly decreasing with energy (from 40%
to 0), while for hadronic models it is high and increasing in energy ($>$60%
to 80%). Even in non optimal conditions (fields not well ordered, alignment to
line of sight not optimal) the polarization signature would allow to identify
unambiguously a hadronic scenario. The best targets for this study are blazars
- both BL Lacs (suitable to study the transition from the synchrotron to the
IC or hadronic-dominated component) and the powerful FSRQs (in which the
transition from the highly polarized SSC radiation to the less polarized
external Compton component can be revealed).

Over 2/3 of the 3033 sources from the 3rd Fermi-LAT Catalog of GeV-band
sources (3FGL), have power-law spectra (at energies larger than 100 MeV)
steeper than $E^{-2}$, implying that their peak energy output is below 100 MeV
(Fig. 6). The subset that is extragalactic hosts BHs with masses reaching
$10^{10}$ $M_{\odot}$, and are often located at high redshift ($z\geq 2-3$).
They are therefore ideal tracers of the formation and history of super-massive
BHs in the Universe Ghisellini2010 ; Ghisellini2013 . In particular, the
sources hosting the most massive BHs are elusive in the GeV band as probed by
Fermi-LAT.

Figure 6: SED of PKS 2149-306 at $z=2.345$ (left panel) and the fitting model
by tagliaferri2015  and GB 1428+4217 at $z=4.72$ (right panel) and the fitting
model by Ghisellini (private communication). Downward arrow is 2$\sigma$ Fermi
upper limit over 7.5 years (see paliya2016 ). The sensitivity of e-ASTROGAM,
calculated for an effective exposure of 1 year, is shown as a purple curve in
the 0.3 MeV – 3 GeV range. The bulk of the power is expected in the MeV band,
around $10^{21}$ Hz.

Recent hard X-ray surveys ajello09 ; Ghisellini2010 ; ajello12  have been
shown to be more effective in detecting higher redshift blazars compared to
GeV $\gamma$-ray surveys.The main reason is that the SEDs of these sources
peak in the MeV region (hereafter, MeV blazars) and detection becomes a
difficult task for $\gamma$-ray instruments, even for Fermi-LAT. e-ASTROGAM
will detect hundreds of these blazars, constraining their SED peaks very
tightly (Fig. 6). These discoveries will revolutionize our understanding of:
(i) how the two populations of AGNs (radio-quiet and radio-loud) evolve with
redshift; (ii) the formation and growth of supermassive BHs; (iii) the
connection between the jet and the central engine and (iv) the role of the jet
in the feedback occurring in the host galaxies volon11 . e-ASTROGAM will very
substantially advance our knowledge of MeV-blazars up to redshift $\sim 4.5$,
with implications for blazar physics and cosmology – the attenuation due to
cosmological absorption is negligible in the MeV region and the sensitivity
has been shown by fits to the SED to be adequate (Fig. 6). These observations
will be invaluable and complementary to data from the future ATHENA mission
for the study of super-massive black holes.

By detecting the population of MeV-blazars up to redshift $\sim 4.5$,
e-ASTROGAM will resolve the extragalactic gamma-ray background (EGB) in the
MeV range (Fig. 3). A possible residual excess in the MeV range may have
cosmological implications related to baryogenesis vonbal14 . Quasar-driven
outflows could also account for the yet-not-understood part of the EGB
loeb2016 .

####  II.1.1 Gamma-Ray Bursts

GRBs are among the most intriguing and puzzling phenomena in astrophysics.
They are believed to originate from coalescing NSs or BHs (short-GRBs, of
duration $\lesssim$2 s) or from the final collapses of massive stars (long-
GRBs). Their radiative output is believed to originate from highly
relativistic outflows.

e-ASTROGAM will allow unprecedented studies of both classes of GRBs, thanks to
the combined interplay between the imaging Tracker, the Calorimeter, and the
AC system, providing excellent sensitivity for spectral and timing studies
combined with polarization capability. It will be possible to detect GRBs with
durations from sub-millisecond to hundreds of seconds and study them in the
energy range where their emission peaks. The e-ASTROGAM spectral performance
will be very relevant, because of the role played by the MeV range in
constraining theoretical models of particle acceleration. The total number of
GRBs detectable by e-ASTROGAM is estimated to be $\sim$600 during the first 3
years (see Sect. IV.2.6).

The e-ASTROGAM imaging Tracker can localize GRBs within 0.1∘-1∘ (depending on
their intensity), and the information can be processed onboard for a fast
communication. The delay of the alerts is similar to $Fermi$-LAT, being the
procedure (event rates onboard, plus simple localization based on onboard fast
reconstruction) the same. This translates into an initial alert with an
accuracy of 1∘-2∘ within (2-4) s, to be confirmed with similar accuracy within
30 s and then made public. Within 4h-8h the final alert accuracy of 0.1∘-1∘
can be reached. The alerts issued by e-ASTROGAM will be extremely valuable for
observatories such as CTA. The Calorimeter can act as an independent detector
extending the energy range down to 30 keV: an on-board trigger logic spanning
timescales from sub-ms up to seconds will be implemented.

For bright GRBs, e-ASTROGAM will detect polarization in the MeV range. The
Tracker can provide information down to (150-200) keV, also for polarization
measurements. We can estimate as 42 GRBs/year the number of events with a
detectable polarization fraction of 20%; for a polarization fraction of 10%
the number is about 16 GRBs/year. The polarization information, combined with
spectroscopy in the MeV-GeV band, will provide a unique diagnostic to address
the role of magnetic fields in the radiative output and dynamics of the most
relativistic outflows in our Universe.

The e-ASTROGAM sensitivity to short and long GRBs will be very useful also for
the detection of electromagnetic counterparts of impulsive gravitational wave
events, as described in Sect. II.1.2.

####  II.1.2  e-ASTROGAM and the new Astronomy

e-ASTROGAM fills the need for an MeV gamma–ray detector operating at the same
time as facilities such as SKA and CTA, as well as eLISA and neutrino
detectors. It guarantees the availability of complementary information to
obtain a coherent picture of the transient sky and the sources of
gravitational waves (GWs) and high-energy neutrinos. This will undoubtedly be
an exciting new landscape for astronomy in the XXI century.

The first detections of GW signals from binary black hole (BH-BH) mergers,
observed by Advanced LIGO Abbott2016a , marked the onset of the era of GW
astronomy. The next breakthrough will be the observation of their
electromagnetic counterparts, which will characterize the progenitor and its
environment. Neutron star (NS)–BH or NS-NS mergers can eject relativistic
outflows Nakar2007 ; Veres2014 ; Wang2016  or produce sub-relativistic
omnidirectional high-energy emission Takami2014 . In both cases emission up to
the MeV energy range can be expected. The expected detection rate of GRB
prompt emission by e-ASTROGAM in coincidence with a GW detection is up to 1.5
events per year Patricelli2016 ; it will double after the incorporation of
KAGRA and LIGO-India into the GW network, which should happen several years
before 2029. e-ASTROGAM will also play a key role in the multiwavelength study
of GW events: in fact, its large FoV will maximize the detection probability
and provide accurate sky localization ($<$ 1 sq. deg at 1 MeV), thus allowing
the follow-up of the GW events by other telescopes. This capability will be
crucial for the identification and the multiwavelength characterization of the
GW progenitor and of its host galaxy. e-ASTROGAM could associate binary
systems to short GRBs, improving the localization of sources and measuring
spectral energy distributions.

e-ASTROGAM may coincide with the third generation of ground-based
interferometer projects, such as the Einstein Telescope Punturo2010  and
Cosmic Explorer McClelland2015 , with an order of magnitude increase in
sensitivity. Furthermore, the space detector eLISA eLISA  will open GW
observations to massive (104-106) $M_{\odot}$ BHs, which could have magnetized
circumbinary disks powering EM emission. Simultaneous GW/EM emission will
transform our understanding of the formation, evolution, properties and
environment of different mass compact objects through cosmic history.

Another important topic in the new astronomy will be neutrino astrophysics.
Although astrophysical neutrinos have been detected by IceCube astronu , no
significant cluster (in space or in time) has been found yet. Coincidences in
time and space of astrophysical neutrino events with exceptional flares of
blazars have been proposed by several authors (e.g. kadler2016 ). Among the
speculated possible origins of the IceCube events are GRBs with jets shocked
in surrounding matter. Models explaining hypernovae and low luminosity GRBs
(e.g. senno2016 ) predict neutrino and gamma-ray emission, with the highest
energy $\gamma$-rays being likely absorbed, and thus a possible cutoff in the
MeV range.

Observations by e-ASTROGAM could therefore open a new avenue within
multimessenger astrophysics, also by allowing multimessenger coincidences for
KM3NeT in the Mediterranean sea, and thus making it possible to remove the
background for neutrinos in the TeV range.

###  II.2 The origin and impact of high-energy particles on Galaxy evolution,
from cosmic rays to antimatter

Relativistic particles permeate the interstellar medium (ISM) of galaxies and
drive their evolution by providing heat, pressure and ionization to the clouds
and to galactic winds and outflows. Sub-GeV particles have a particularly
important role and understanding their origin and transport has profound
implications. High-energy particles also signal the presence of antimatter and
potential sources of dark matter. Observations with e-ASTROGAM can advance our
knowledge on all fronts by observing the radiation borne from particle
interactions with interstellar gas: electrons emitting bremsstrahlung $\gamma$
rays (often dominant below 50-100 MeV); nuclei producing $\pi^{0}$s decaying
into $\gamma$ rays (with the characteristic “pion bump” in energy density
below one GeV); nuclear excitation lines, and the 511-keV line from positron
annihilation. In addition to the the gas-related emission, e-ASTROGAM will
also observe the large-scale emission due to IC scattering of CR electrons on
the interstellar radiation field and cosmic microwave background. This is a
significant component above $\sim$100 MeV, and it is believed to be the
dominant interstellar diffuse component below few tens of MeV.

We have fair measurements of the local spectrum of Galactic cosmic-rays (CRs),
from GeV to PeV energies, but not at lower energies. There is only a very
coarse description of their flux both radially and vertically across the Milky
Way. Their diffusion processes in and out of the spiral arms and star-forming
regions are poorly understood, as is their penetration through dense clouds as
a function of energy. Convincing, albeit not definitive, observational
evidence is available for CR acceleration by supernova shockwaves (via
diffusive shock acceleration), but little understanding of the total energy
imparted to CRs, of their escape into the ambient medium, of their diffusion
through stellar-wind-driven turbulence in starburst regions, and of their role
in the self-regulation of the Galactic ecosystem.

####  II.2.1 What are the CR energy distributions produced inside SNRs and
injected into the surrounding ISM?

Figure 7: e-ASTROGAM sensitivity for 1-year exposure (thick purple line)
compared to typical $\gamma$-ray energy spectra for several SNRs; young SNRs
($<$1000 years) are shown in green. High-energy data ($E>100$ MeV) are taken
from funk2015 ; low-energy data (related to RX J1713.7-3946) from masaaki .

Fermi-LAT has barely resolved a handful of SNRs, indicating that younger ones
tend to emit harder gamma–rays FermiSNRcat , and detected curved spectra
compatible with pion bumps in only three sources (IC 443, W44, W51C,
FermiW44IC443 ; FermiW51C ). The performance of e-ASTROGAM will open the way
for spectral imaging of a score of SNRs, spanning ages from $10^{3}$ to
$10^{5}$ years. The bremsstrahlung emitting electrons seen with e-ASTROGAM
have energies close to the radio synchrotron emitting ones, and lower than
those seen in synchrotron X-rays, thus permitting tomographic reconstruction
of the magnetic field and electron distributions inside the remnant. Fig. 7
indicates that e-ASTROGAM has the sensitivity in one year of exposure to
detect CR electrons even for a strong mean magnetic field. The sub-GeV part of
the gamma radiation is essential to separate the emission from relativistic
electrons and nuclei above 100 MeV, so the new data can constrain how
electrons and protons are differentially injected into the shock, how large
and sometimes highly intermittent magnetic fields build up near the shock, and
how the acceleration efficiency and the total CR content of a remnant evolves
as the shockwave slows down.

Figure 8: Imaging the inner Galaxy region: simulated performance of e-ASTROGAM
in one year of effective exposure (right panels) compared to 8 years of Fermi
(left panels) for the energy ranges 150 MeV - 400 MeV (top) and 400 MeV - 800
MeV (bottom).

Older remnants often interact with molecular clouds that provide target gas
for CRs escaping the remnant Uchiyama2012 . Resolving the diffuse pion
emission produced in those clouds against the bright Galactic background is
essential to probe the CR spectra that are actually injected into the ISM.
Imaging the remnant and shocked clouds both require an angular resolution
better than 0.2∘ around one GeV and a sensitivity below $10^{-11}$ erg cm-2
s-1 above 50 MeV that e-ASTROGAM will achieve. The instrument may also detect
line emission from nuclear excitation caused by low-energy CR nuclei, opening
a new and unique way to remotely measure the flux and elemental composition of
low-energy particles. The comparison with direct composition measurements near
the Earth would provide valuable clues as to the type of supernovae that
dominate CR production in the Galaxy.

A conservative estimate on the number of SNRs studied in detail by e-ASTROGAM
can be obtained starting from the 3FGL catalog of sources in the 100 MeV–300
GeV energy range based on the first four years of science data from the
$Fermi$ mission 3FGL . This catalog contains 12 SNRs, 9 PWNe, the Cygnus
superbubble produced by multiple supernovae FermiCyg , as well as 62
additional sources potentially associated to SNRs. Given that e-ASTROGAM will
have a comparable or better sensitivity, FoV and observation strategy than
$Fermi$-LAT in the 100 MeV–1 GeV energy range, and accounting for the measured
spectra of SNRs, we expect the detection of at least the same amount of
objects of this class in this energy range.

However, e-ASTROGAM will have a much improved sensitivity with respect to
$Fermi$-LAT below 100 MeV. This will allow bright SNRs like IC 443, W44, W51C,
and W28 to be studied for the first time with good statistics spectra in the
very important energy band below the pion-decay bump, which is crucial to make
firm conclusions about the hadronic and leptonic contributions to the observed
emission. This is very important to constrain the CR acceleration physics.

Another definitive advantage of e-ASTROGAM is its very good sensitivity in the
MeV energy domain, where the 44Ti 1157 keV line measurements should uncover
about 10 new, very young SNRs (of ages up to 500 yr) presently hidden in
highly obscured clouds (see also Sect. II.3.2). These objects are thought to
accelerate CR up to PeV energies and will be prime targets for the Cherenkov
Telescope Array (CTA) as well.

####  II.2.2 How do CR fluxes vary with Galactic environments, from passive
interstellar clouds to active starburst regions and near the Galactic Center?

The gamma-rays produced by CR nuclei along their interstellar journey can
remotely probe the CR flux and spectrum across Galactic spiral arms, inside
young stellar clusters and superbubbles, in the central molecular zone, and
down to parsec scales inside nearby clouds. The current picture provided by
the Fermi-LAT analyses does not provide sufficient resolution to probe
theoretical expectations based on spiral distributions of CR sources and
environmental changes in CR transport due to their streaming in the varying
Galactic magnetic field Recchia2016  or to different levels of interstellar
MHD turbulence powered by massive stars and supernovae PICARD ; Grenier15 .
With their much improved angular resolution and lower energy band, e-ASTROGAM
observations can complement the Fermi-LAT archival data to probe the
heterogeneity of the CR population in a variety of Galactic environments,
across four decades in momentum around the maximum energy density. The
e-ASTROGAM sub-degree resolution is essential to map the structured CR
emission and avoid confusion with Galactic point sources (Fig. 8). It is also
key to resolve starburst regions hosting cocoons of young energetic CRs
FermiCyg  to reveal the impact on the CR properties (energy and diffusion) of
the large level of supersonic turbulence driven by the massive stars. How is
the emerging CR spectrum modified by confinement, re-acceleration and enhanced
losses in the turbulent medium? Since most CR sources occur in star-forming
regions, these questions challenge our global understanding of the early steps
of CR propagation and our use of observational diagnostics in the Galaxy at
large and in starburst galaxies in particular. The sensitivity and PSF of
e-ASTROGAM permit searches for Galactic CR cocoons other than Cygnus X and an
extensive characterization of the particle emissions across four decades in
energy (with Fermi archives and TeV data from CTA and HAWC).

With a sensitivity of $10^{-12}$ erg cm-2 s-1 around 100 MeV, e-ASTROGAM can
also detect the pion bump from CRs in the Large Magellanic Cloud (LMC), in
particular in the star-forming regions of 30 Doradus and N11 FermiLMC , and in
nearby starburst galaxies FermiStarburst . Such observations will provide
insight on the CR in external galaxies at energies relevant for the physics of
their ISM. The detection of the pion bump is crucial to disentangle the CR
origin of the emission from other sources such as pulsars.

####  II.2.3 Where are the low-energy CRs and how do they penetrate dense
clouds?

Locally, the CR energy density is dominated by sub-GeV and GeV protons. It is
comparable to the energy densities of the interstellar gas, magnetic field,
and stellar radiation. Low-energy cosmic rays (LECRs) influence galactic
evolution by changing the thermodynamical state (pressure, heat) and chemical
evolution (via the ionization rate) of the dense clouds that lead to star
formation. They also provide critical pressure support in starburst regions to
launch galactic winds into Galaxy halos Recchia16wind . Yet, our knowledge of
the production pathways and transport properties of LECRs is very rudimentary
in our Galaxy, and even more so in the conditions of merger/starburst
galaxies. In the Local Bubble, the pronounced break in CR momentum implied
near one GeV by the Voyager and Fermi LAT data suggests that LECRs are
advected off the plane by a local Galactic wind Schlick2014 ; Grenier15 .
Beyond Voyager, indirect measures of the LECR flux at the low, ionizing,
energies are uncertain by several orders of magnitude, even in the local ISM.
Whereas multi-GeV CRs appear to penetrate deeply and rather uniformly into
molecular clouds, molecular line observations suggest strong spatial
variations in the ionization induced by LECRs Indriolo2012 .

The energy band and performance of e-ASTROGAM are well suited to probe the
distribution of LECRs in different Galactic environments, both from the
bremsstrahlung radiation of low-energy electrons and the pion bump from low-
energy nuclei. Another long-awaited goal is to test the concentration and
exclusion processes that govern the penetration of CRs into dense clouds,
which are predicted to leave an energy-dependent signature below 1 GeV
Skilling1976 .

Inelastic collisions of LECRs with interstellar gas should produce a rich
spectrum of gamma-ray lines between 0.3 and 10 MeV ben13 . Spectroscopic
observations of these lines with e-ASTROGAM are the only direct way to detect
these elusive particles, to measure their energy density in and out of dense
clouds, and to measure the production rate of light elements (Li, Be, and B)
resulting from their interactions with gas. Fig. 9 shows that e-ASTROGAM
should allow a firm detection of $\gamma$-ray line complexes from the inner
Galaxy with a total flux of $\sim 2\times 10^{-4}$ cm-2 s-1 sr-1 in the
(0.3-10) MeV band, and possibly also from superbubbles, hypernovae, and active
star forming regions.

####  II.2.4 The origin and energy content of Galactic wind and Fermi bubbles

There is increasing evidence, observationally (e.g. Crocker11 ; kre13 ) and
theoretically, for the emergence from the inner 200 pc of the Galaxy of a
Galactic wind flowing to large height ($\sim 10$ kpc) into the halo and partly
accelerated by the pressure gradient supplied by CRs Breit91 ; Everett08 . The
relation with the Fermi Bubbles seen in gamma rays and possibly in microwaves
and polarized radio waves is unclear despite their biconical structure and the
finding of gas at large velocities ($>$ 900 km/s) in their direction Fox15 .
The Bubbles may be the few-Myr-old relics of past accretion-driven outflow(s)
from Sgr A* or the CR-driven Galactic wind powered by the starburst activity
in the central nucleus. They are filled with relativistic particles of unknown
nature (electrons or nuclei) and origin. The bubble shapes near the Galactic
disc and their spectrum below 0.5 GeV are quite uncertain because of the large
confusion with Galactic foregrounds FermiBubbles . The improved angular
resolution of e-ASTROGAM will reveal the geometry and sub-GeV spectrum of the
Bubbles down to their base to help identify the dominant particles, study
particle ageing in the outflow, distinguish between impulsive and wind models,
and estimate the total power expelled from the modest nucleus of our Galaxy.

Figure 9: Predicted gamma-ray emission due to nuclear interactions of CRs in
the inner Galaxy. The gamma-ray line emission below 10 MeV is due to LECRs,
whose properties in the ISM have been adjusted such that the mean CR
ionization rate deduced from H${}_{3}^{+}$ observations and the Fermi-LAT data
(magenta band) at 1 GeV are simultaneously reproduced (adapted from ben13 ).
The 1-year sensitivity of e-ASTROGAM (for Galactic background) is
superimposed.

####  II.2.5 Antimatter and WIMP Dark Matter

The detection of a bright gamma-ray $e^{+}e^{-}$ annihilation line at 511 keV
places strong constraints in many emission models for high-energy
astrophysical sources. The detection of $e^{+}e^{-}$ annihilation radiation
can thus serve as an important and unambiguous calorimeter, e.g., to track
supernova nucleosynthetic activity or constrain the presence of dark matter at
the center of our Galaxy. e-ASTROGAM will have significantly better
sensitivity than INTEGRAL/SPI in the 511 keV line (Table 5). It would
represent a major step forward, for example, in answering the many questions
posed by INTEGRAL’s detection of spatially extended $e^{+}e^{-}$ annihilation
in our Galaxy. Note also that e-ASTROGAM’s energy coverage is ideally suited
(compared to Fermi) to capture the radiation from decaying pions and anti-
proton annihilation, especially if the relevant sources are cosmological and
redshifted. By either resolving the extragalactic gamma-ray background at
$\sim$1-100 MeV or constraining its angular anisotropy, e-ASTROGAM could thus
directly detect or, at least, significantly improve on COMPTEL constraints for
anti-matter domains in the Universe left over from the Big Bang and the
process of baryogenesis.

The theoretical paradigm about dark matter (DM) most popular today is
motivated by the so-called “WIMP miracle” (i.e., the fact that we can explain
the missing mass with just one more particle, and without the need for new
interactions). Such a Weakly Interacting Massive Particle, or WIMP, is present
in many extensions of the Standard Model (SM) of particle physics (e.g. the
neutralinos in supersymmetry, or the Kaluza-Klein states in theories with
extra dimensions, see e.g. book ). To account for the missing matter, a WIMP
that comes from an equilibrium state in the early Universe should have mass
O(100 GeV/$c^{2}$), and a self-annihilation cross section $\sigma$ and a
typical velocity $v$ such that $<\sigma v>\sim 3\times 10^{-26}$ cm3/s (this
value is a kind of “thermal benchmark”). The annihilation of pairs of WIMPs in
high-density DM regions could result in an excess of photons and antimatter
coming mainly from the hadronic cascades; for $m_{\rm WIMP}<100$ GeV/$c^{2}$
the bulk of photons is expected below 1 GeV photonsfromdm . Monochromatic
lines from 2-body final states (either of which being a photon, or both) are
not excluded.

In final states characterized by a photon continuum or by $\gamma$-ray lines,
the e-ASTROGAM sensitivity for the detection of DM is complementary to Fermi
and CTA because it covers with larger sensitivity the low-mass interval.
Moreover, the possibility of Fermi (and in the future of CTA) to detect DM
requires that these experiments demonstrate that there is an excess of
photons. Modeling the photon background from astrophysical sources is where
e-ASTROGAM is fundamental, since the MeV/GeV range is probably the most
constraining for the SED of astrophysical sources. In addition, e-ASTROGAM can
constrain the signal from nearby pulsars.

The region of the Galactic Center (GC) is expected to host the highest density
of DM in the vicinity of the Earth, but the many astrophysical processes at
work in the crowded inner Galaxy make it extremely difficult to disentangle
the possible DM signal from conventional emissions Prada:2004 . e-ASTROGAM
will improve our understanding of the origin of particles in the inner Galaxy,
therefore reducing the uncertainties associated to DM searches.

Other targets for the search of DM are dwarf spheroidal galaxies (dSphs,
Walker:2009zp ), whose otherwise mysterious dynamics can be simply explained
if they are highly DM-dominated ($M/L\sim 10^{3}M_{\odot}/L_{\odot}$). dSph
allows an almost background free observation, since they are not expected to
be $\gamma$-ray emitters unless a sizable WIMP pair annihilation takes place.
There are currently about 30 known dSphs, with new objects in this class being
discovered. For a low WIMP mass, e-ASTROGAM will have a discovery potential
comparable to Fermi-LAT – stronger if the DM mass is on the few GeV scale.

In some extensions of the SM, DM is on the MeV scale. Theories predict a
stable relic particle, in thermal equilibrium during the early Universe, with
mass between 1 and 100 MeV Boehm:2002yz . Such models attracted some interest
about a decade ago because they would naturally explain the 511-keV emission
line toward the galactic bulge Boehm:2003bt ; diehl2016 . In this case
e-ASTROGAM could make a direct detection or provide constraining limits
Boddy:2015fsa . e-ASTROGAM outperforms Fermi and has a sensitivity much better
than the “thermal benchmark” for masses below 5 GeV for the search of a
continuum signal in dSphs and below 200 MeV for line searches FermiDM .

Besides the popular paradigms outlined above, other scenarios are discussed in
the literature for DM, in which e-ASTROGAM has the best sensitivity, in
particular:

  * •

Monochromatic photons at MeV energies may result from DM annihilation to
quarkonium Rudaz:1986db , as well as step-like features from the decay $b\to
s+\gamma$ or $b^{\prime}\to b+\gamma$, where $b^{\prime}$ is a hypothetical
4th generation quark Bergstrom:1988jt . Another possibility is the decay of DM
candidates like the gravitino, which has motivated line searches with the
Fermi Large Area Telescope down to energies of 100 MeV Albert:2014hwa . It was
also pointed out that for DM lighter than around 100 MeV, the only
kinematically accessible non-leptonic states are photons and neutral pions,
leading to clear gamma-ray signatures to look for Boddy:2015fsa . Due to the
gap in sensitivity at the MeV, very weak limits on DM signals exist in this
range Essig:2013goa .

  * •

A new class of potential smoking-gun signatures for DM signals in the range 10
MeV - 100 MeV is pointed out in mevdm  and involve transitions between meson
states and, in their simplest realization, do not require any new physics
(beyond, obviously, the DM particle itself) but inevitably arise in certain
kinematical situations for GeV-scale DM annihilating or decaying to heavy
quarks. Unlike direct detection or collider experiments, these signatures are
thus very sensitive to DM coupling with third or second generation quarks.

  * •

Axion-Like-Particle DM (of extremely low mass); this is discussed in the
“observatory science” (Sect. II.4).

e-ASTROGAM can also shed new light on dark matter by the study of antimatter –
an excess of antimatter is expected from DM annihilation. The case of signals
from excesses of antimatter is particularly intriguing: the presently measured
flux of mildly relativistic cosmic rays (anti-electrons in particular) cannot
be explained on the basis of present knowledge, and the data show an excess
with respect to known astrophysical sources (PAMELA, AMS02; see pdg2015  for a
review). Is this excess due to presently unknown sources, e.g. as yet unknown
pulsars or past activity of the GC Petrovic14 ; CarlsonProfumo , or are we
detecting evidence of new physics at the fundamental scale? This question can
be answered by observations of nearby pulsars with e-ASTROGAM.

Finally an improved angular resolution with respect to AGILE and Fermi-LAT in
the inner Galaxy region and in regions closer to Earth in the 5 MeV–100 MeV
energy range can disentangle the possible contributions from the diffuse
background, from point sources, and other possible emitters. Overall, a large
class of spectral features in the MeV-GeV range can result in indications for
WIMP DM particles, or significantly reduce the astrophysical background
uncertainties to identify genuine DM signatures in VHE photon spectra mevdm .

###  II.3 Nucleosynthesis and the chemical evolution of our Galaxy

The origins of the cosmic atomic nuclei and their variety is one of the main
themes of astrophysical research, and has been studied through nuclear
gamma–ray emission for several decades. The Compton Gamma Ray Observatory
(CGRO) provided the first sky survey of nuclear emission from cosmic sources
scho96 ; die98 , INTEGRAL added high-resolution spectroscopy die13 . From
these missions, however, only the brightest sources of their class have been
seen, and a deeper survey as proposed with e-ASTROGAM will address fundamental
issues in nuclear astrophysics, exploring the variability of nuclear emission
from supernovae (SNe), localized stellar groups, and other transients related
to compact stars and nuclear processes therein. Key science goals of
e-ASTROGAM focus on the astrophysics of SNe, for which a validated and self-
consistent model has not yet been established, neither for thermonuclear
supernovae (SN Ia) nor for core collapse supernovae (SN Ib/c and II).

####  II.3.1 What are the progenitor system(s) and explosion mechanism(s) of
thermonuclear SNe? Can we use SN Ia for precision cosmology?

SN Ia are the outcome of a thermonuclear burning front that sweeps a
carbon/oxygen white dwarf in a close binary system. But exactly how the
ignition conditions are obtained, and on which white dwarfs, and more so how
the thermonuclear runaway proceeds through the white dwarf and turns it into a
variety of isotopes that are ejected, are all questions that are subject to
considerable debate (e.g. hil00 ; hi13  and references therein). It seems that
several candidate evolutionary channels may all contribute, from the _double
degenerate_ variant of merging white dwarf binaries disrupting one of the
dwarfs through tidal forces or a hard collision, to a variety of _single
degenerate_ models where accretion of material from a companion star may lead
to either the white dwarf reaching the critical Chandrasekhar mass stability
limit, or be ignited earlier through a surface explosion from a helium flash.

Such uncertainties are troublesome for cosmology since the use of SN Ia as
standard candles depends on an empirical relationship between the shape and
the maximum of the light curve phi93 . Although useful up to now, in view of
the development of precision cosmology, a better, astrophysically supported
understanding of thermonuclear SNe, as well as their evolutionary effects at
large distances and low metallicities, are mandatory. The brightness-decline
relation phi93  is closely related to the mass of synthesized 56Ni, and
factors like the progenitor evolution, ignition density, flame propagation,
mixing during the burning, completeness of burning in outer, expanding
regions, all lead to different amounts of 56Ni, which is measured directly
through gamma–ray lines. On the other hand, radiation transport from
radioactivity to optical light and their spectra depend on complex atomic line
transitions in the expanding supernova as well as total mass burned, the
amount and distribution of radioactive nickel and intermediate mass elements,
all of which must combine in quite a tight way to reproduce the observations
woo07 ; ker14 . Some of these factors depend on the evolution of the white
dwarf prior to the explosion and cast some doubt on the use of SN Ia as high
precision distance indicators. It is thus of critical importance to
disentangle the role of these factors to understand the limits of the Phillips
phi93  relation.

Figure 10: The evolution of the 847 keV line brightness from 56Co decay
reflects how radioactive energy is deposited within a supernova to make it
shine. Here, INTEGRAL data from the Type Ia supernova SN 2014J (adapted from
Fig. 4 in Ref. die15 , red data points) are compared to a family of candidate
models the14 . A simulation of the response to a time evolution such as in the
W7 model nom84  shows that the sensitivity improvement by e-ASTROGAM (blue
points) will lead to breakthrough science.

With SN2014J, for the first time a SN Ia occurred close enough for current
generation $\gamma$ray telescopes, at 3.5 Mpc in the starburst galaxy M82.
INTEGRAL could detect the long awaited gamma–ray signatures of the
thermonuclear runaway, through the early emission from the decay of 56Ni (mean
lifetime $\tau\simeq$8.8 days) about 20 days after the explosion, and the main
gamma-ray lines at 847, 1238, and 511 keV from the decay of 56Co
($\tau\simeq$111 days). These data suggested either a surface explosion or
some unusual morphology of the runaway die14 ; chu14 ; die15 ; chu15 ; ise16 ,
either case in stark contrast to the conventional Chandrasekhar model.
Clearly, the glimpse offered by SN2014J observations underlines the importance
of gamma–ray line diagnostics in these systems and emphasize that more and
better observations hold the key to a deeper understanding of how the
thermonuclear explosion of a white dwarf star unfolds.

e-ASTROGAM will achieve a major gain in sensitivity compared to INTEGRAL for
the main gamma–ray lines arising from 56Ni and 56Co decays. Thus with the
expected sensitivity of $3.5\times 10^{-6}$ ph cm-2 s-1 for the 847 keV line
in 1 Ms of integration time (see Table 5), e-ASTROGAM will detect ($10\pm 3$)
SNe Ia in 3 years of nominal mission lifetime, up to a distance of $\sim$35
Mpc (for the brightest SNe). As illustrated in Fig. 10, e-ASTROGAM will
provide much better data than we have now with INTEGRAL for SN 2014J from
similarly nearby events. These data will allow us to probe the explosion
mechanism in detail, and compare with astrophysical models for each event to
better understand the progenitor system(s) and the thermonuclear explosion
process.

Gamma rays escaping from supernovae contribute to the diffuse background in
the MeV range. The contribution from Type Ia supernovae dominates, potentially
providing a significant fraction of the observed flux in the MeV band. (see,
e.g., ruizl ).

####  II.3.2 How do core-collapse supernovae (CCSNe) explode? What is the
recent history of CCSNe in the Milky Way?

Similar to SN Ia, core collapse physics is also not well understood in terms
of an astrophysical model. But these events are more common, being the end
states of the evolution of massive stars, and are key to understanding the
diversity of elements in the universe. Also here, deviations from spherical
symmetry are the rule. The goal is to explain a tremendous variety of core
collapse events, e.g. electron capture supernovae such as the Crab, clumpy
explosions such as Cas A, collapsars that appear as GRB sources and produce
stellar mass black holes, superluminous supernovae that may be powered
entirely differently by magnetar rotational energy, or pair instability
supernovae that create huge amounts of radioactive 56Ni.

Stellar rotation is known to exist but is complicated to track in its effects
on stellar evolution, yet important for many of the above outcomes:
nucleosynthesis, pre-supernova structure, core collapse. Measuring
nucleosynthesis products such as 56Ni, 56Co and 44Ti is one of the more direct
ways to extract information on the inner processes triggering the explosion
near the newly forming compact stellar remnant (e.g., gre14 ) – other
observables are indirect, and mostly reflect interactions within the envelope,
or with circumstellar, pre-explosively ejected, or ambient gas. e-ASTROGAM
will detect the signatures of 56Ni and 56Co decay from several CCSNe in nearby
galaxies. Comparing $\gamma$-ray characteristics of different classes of
CCSNe, possibly including the pair instability SNe with their order of
magnitude higher 56Ni production gal09 , will probe potentially large
variations in their progenitors and offer a direct view of their central
engines.

With a gain in sensitivity for the 44Ti line at 1157 keV by a factor of 27
compared to INTEGRAL/SPI (see Table 5), e-ASTROGAM should also detect the
radioactive emission from 44Ti ($\tau\simeq$87 years) from most of the young
(age $\lesssim$ 500 yr) SNRs in the Milky Way, thus uncovering about 10 new
SNRs in the Galaxy, as well as the youngest SNR(s) in the LMC. Among the
youngest Galactic SNRs, only Cassiopeia A has been firmly detected in 44Ti
surveys carried out up to now tsy16 , which is surprising in view of an
otherwise inferred rate of one CCSN every 50 years in our Galaxy. e-ASTROGAM
will measure in particular the amounts of 44Ti in the SNRs G1.9+0.3, Tycho and
SN 1987A (LMC), which are currently disputed in the literature (see, e.g.,
tsy16 ). Besides giving new insights on the dynamics of core collapse at the
mass cut, e-ASTROGAM observations of young SNR will be very important for
cosmic-ray physics. Indeed, only young SNRs can accelerate CR up to PeV
energies and above, and the identification of new Pevatrons would be
invaluable when the Cherenkov Telescope Array (CTA) will be fully operational.

####  II.3.3 Nova explosions

e-ASTROGAM will also contribute to the study of novae, which are responsible
for the enrichment of the Galaxy in some species and for the peculiar isotopic
signatures found in some pre-solar meteoritic grains jos07 . Emission is
expected gom98 ; her04  from positron-electron annihilation (e+ being emitted
by the $\beta^{+}$-unstable short-lived isotopes 13N and 18F), and from the
decay of the medium-lived isotopes 22Na, which is produced in ONe novae, and
7Be, produced in CO novae. The first type of emission consists of a 511 keV
line plus a continuum between about 20 and 511 keV, whereas the second, long-
lasting emission consists of two $\gamma$-ray lines at 1275 keV (22Na) and 478
keV (7Be). Both provide a direct insight on the amount of radioactive nuclei
in the expanding nova envelope, an information only obtainable through the
observation of $\gamma$-rays. The large FoV of e-ASTROGAM is crucial for these
observations, since the 511 keV line emission happens before the nova is
discovered optically, preventing pointed observations. The detectability
distance with e-ASTROGAM is around 3 kpc for both the 478 and 1275 keV line,
and 4–6 novae at distance $D<3$ kpc are expected in the 3 years of nominal
mission lifetime.

e-ASTROGAM will also help to disentangle the current puzzle posed by the
detection of novae at GeV energies with Fermi-LAT Ferminovae ; cheung16 . Some
novae have been identified theoretically as sites of particle acceleration, in
the shocks within the ejecta and/or between the ejecta and the circumstellar
matter, making them responsible for a fraction of the Galactic cosmic rays
tat07 . These “miniature supernovae” are key systems to study the time
dependence of diffusive shock acceleration of CR. An important consequence of
the production of high-energy particles is that photons with energies higher
than about 100 MeV are emitted, both via neutral pion decay and IC processes.
With the e-ASTROGAM sensitivity, it will be possible to disentangle the origin
of this high-energy emission, hadronic and/or leptonic, and thus understand
better the properties of the nova ejecta and the shocks.

####  II.3.4 How are cosmic isotopes created in stars and distributed in the
interstellar medium?

The cycle of matter proceeds from the formation of stars through nuclear
fusion reactions within stars during their evolution, towards the ejection of
stellar debris into interstellar space in winds and SN explosions.
Interstellar gas, enriched with some newly produced nuclei, eventually cools
down to form new stars, closing and starting the cycle again. The cooling down
of hot nucleosynthesis ejecta and their trajectories towards new star
formation are particularly hard to constrain through observations. The
recycling time scale in the interstellar medium is of the order of tens of
Myr, but SNRs can be seen over time scales of few 105 yr at most. Long-lived
radioactive gamma–ray emitters 26Al ($\tau\simeq$1.0$\times$106 yr) and 60Fe
($\tau\simeq$3.8$\times$106 yr) can trace mixing processes of ejecta into the
next generation star forming regions over much longer time, testing, among
other things, molecular-cloud lifetime and models for stimulated/triggered
star formation.

INTEGRAL/SPI data for the 1809 keV 26Al line suggest that on the global,
Galactic scale, superbubbles are key structures in the transport of fresh
ejecta towards new star forming regions kre13 ; kra15 . With its huge increase
in sensitivity, e-ASTROGAM will provide a detailed view of the morphology of
this emission, with high precision measurements of the line flux from many
regions of the Galaxy. Thus, e-ASTROGAM will observe the 26Al radioactivity
from dozens of nearby ($\sim$kpc) stellar objects and associations. In
particular, it will measure precisely the amount of 26Al ejected by the Wolf-
Rayet star WR11 in the $\gamma^{2}$-Velorum binary system (expected line flux
of $\sim$10-5 ph cm-2 s-1), thus providing a unique calibration of the 26Al
production during the Wolf-Rayet phase of a massive star. e-ASTROGAM has also
the capability of detecting 26Al emission from the LMC (expected line flux of
$\sim$10-6 ph cm-2 s-1), thus providing new insight into stellar
nucleosynthesis outside the Milky Way.

For the first time, e-ASTROGAM will provide the sensitivity needed to
establish the Galactic 60Fe emission and build an accurate map of the 60Fe
flux in the Milky Way, enabling its comparison with the 26Al map to gain
insight into the stellar progenitors of both radioisotopes. In particular,
measuring $\gamma$-ray line ratios for specific massive-star groups will
constrain 60Fe production in massive stars beyond $\sim$40 $M_{\odot}$, which
directly relates to stellar rotation and uncertain convective-layer evolution
in massive star interiors lim06 .

###  II.4 Observatory science in the MeV - GeV domain

During the first phase e-ASTROGAM will collect data especially for the core
science topics, as described above. However, given the very large sky coverage
and the accumulated exposure, a very large number of sources can be detected
and monitored. e-ASTROGAM has the capability of studying thousands of sources
both Galactic and extragalactic of which many are expected to be new
detections. Therefore, a very large community of astronomical users will
benefit from e-ASTROGAM data available for multifrequency studies through a
Guest Investigator programme managed by ESA.

e-ASTROGAM will detect with highly improved sensitivity in the MeV-GeV domain
phenomena characterized by: (1) rapid and very rapid variability timescales
(sub-second, second, minutes, hours), and (2) steady sources. The e-ASTROGAM
sensitivity to pointlike sources varying on timescales of seconds (for GRBs)
and minutes-hours-days (compact objects, novae, magnetars, blazars) will
provide unique information about outstanding physical processes including jet
processes, shock accelerations and magnetic field reconnection. The study of
steady sources (diffuse emission, pulsars, PWNe, SNRs, extragalactic
background) will provide a detailed diagnostic of fundamental processes that
operate in quasi-stable regimes. The e-ASTROGAM Observatory science program
will emphasize multifrequency response to both variable and steady sources in
a decade that will benefit from the operations of many other observatories
planned to be operative in the 2030s, that include LIGO-Virgo-GEO600-KAGRA,
SKA, ALMA, E-ELT, LSST, ATHENA, CTA and possibly e-LISA. e-ASTROGAM will
provide unique data for multifrequency science, triggering other instruments
and reacting rapidly to transient detections.

We summarize here the most relevant classes of phenomena or sources in
addition to the e-ASTROGAM “core science” topics.

  * •

Diffuse Galactic gamma-ray background, for which e-ASTROGAM is in a position
to determine the underlying cosmic ray population and spatial and spectral
variations across the Galaxy.

  * •

Pulsars and millisecond pulsars both isolated and in binaries, whose (pulsed
or unpulsed) emission will be observable in a spectral range rich in
information to discriminate between particle acceleration models.

  * •

Pulsar wind nebulae, a product of the interaction between shocked relativistic
pulsar winds and the ISM, for which e-ASTROGAM will obtain crucial data on
particle acceleration and propagation.

  * •

Magnetars, enigmatic and strongly variable compact stars characterized by very
strong magnetic fields that exhibit special phenomena exclusively in the MeV
energy range.

  * •

Galactic compact binaries, including white dwarfs, neutron stars and stellar
mass black holes whose spectral transitions and outbursts in the MeV range
will be systematically monitored by e-ASTROGAM.

  * •

Novae, that in addition to line emission in the MeV range can also be studied
for their surprising and poorly understood $\gamma$-ray emission up to
hundreds of MeV, a product of shock interaction within the nova ejecta and/or
of the nova ejecta with the circumstellar matter (see Sect. II.3.3).

  * •

Massive binary stars with colliding winds of the Eta-Carinae type, whose MHD
shocks are predicted to produce particle acceleration and $\gamma$-ray
emission, a topic of great interest and yet unsettled.

  * •

Interstellar shocks, such as the Cygnus cocoon showing the existence of
particle acceleration over large distances in the ISM, for which the spectral
and angular resolution of e-ASTROGAM will be unique.

  * •

Blazar population studies in the MeV range, to be obtained by the detection
capability of thousands of sources by e-ASTROGAM.

  * •

Studies of the propagation of $\gamma$-rays over cosmological distances, for
which the attenuation is predicted to be negligible in standard QED - effects
of absorption might indicate new physics at work, possibly the existence of
axion-like-particles (ALPs) coupling to gamma–rays dmr2008 . ALPs $a$ are
spin-0, neutral and very light particles predicted by many extensions of the
SM alp1 , coupling to two photons through an amplitude $g_{a\gamma}\,{\bf
E}\cdot{\bf B}$ (presently bound by $g_{a\gamma}<0.66\cdot 10^{-10}\,{\rm
GeV}^{-1}$ cast ; pdg2016  for a wide mass range.) In a photon beam emitted by
a far-away source (blazar, GRB), $\gamma\to a$ and $a\to\gamma$ conversions
can take place, resulting in photon-ALP oscillations. These may show up in the
e-ASTROGAM energy spectrum about $E_{L}=2.56\cdot 10^{21}\left(m/{\rm
eV}\right)^{2}\,\xi^{-1}\,{\rm GeV}$ for an ALP mass $3.42\cdot
10^{-13}\,\xi^{1/2}\,{\rm eV}<m<3.42\cdot 10^{-11}\,\xi^{1/2}\,{\rm eV}$,
where $\xi\equiv\left(g_{a\gamma}\,10^{11}\,{\rm GeV}\right)\left(B/{\rm
nG}\right)$ dmr2008 . For a $100\%$ polarized beam the amplitude of the
fluctuations can be twice then for an unpolarized beam wb2012 ; gr2013 . CCSNe
can also be a source of ALPs, produced in the few seconds after the explosion
inside the core by Primakoff effect, and reconverted to photons of the same
energy (peaking at $\sim$50 MeV) in the Milky Way magnetic field. The arrival
time of these photons would be the same as for neutrinos, providing a clear
signature; e-ASTROGAM would have a sensitivity better than Fermi-LAT meyer
and access to much smaller mass/coupling values than dedicated laboratory
experiments.

  * •

Solar flares and contribution to “Space Weather”, that will be studied with
unprecedented line emission and continuum capability for theoretical modeling
as well as fast reaction for alerts.

The Sun is an efficient particle accelerator during flares. A possible
mechanism requires that ions and electrons are accelerated by the release of
energy during magnetic reconnection in the upper corona; they can then
propagate into the chromosphere where they interact to produce a strong
X/$\gamma$-ray bremsstrahlung continuum, nuclear lines (through nuclear
interactions with the solar atmosphere: prompt deexcitation lines from e.g.
12C* at 4.44 MeV and 16O* at 6.2 MeV but also a neutron capture line at 2.2
MeV from $n$ H $\rightarrow$ D + 2.2 MeV photons), and pion-decay components.

After a solar flare, the Sun is predicted to produce $\gamma$-ray emission
through radioactive decays (e.g. lines at 847 and 1434 keV from the decay of
56Co and 52Mn, respectively). The detection of these lines would provide
additional information on energy spectra and composition of flare-accelerated
particles, and on mixing processes in the solar atmosphere. e-ASTROGAM can
study for the first time solar-flare radiation from 300 keV to $>$100 MeV.

To summarize, the continuum and line sensitivity of e-ASTROGAM in the
(0.3-100) MeV energy range will provide a diagnostic for acceleration
mechanisms with simultaneous information at GeV energies. MeV polarization
will add important information on the in-situ magnetic properties.

The solar emission produced by interactions of CRs with the surface and the
heliosphere or3 ; or1 ; or2  will be also observed. This will allow precise
studies on the CRs and their propagation very close to the Sun.

  * •

Terrestrial Gamma-Ray Flashes, an atmospheric phenomenon with possible
environmental impact for which e-ASTROGAM can provide continuous monitoring
(including the 511-keV line detection).

Atmospheric lightning and thunderstorms can produce particle acceleration in
TGFs up to the GeV; the overall expected TGF detection rate by e-ASTROGAM is
of about 60 TGFs per day, improving by a factor of 30 $Fermi-$LAT.
Observations in the Compton and pair production regime will provide TGF
imaging in the MeV - 100 MeV range. The optimal sensitivity of e-ASTROGAM in
the MeV-GeV range will clarify the physics and the many implications of this
puzzling atmospheric phenomenon.

##  III Scientific Requirements

e-ASTROGAM’s requirements to achieve its core science objectives, such as the
angular and energy resolution, the field of view, the continuum and line
sensitivity, the polarization sensitivity, and the timing accuracy, are
summarized in Table 1.

Table 1: e-ASTROGAM scientific requirements.

  * •

The very large spectral band covered by the telescope in the standard gamma-
ray acquisition mode will give a complete view of the main nonthermal
processes at work in a given astrophysical object, for the first time with a
single instrument. The e-ASTROGAM energy band includes the 511 keV line from
$e^{+}e^{-}$ annihilation, the nuclear de-excitation lines, the characteristic
spectral bump from pion decay, the typical domains of nonthermal electron
bremsstrahlung and IC emission, as well as the high-energy range of
synchrotron radiation in sources with high magnetic field ($B\lower
2.15277pt\hbox{$\;\buildrel>\over{\sim}\;$}1$ G). The designed wide energy
band is particularly important for the study of blazars, GRBs, Galactic
compact binaries, pulsars, as well as the physics of CRs in SNRs and in the
ISM.

  * •

The large energy band covered by the Calorimeter in the burst search mode of
data acquisition (Sect. IV.1.5) is primarily designed for the triggering and
study of GRBs. It is also well adapted to the broadband emissions of TGFs and
solar flares.

  * •

The wide field of view of the telescope is especially important to enable the
measurement of source flux variability over a wide range of timescales both
for a-priori chosen sources and in serendipitous observations. Coupled with
the scanning mode of operation, this capability enables continuous monitoring
of source fluxes that will greatly increase the chances of detecting
correlated flux variability with other wavelengths. The designed wide field of
view is particularly important for the study of blazars, GRBs, Galactic
compact objects, supernovae, novae, and extended emissions in the Milky Way
(CRs, radioactivity). It will also enable, for example, searches of
periodicity and orbital modulation in binary systems.

  * •

One of the main requirements of e-ASTROGAM is to improve dramatically the
detection sensitivity in a region of the electromagnetic spectrum, the so-
called MeV domain, which is still largely unknown. The sensitivity requirement
is relevant to all science drivers discussed above. Thus, the goal of
detecting a significant number ($N>5$) of SN Ia in gamma rays after 3 years
requires a sensitivity in the 847 keV line $<5\times 10^{-6}$ ph cm-2 s-1 in 1
Ms of integration time (Table 1).

  * •

Another major requirement for a future gamma-ray observatory is to improve
significantly the angular resolution over past and current missions, which
have been severely affected by a spatial confusion issue. Thus, the e-ASTROGAM
angular resolution will be excellent in the MeV range and above a few hundreds
of MeV, improving $CGRO$/COMPTEL and $Fermi$-LAT by almost a factor of 4 at 1
MeV and 1 GeV, respectively. The targeted angular resolution given in Table 1
is close to the physical limits: for Compton scattering, the limit is given by
the Doppler broadening induced by the velocity of the atomic electrons, while
for low-energy pair production, the limit is provided by the nuclear recoil.
e-ASTROGAM angular resolution will allow a number of currently unidentified
gamma-ray sources (e.g. 992 sources in the 3FGL catalog 3FGL ) to be
associated with objects identified at other wavelengths. The GC region is the
most challenging case, for which the e-ASTROGAM capability will be fully
employed.

  * •

The polarization sensitivity of e-ASTROGAM is designed to enable measurements
of the gamma-ray polarization fraction in more than 20 GRBs per year (GRBs
being promising candidates for highly gamma-ray polarized sources, see, e.g.,
mcc16 ). Such measurements will provide important information on the
magnetization and content (leptons, hadrons, Poynting flux) of the
relativistic outflows, and, in the case of GRBs at cosmological distance, will
address fundamental questions of physics related to vacuum birefringence and
Lorentz invariance violation (e.g., got14 ). With the designed polarization
sensitivity, e-ASTROGAM will also be able to study the polarimetric properties
of more than 50 pulsars, magnetars, and black hole systems in the Galaxy.

  * •

The spectral resolution of e-ASTROGAM is well adapted to the main science
drivers of the mission. Thus, the main gamma-ray lines produced in SN
explosions or by LECR interactions in the ISM are significantly broadened by
the Doppler effect, and a FWHM resolution of 3% at 1 MeV is adequate. In the
pair production domain, an energy resolution of 30% will be more than enough
to measure accurately putative spectral breaks and cutoffs in various sources
and distinguish the characteristic pion-decay bump from leptonic emissions.

  * •

The timing performance of e-ASTROGAM is mainly driven by the physics of
magnetars and rotation-powered pulsars, as well as by the properties of TGFs.
The targeted microsecond timing accuracy is already achieved in, e.g., the
AGILE mission tav09 .

The e-ASTROGAM requirements reflect the dual capacity of the instrument to
detect both Compton scattering events in the 0.3 (and below) – 10 MeV range
and pair-producing events in the 10 MeV – 3 GeV energy range; a small overlap
around 10 MeV allows (although in a limited energy band) cross-calibration,
thus reducing systematic uncertainties. The main instrument features of
e-ASTROGAM necessary to meet the scientific requirements in Table 1, are
described in Sect. IV.2.

The sensitivity performance is consistent with the requirement of an
equatorial low-Earth orbit (LEO) of altitude in the range 550 – 600 km. Such
an orbit is preferred for a variety of reasons. It has been demonstrated to be
only marginally affected by the South Atlantic Anomaly and is therefore a low-
particle background orbit, ideal for high-energy observations. The orbit is
practically unaffected by precipitating particles originating from solar
flares, a virtue for background rejection. Finally, both ESA and ASI have
satellite communication bases near the equator (Kourou and Malindi) that can
be efficiently used as mission ground stations.

Table 1 also includes the most important system requirements such as the
satellite attitude reconstruction, telemetry budget, and pointing capability.
e-ASTROGAM is a multi-purpose astrophysics mission with the capability of a
very flexible observation strategy. Two main scientific observation modes are
to be managed by the Mission Operation Center (MOC):

  * $\bullet$

pointing mode;

  * $\bullet$

survey mode.

The pointing mode can be implemented either in a fixed inertial pointing or in
the more efficient double-pointing per orbit mode. In the latter case, the
e-ASTROGAM satellite is required to be able to perform two sky pointings per
orbit, lasting approximately 40 minutes each. The survey mode consists in a
continuous pointing to the zenith to perform a scan of the sky at each orbit.
This mode can be activated at any time in principle, and depending on the
scientific prioritization and on the mission schedule foreseen by the Science
Management Plan, can lead to an optimized all-sky survey.

Requirements for the Ground Segment are standard for an observatory-class
mission. Target of Opportunity observations (ToOs) are required to follow
particularly important transient events that need a satellite repointing. The
e-ASTROGAM mission requirement for ToO execution is within 6–12 hours, with
the goal of reaching 3–6 hours. The speed of repointing depends on the torque
of the reaction wheels. We expect a repointing velocity similar to Fermi
($\sim 30$ degrees/min, which grants to have a visible object in FoV within
less than 5’).

e-ASTROGAM does not use any consumable and could in principle be operated for
a duration up to 10-20 years (well within the foreseen operation duration of 3
years with a possible extension of two), limited mainly by orbital
instabilities and by the risk of accidents. Radiation damage in LEO, with
almost equatorial inclination, is negligible. As an example, the degradation
of Fermi, whose inclination implies significant crossing of the South Atlantic
Anomaly, is negligible for what concerns electronics, negligible for what
concerns Tracker aging, and around 1%/year in terms of loss in light yield of
the Calorimeter crystals.

Table 2 summarizes our conservative estimates of the number of sources
detectable by e-ASTROGAM in 3 years, based on current knowledge and $\log
N-\log S$ determinations of Galactic and extragalactic sources, including
GRBs. It takes information from the the Swift-BAT 70-Month Hard X-ray survey
catalog b70h , the 4th INTEGRAL-IBIS catalog ibisc , and the 3rd Fermi-LAT
catalog 3FGL . Noteworthy, the latter catalog contains more than 1000
unidentified sources in the 100 MeV – 300 GeV range with no counterparts at
other wavelength, and most of them will be detected by e-ASTROGAM, in addition
to a relevant number of new unidentified sources. The discovery space of
e-ASTROGAM for new sources and source classes is very large.

Type | 3 yr | New sources
---|---|---
Total | 3000 – 4000 |  $\sim$1800 (including GRBs)
Galactic | $\sim 1000$ |  $\sim$400
MeV blazars | $\sim 350$ | $\sim 350$
GeV blazars | 1000 – 1500 | $\sim 350$
Other AGN ($<$10 MeV) | 70 – 100 | 35 – 50
Supernovae | 10 – 15 | 10 – 15
Novae | 4 – 6 | 4 – 6
GRBs |  $\sim$600 |  $\sim$600
Table 2: Estimated number of sources of various classes detectable by
e-ASTROGAM in 3 years. The last column gives the number of sources not known
before in any wavelength.

##  IV The Scientific Instrument

###  IV.1 Measurement principle and payload overview

The e-ASTROGAM payload is shown in Figure 11. It consists of three main
detectors:

  * •

A silicon Tracker in which the cosmic gamma-rays undergo a first Compton
scattering or a pair conversion; it is based on the technology of double sided
Si strip detectors to measure the energy and the 3D position of each
interaction with an excellent energy and spatial resolution;

  * •

A 3D-imaging Calorimeter to absorb and measure the energy of the secondary
particles; it is made of an array of small scintillation crystals (33,856 CsI
(Tl) bars of 5$\times$5$\times$80 mm3) read out by silicon drift
photodetectors to achieve the required energy resolution (4.5% at 662 keV);

  * •

An Anticoincidence system (AC), composed of a standard AC shielding
surrounding the top and four lateral sides of the instrument, and a Time-of-
Flight unit located below the instrument, to veto the particle background
arising from the platform; it is made of plastic scintillator tiles with a
detection efficiency exceeding 99.99%.

Figure 11: Overview of the e-ASTROGAM payload showing the silicon Tracker, the
Calorimeter and the Anticoincidence system.

The payload is completed by a Payload Data Handling Unit (PDHU) and a Power
Supply Unit (PSU) located below the Calorimeter inside the platform together
with the back-end electronics (BEE). The PDHU is in charge of the payload
internal control, the scientific data processing, the operative mode
management, the on-board time management, and the telemetry and telecommand
management. The total payload mass and power budget (including maturity
margins) are 999 kg and 1340 W, respectively.

Interactions of photons with matter in the e-ASTROGAM energy range is
dominated by Compton scattering from (below) 0.2 MeV up to about 15 MeV in
silicon, and by $e^{+}e^{-}$ pair production in the field of a target nucleus
at higher energies. e-ASTROGAM maximizes its efficiency for imaging and
spectroscopy of energetic gamma-rays by using both processes. Figure 12 shows
a schematic representation of topologies for Compton and pair events.

For pair-production events, e-ASTROGAM is similar in design to AGILE and
$Fermi$-LAT, but optimized for lower energy. This goal is achieved by
eliminating the passive converters used in both these instruments. This
approach reduces gamma-ray conversion efficiency, but it improves the
instrument point-spread function (PSF) by reducing absorption and multiple
Coulomb scattering of the electron and positron. The broad PSF is a primary
limiting factor in the science that can be done at energies below 100 MeV by
AGILE and $Fermi$-LAT. Pair events produce two main tracks from the created
electron and positron. Tracking of the initial opening angle and of the plane
spanned by the electron and positron tracks enables direct back-projection of
the source position. Multiple scattering of the pair in the tracker material
leads to broadening of the tracks and limits the angular resolution. The
nuclear recoil taking up an unmeasured momentum results in an additional small
uncertainty. The energy of the gamma-ray is measured using the Calorimeter and
information on the electron and positron multiple scattering in the Tracker.
Polarization information in the pair domain is given by the azimuthal
orientation of the electron-positron plane; in addition to improving the PSF,
the use of low-mass tracker planes also enables photon polarization
measurements.

Figure 12: Representative topologies for a Compton event (left) and for a pair
event (right). Photon tracks are shown in pale blue, dashed, and electron
and/or positron tracks in red, solid. From alexnew .

Detecting gamma-rays by Compton scattering is more complicated than for pair
production, because the scattered photon carries a significant amount of the
information about the incident photon and thus it needs to be detected too. In
practice, a Compton telescope requires two separate photon interactions in
order to have a clear detection. The first Compton scattering of the incident
photon occurs in one of the Tracker planes, creating an electron and a
scattered photon. The Tracker measures the interaction location, the electron
energy, and in some cases the electron direction. The scattered photon can be
absorbed in the Calorimeter or (with smaller probability) scattered a second
time in the Tracker before being absorbed in the Calorimeter where its energy
and absorption position are measured.

The basic principle of the Compton mode of operation is illustrated in Figure
12, left. An incident gamma-ray Compton scatters by an angle $\Theta$ in one
layer of the Tracker, transferring energy $E_{1}$ to an electron. The
scattered photon is then absorbed in the Calorimeter, depositing an energy
$E_{2}$, and the scattering angle is given by
$\cos\Theta={m_{e}c^{2}}/{E_{2}}+{m_{e}c^{2}}/{(E_{1}+E_{2})}$, where $m_{e}$
is the electron mass. With this information, one can derive an “event circle”
from which the original photon arrived. We will call “untracked” this sort of
Compton events. The uncertainty in the event circle reconstruction is
reflected in its width and is due to the uncertainties in direction
reconstruction of the scattered photon and the energy measurements of the
scattered electron $(E_{1})$ and the scattered photon $(E_{2})$. Multiple
photons from the same source enable a full deconvolution of the image, using
probabilistic techniques.

For energetic Compton scatters (above $\sim$1 MeV), measurement of the track
of the scattered electron becomes possible, resulting in a reduction of the
event ring to an arc, hence further improving event reconstruction. If the
scattered electron direction is measured, the event circle reduces to an event
arc with length due to the uncertainty in the electron direction
reconstruction, allowing improved source localization. This event is called
“tracked”, and its direction reconstruction is somewhat similar to that for
pair event – the primary photon direction is reconstructed from the direction
and energy of two secondary particles: scattered electron and photon.
Redundant kinematic information from multiple interactions could also help.
Compton scattering angles depend on polarization of the incoming photon, hence
careful statistical analysis of the photons for a strong (e.g., transient)
source yields a measurement of the degree of polarization of its high-energy
emission (e.g. for08 ).

Especially for the Compton mode at low energies, but also more broadly over
the entire energy range covered by e-ASTROGAM, it is important to keep the
amount of passive materials on the top and at the sides of the detector to a
minimum, to reduce background in the field of view and to optimize angular and
energy resolutions. In addition, the passive materials between the Tracker
layers, and between the Tracker and the Calorimeter, must be minimized for
best performance.

####  IV.1.1 Silicon Tracker

The Si Tracker is the heart of the e-ASTROGAM payload. It is based on the
silicon strip detector technology widely employed in medical imaging and
particle physics experiments (e.g. ATLAS and CMS at LHC), and already applied
to the detection of gamma-rays in space with the AGILE and Fermi missions. The
e-ASTROGAM Tracker needs double sided strip detectors (DSSDs) to work also as
a Compton telescope.

The essential characteristics of the e-ASTROGAM Tracker are:

  * •

its light mechanical structure minimizing the amount of passive material
within the detection volume to enable the tracking of low-energy Compton
electrons and $e^{+}e^{-}$ pairs, and improve the point spread function in
both the Compton and pair domains by reducing the effect of multiple Coulomb
scattering;

  * •

its fine spatial resolution of less than 40 $\mu$m ($<1/6$ of the microstrip
pitch) obtained by analog readout of the signals (as in the AGILE Tracker);

  * •

its charge readout with a very good spectral resolution of $\sim$6 keV FWHM
(noise level in the baseline configuration; the statistical contribution to
the energy resolution is negligible in the relevant range of deposited energy
($E<500$ keV), so the energy resolution is practically independent of energy
and corresponds to the noise level) obtained with an ultra low-noise FEE, in
order to accurately measure low-energy deposits produced by Compton events;
the energy threshold is 15 keV.

The Si Tracker comprises 5600 DSSDs arranged in 56 layers (100 DSSDs per
layer). It is divided in four towers of 5$\times$5 DSSDs. The spacing of the
Si layers is of 10 mm. The total detection area amounts to 9025 cm2 and the
total Si thickness to 2.8 cm, which corresponds to 0.3 radiation length on
axis, and a probability of a Compton interaction at 1 MeV of 40%. Such a
stacking of relatively thin detectors enables an efficient tracking of the
electrons and positrons produced by pair conversion, and of the recoil
electrons produced by Compton scattering. The DSSD signals are read out by 860
160 independent, low-power, electronics channels with self-triggering
capability.

##### Silicon detectors

The active element is a Si DSSD of 500 $\mu$m thickness and $9.5\times 9.5$
cm2 area, with electrodes of 100 $\mu$m width, and 240 $\mu$m pitch
(corresponding to 384 microstrips per side), a guard ring of 1.5 mm, and
polysilicon resistors for the bias. It can be manufactured from high
resistivity ($R\geq 5$ k$\Omega$ cm) 6” substrate by, e.g., the Silicon
Radiation Sensors© group of the Fondazione Bruno Kessler FBK (SRS-FBK) or
Hamamatsu Photonics©.

Each layer of a tower contains $5\times 5$ DSSDs, which are chained together
with wire bonding strip to strip. Ladders of five Si tiles are first assembled
and then bonded to five other ladders in the orthogonal direction. Si strip
bonding is now a standard technology previously used in, e.g., the Fermi/LAT
and AGILE Tracker and the PAMELA and AMS-02 cosmic-ray experiments.

##### Mechanical structure

The mechanical structure holding a tray of $5\times 5$ DSSDs with the
associated FEE is composed of two frames sandwiching the Si detectors, the
rods direction of the upper frame being orthogonal to that of the lower frame,
to form a grid. On each side, the support rods are parallel to the DSSD strips
to enable the wire bonding. The DSSDs are glued onto the frames with a
structural adhesive, and a Kapton© foil is added in the middle of the glue
thickness to ensure electrical insulation. The frames, which are 2 mm thick,
are made of a polymeric resin reinforced by high modulus carbon fibers woven
into fabrics. Carbon fiber spacers determine a total spacing of the Si layers
of 10 mm; placed at the crossing between frames, they limit the vertical
displacements under loads achieving a uniform distribution of the displacement
among them and realize unilateral connections between the trays. The towers of
the tracker are assembled together by a structural mainframe composed of
vertical fixing aluminum columns. In order to increase the stiffness of each
tower, the baseline configuration of the Tracker comprises two honeycomb
panels (e.g. Hexel 3/16-5052 + 1 mm thick carbon fiber foils), one at the top
(4 cm thickness) and one at the bottom (1 cm thickness).

The Tracker mechanical design is the result of detailed structural calculation
based on a simplified Finite Element Model (FEM) and including both static and
modal analyses. The maximum vertical displacement obtained by FEM is of 280
microns (Figure 13), far below the rupture limit of Si detectors.

Figure 13: Left panel – Overview of the Tracker mechanical design. Right panel
– Illustration of the Tracker structural analysis showing normalized
deformation of elements.

##### Front-End Electronics

The FEE is distributed over the four sides of the Tracker (there are no
electronics between the four towers), the detector microstrips being connected
to the FEE ASICs through pitch adapters. The FEE ASICs are the 32-channels
IDeF-X HD circuits developed at CEA/Saclay. The 860,160 DSSD signals are read
by a total of 26,880 ASICs (4$\times$120 ASICs per layer).

IDeF-X HD is a $5.8\times 2.5$ mm2 chip operated at 3.3 V and consuming 800
$\mu$W/channel (static dissipated power). It was built based on the AMS CMOS
0.35 $\mu$m technology using the full custom Rad-Hard libraries, and combine
the most recent developments for space applications, including fully space-
qualified devices lim05 ; gev12 . In particular, IDeF-X HD was extensively
studied with respect to radiation tolerance (Single Event Latchup (SEL) –
free; Single Event Effect (SEE) $>9$ MeV cm2 mg-1; Total Ionizing Dose (TID)
$>300$ krad w/o effect on noise response), as it was selected for the FEE of
the STIX instrument of the Solar Orbiter mission.

Each individual channel of IDeF-X HD is made of: a charge sensitive
preamplifier (CSA) optimized for low current ($<1$ nA) and capacitance around
10 pF, a variable gain (inverting or non inverting) stage, a pole-zero
cancellation stage, an adjustable shaper in the range from 1 to 11 $\mu$s
(peaking time), a baseline holder, providing a stable offset whatever the
leakage current into a channel, a peak detector and hold, and a discriminator
(each channel, 6 bit DAC) in the range from 0 to 13 keV for Si (the settings
is not linear to allow fine tuning in the low range and coarse in the high
range). Together with the 32 inputs (DC or AC coupled), the other interfaces
of IDeF-X HD are two differential analog output and two independent slow
controls. The CSA includes a continuous reset feedback circuitry, which
includes a “non stationary noise suppressor” to optimize the noise response
into the whole dynamic range. The dynamic range is 10 fC (but is programmable
up to 40 fC).

The analog output signals of IDeF-X will be converted to digital signals with
the OWB-1 ADC integrated system. OWB-1 is a low noise (0.6 LSB), low power (1
mW per active channel) chip of 11.55 mm2 area, including parallel Wilkinson
ADC for 16 differential channels with a real 13 bits resolution at a
conversion rate of 2.8 $\mu$s (and 11 bits resolution at 0.9 $\mu$s conversion
rate). The chip was also built from the AMS CMOS 0.35 $\mu$m technology and it
is radiation hard by design (SEL hardened + Single Event Upsets flag).
Qualification procedure of this ASIC is on going.

####  IV.1.2 Calorimeter

The e-ASTROGAM Calorimeter is a pixelated detector made of a high-$Z$
scintillation material – Thallium activated Cesium Iodide – for an efficient
absorption of Compton scattered gamma-rays and electron-positron pairs. It
consists of an array of 33,856 parallelepiped bars of CsI(Tl) of 8 cm length
and 5$\times$5 mm2 cross section, read out by silicon drift detectors (SDDs)
at both ends gat84 , arranged in an array of 529 ($=23\times 23$) elementary
modules comprising each 64 crystals (see Figure 14). The Calorimeter thickness
– 8 cm of CsI(Tl) – makes it a 4.3 radiation-length detector having an
absorption probability of a 1-MeV photon on axis of 88%.

The Calorimeter detection principle and architecture are based on the heritage
of the space instruments INTEGRAL/PICsIT, AGILE/MCAL and Fermi/LAT, as well as
on the particle physics experiment LHC/ALICE at CERN. However, the e-ASTROGAM
calorimeter features two major improvements with respect to the previous
instruments:

  * $\bullet$

the energy resolution is optimized to a FWHM of 4.5% at 662 keV (scaling with
the inverse of the square root of the energy) by the use of low-noise SDDs for
the readout of the scintillation signals, combined with an appropriate ultra
low-noise FEE;

  * $\bullet$

the spatial resolution is improved by measuring the depth of interaction in
the detector from a suitable weighting function of the recorded scintillation
signals at both ends; the position resolution along the CsI(Tl) bars is
$\sim$5 mm FWHM, i.e. comparable to the resolution in the X–Y plane given by
the crystal cross section ($5\times 5$ mm2). Accurately measuring the 3D
position and deposited energy of each interaction is essential for a proper
reconstruction of the Compton events.

Figure 14: Overview of the Calorimeter and of one of its 529 ($=23\times 23$)
basic modules comprising 64 CsI(Tl) crystals.

The simultaneous data set provided by the Silicon Tracker, the Calorimeter and
the Anticoincidence system constitutes the basis for the gamma-ray detection.
However, thanks to the detector excellent granularity, Calorimeter-only events
can be used on board to provide a burst notice and a first approximate
localization via fast onboard reconstruction even in the absence of a signal
from the Tracker. This data acquisition mode is dedicated to the search for
fast transient events such as GRBs and TGFs. The corresponding trigger
condition is explained in Sect. IV.1.5.

##### Scintillation crystals and housing

The choice of CsI(Tl) as the scintillation material in the baseline design is
motivated by the facts that (i) it is one of the brightest scintillator (light
yield of 54 photons per keV), (ii) it matches well to the response of Si
photodiodes (broad emission spectrum with a maximum at 540 nm), (iii) it is
only slightly hygroscopic (much less than, e.g., LaBr3 and CeBr3), (iv) it has
good radiation hardness properties, and (v) it has already flown on, e.g., the
INTEGRAL, AGILE and Fermi satellites. The required 33,856 CsI(Tl) bars could
be produced by, e.g., Saint Gobain Crystals© or Detec Europe©.

The possibility of using a new scintillator technology that could offer an
improved energy resolution and a reduced scintillation decay time will be
studied during the assessment phase. Several potential candidates exist,
including RGB (RbGd2Br7:Ce) and lutetium-based scintillators. New developments
in nano-crystalline glass ceramic scintillators also offer sufficient promise
as to be worthy of further investigation. There is a very active development
ongoing within the e-ASTROGAM consortium on this topic, with ESA support, and
the results are encouraging.

Each crystal will be wrapped with a reflective material in order to optimize
the light collection and reduce the optical cross talk. The selected
reflective material is the Radiant Mirror Film ESR from 3M, with a thickness
of 63 $\mu$m. This wrapping material, which is a non-metallic multilayer
polymer, was previously employed for the CsI crystals of the Fermi/LAT
calorimeter, as well as, e.g., for the calorimeters of the JLAB/DVCS and
GSI/PANDA particle physics experiments.

##### Photosensors: silicon drift detectors

Silicon Drift Detectors (SDDs) are solid-state devices suitable for both
direct X-ray and scintillation light detection gat84 . Their peculiar
characteristic is the fact that the signal charge is driven toward a small
collecting anode by means of a suitably tailored electric field within the
depletion region. Under these conditions, a very small output capacitance can
be obtained, almost independent from the active area, and hence an improvement
of more than an order of magnitude in noise performance with respect to PIN
photodiode of equivalent active area. When SDDs are used as readout devices
for CsI(Tl) scintillation crystals, the improved noise performance results in
a very good energy resolution (4.5% FWHM at 662 keV) and an energy threshold
lower than 30 keV mar05 . For the e-ASTROGAM calorimeter we foresee to use
SDDs designed and developed at INFN Trieste, and fabricated at the FBK (SRS-
FBK). A 6” wafer processing line was successfully set up at SRS-FBK, with mass
production capabilities.

In the e-ASTROGAM calorimeter, the basic detector element is formed by the
coupling of four CsI(Tl) bars to two square arrays of 2$\times$2 SDDs of 5 mm
side. The CsI(Tl) scintillator bars will be optically coupled to two SDD
arrays, one on top and the other at the bottom of the crystals, to enable the
reconstruction of the energy and position of interaction along the bars by
means of a suitable weighting function of the recorded signals at both ends.
This hodoscopic architecture was already employed for the AGILE and Fermi LAT
calorimeters, but using much larger crystals and PIN photodiodes as readout
devices. The improved noise performance of the SDDs will allow us to obtain a
depth-of-interaction resolution of about 5 mm FWHM when coupled to 80 mm long
scintillator bars, as already demonstrated at laboratory level lab08 .

##### Mechanical structure

The e-ASTROGAM Calorimeter is made of 529 ($=23\times 23$) elementary modules,
each of which comprising 16 basic elements of four CsI(Tl) bars coupled to two
square arrays of 2$\times$2 SDDs. This gives a total of 33,856 identical
crystals and 67,712 SDDs. The surface covered by the calorimeter is about
100$\times$100 cm2. The spacing between the Calorimeter and the Tracker (i.e.
the distance between the Calorimeter and the bottom honeycomb panel of the
Tracker structure) is 2 mm.

Figure 15: Exploded view drawing of a elementary module of the Calorimeter
with its alveolus structure.

An exploded view of a basic module of the Calorimeter is shown in Figure 15.
The crystals are supported by an alveolar structure made of a polymer resin
reinforced by high modulus carbon fibers, with four crystals (i.e. one
detector element) contained in each cell. The thickness of the structure walls
is 200 $\mu$m. Each alveolus is epoxy-glued to an insert in carbon fibers,
which makes the interface with the support elements at the bottom of the
module. Each module is fixed to a bottom sandwich panel composed of a 3 cm
honeycomb core covered by carbon fiber skins. The sandwich panel is stiff
enough to hold the weight of the 33,856 crystals of the calorimeter (305 kg).
At the top of the alveolar structure, a carbon plate is added to further limit
the displacement of the crystals.

The FEE boards with the OWB-1 ADC ASICs (see below) are positioned under the
crystals on dedicated supporting circuits. The SDD boards with the VEGA analog
ASICs in contact of the collecting anodes are connected to the ADC ASIC boards
via Kapton foils (Figure 15). The low heat generated by the SDD boards
($2\times 27$ mW per module) is also evacuated by the Kapton circuits.

#####  Front-End Electronics

We foresee using for the FEE ASICs a modified version of the VEGA circuits
developed by Politecnico di Milano and Università di Pavia. With the
development of a new chip of 4 channels, a total of 16,928 ASICs will be
needed to read the 67,712 SSD signals.

The VEGA ASIC aha15 , which was specifically developed for SDD readout, is
realized in CMOS AMS 0.35 $\mu$m technology. It is a low power circuit,
consuming 420 $\mu$W/channel, which contains a preamplifier, a programmable
shaper-amplifier, a discriminator and a peak stretcher. It provides an
Equivalent Noise Charge (ENC) of 16 electrons rms at 3 $\mu$s shaping time
when coupled to a 350 fF input capacitance SDD. With this system, an energy
resolution of 183 eV FWHM at 5.9 keV has been obtained. The ASIC includes a
linear array of 32 independent channels with dimensions of 200$\times$500
$\mu$m per channel, each with a dynamic range of 60 keV for X-ray direct
detection in Si (equivalent to about 16,400 electrons).

A minimal set of optimization steps will be carried out to tailor the
characteristics of the VEGA ASIC to the e-ASTROGAM calorimeter requirements.
In particular, considering a typical photoelectron yield of about 30 electron
per keV obtainable for CsI(Tl) scintillators coupled to silicon devices, the
shaping-amplifier dynamic range should be increased by a factor of $\sim$20
(equivalent to $\sim$10 MeV detection in CsI(Tl)).

The analog output signals of the new VEGA ASICs will be converted to digital
signals with the same OWB-1 ADC integrated system as used for the Tracker FEE.
The 2$\times$64 signals of a module will be processed by four 32-channels
OBW-1 ASICs placed on a dedicated board below the crystals (Figure 15).

####  IV.1.3 Anticoincidence System

The third main detector of the e-ASTROGAM payload consists of an
Anticoincidence system composed of two main parts: (1) a standard
Anticoincidence, named Upper-AC, made of segmented panels of plastic
scintillators covering the top and four lateral sides of the instrument,
requiring a total active area of about 5.2 m2, and (2) a Time of Flight (ToF),
aimed at rejecting the particle background produced by the platform. The
Upper-AC detector is segmented in 33 plastic tiles (6 tiles per lateral side
and 9 tiles for the top). All scintillator tiles are coupled to silicon
photomultipliers (SiPM) by optical fibers. The architecture of the Upper-AC
detector is fully derived from the successful design of the AGILE per06  and
Fermi/LAT moi07  AC systems. In particular, their segmentation has proven
successful at limiting the “backsplash” self-veto, therefore the dead time of
the instrument. The Upper-AC particle background rejection is designed to
achieve a relativistic charged particle detection inefficiency lower than
10-4, a standard value already realized in current space experiments. In
addition to the panel segmentation, providing coarse information on the part
of the detector that has been hit, we are also considering the possibility of
even finer position resolution based on the analysis of the relative light
output of multiple fibers.

In the baseline design, the Upper-AC system covers the entire instrument from
five sides, leaving open the bottom for design considerations of cabling to
the S/C bus, the layout of heat pipes, etc. The bottom side of the instrument
is protected by the ToF to discriminate the particles coming out from the
instruments from those entering the instrument from below. The ToF is composed
by two scintillator layers separated by 50 cm. The required timing resolution
is of 300 ps. The readout will be performed by SiPM connected with Time
Digital Converter (TDC). The ToF will be based on technologies well proven in
space (AMS and PAMELA satellites).

The plastic scintillator type that was selected for both Upper-AC and ToF is
the BC400 or BC408 from Saint Gobain Crystals© or the equivalent EJ212 or
EJ200 from Eljen Technology©. For all these materials, the scintillator peak
emission is around 425 nm. The scintillator thickness has been set to a
minimum of 5 mm for the Upper-AC lateral panels and 6 mm for the Upper-AC top
panel and ToF in order to get enough light to detect more than 99.99% of the
passing through relativistic charged particles. The energy threshold of the AC
detectors is set to 100 keV. The light emitted by the plastic is transferred
to the SiPM through wavelength shifting optical fibers. The combination of
optical fibers and SiPM provides the best solution to collect the scintillator
optical light, as it has the higher gain and gives the best efficiency of
charged particle rejection. SiPM has also the advantage over traditional
photomultiplier tubes to work at low bias (few tens of volts), so without the
need of high voltage. The selected SiPM could be from the B-Series (blue
sensitive) of the SensL© company. The peak sensitivity of these photosensors
is at 420 nm (within a sensitivity range of 300 - 800 nm), which copes well
with the plastic scintillator emission. SensL© SiPMs were recently studied
with respect to radiation tolerance in an ESA-led program and are now at
technology readiness level (TRL) 5. For the Upper-AC readout we foresee the
selection of the VATA64 ASIC from the Ideas Company, this chip being optimized
for SiPM readout and already space qualified (see Ref. bag11 ). Alternative
options will be considered for the ToF, such as the MUSIC ASIC, which has also
been developed for SiPM readout, and has a Single Photon Resolution Time of
about 100 ps rms gom16 .

####  IV.1.4 Data Handling and Power Supply

The e-ASTROGAM payload is completed by a Payload Data Handling Unit (PDHU) and
a Power Supply Unit (PSU). The PDHU is in charge of carrying out the following
principal tasks: (i) payload internal control; (ii) scientific data
processing; (iii) operative modes management; (iv) on board time management;
(v) Telemetry and Telecommand management. The main functions related to the
scientific data processing are: (i) BEE interfacing through dedicated links to
acquire the scientific data; (ii) the real-time software processing of the
collected silicon Tracker, Anticoincidence and Calorimeter scientific data
aimed at rejecting background events to meet the telemetry requirements; (iii)
scientific data compression; (iv) formatting of the compressed data into
telemetry packets.

The heart of the PDHU architecture is based on a powerful Digital Signal
Processor (DSP) running the payload on-board software. Considering the large
amount of events to be processed, a possible DSP could be the HiRel component
C6727B-250 produced by Texas Instruments©. This floating point CPU running at
250 MHz is capable of 500 MMACS (2000 MIPS / 1500 MFLOPS) by executing up to 8
instructions in parallel (6 of which floating points).

The PSU is in charge of generating the required payload voltages with high
DC/DC conversion efficiency and distributing them to the other sub-systems.

####  IV.1.5 Trigger logic and data flow architecture

The e-ASTROGAM on-board scientific data processing is composed of two main
trigger pipelines, the gamma-ray acquisition mode and the Calorimeter burst
search. Both are based on the experience of the AGILE and Fermi missions. The
simultaneous data sets provided by the silicon Tracker, the Calorimeter and
the AC constitute the basis for the gamma-ray detection and processing. The
gamma-rays trigger logic is structured on two main levels: Level-1 (fast: 5-10
$\mu$s logic, hardware); and Level-2 (asynchronous, 50 $\mu$s processing,
software). Figure 16 shows the expected data rates at the input of the Level-1
and at the output of the Level-1 and Level-2 trigger stages.

Level-1 is a hardware trigger logic with fast response implemented in the
silicon Tracker BEE providing a preliminary discrimination between Compton and
pair-producing photon events and a first cut of background events.
Discrimination criteria based on the hit multiplicity in the Tracker and in
the Calorimeter can provide optimal algorithms to identify Compton events. The
Level-1 trigger configuration is defined to save the largest possible number
of potential Compton events.

Figure 16: Expected data flow of the on-board e-ASTROGAM gamma-ray data
acquisition system.

Level-2 is a software trigger stage carried out by the PDHU and aimed, at
further reducing the residual particle and photon background of the pair data
set and at finalizing the selection of the Compton events. The heart of the
Level-2 trigger stage consists of the track reconstruction of the candidate
pair events implemented with Kalman Filter techniques. The Level-2 trigger is
a full asynchronous processing stage and does not increase the dead time of
the instrument. At the end, the Compton events and the pair events surviving
the Level-2 trigger are collected in dedicated telemetry packets and sent to
the ground.

Level-1 is has a rate tuned to limit the dead time and to fit the accepted
event rate into the acceptance capability of the onboard reconstruction stage.
Level-2, which is fed by events passing Level-1, is tuned so that the rate of
accepted events fits into telemetry bandwidth. Only events passing Level-2 are
downlinked.

The Calorimeter burst search is a software algorithm implemented by the PDHU.
The burst search is based on the integration and processing of a proper set of
rate meters measuring the trend of the background and foreground counting
rates. Since the expected impulsive signals (GRBs and TGFs) are strongly
energy- and timescale-dependent, the rate meters are integrated on a wide
range of timescales (0.1 ms – 10 s) and energy ranges (in the overall range 30
keV – 200 MeV). Assuming a typical GRB spectrum, the energy channels are
defined in order to obtain a homogeneous threshold over the whole energy
range.

The burst search logic routinely compares the background and foreground rate
meters. Elementary triggers are generated when foreground rate meters show an
over-threshold counting excess respect to the background estimation. Finally,
the triggers have to be validated by satisfying coincidence conditions on the
different energy channels aimed at spurious trigger rejection. These
conditions are implemented using look-up tables fully programmable from
ground.

A cyclic buffer is required to routinely save the events; the size of this
buffer is defined in order to store 100 s of background. Following a burst
trigger, the logic will be able to identify the beginning and the end of the
segment of data acquisition and then transfer it to ground by telemetry.

###  IV.2 Performance assessment

The scientific performance of the e-ASTROGAM instrument was evaluated by
detailed numerical simulations with the software tools MEGAlib and BoGEMMS.
The MEGAlib package zog06  was originally developed for analysis of simulation
and calibration data related to the Compton scattering and pair-creation
telescope MEGA kan05 . It has then been successfully applied to a wide variety
of hard X-ray and gamma-ray telescopes on ground and in space, such as
COMPTEL, NCT, and NuSTAR. BoGEMMS (Bologna Geant4 Multi-Mission Simulator) is
a software for simulation of payload of X- and gamma-ray missions, which has
been developed at the INAF/IASF Bologna bul12 . It has already been applied to
several hard X-ray/gamma-ray instruments and mission projects, including
Simbol-X, NHXM, Gamma-Light, AGILE, and GAMMA-400. Both software packages
exploit the Geant4 toolkit to model the geometrical and physical parameters of
the detectors and simulate the interactions of photons and particles in the
instrument.

The numerical mass model of e-ASTROGAM used to simulate the performance of the
instrument is shown in Figure 17. An accurate mass model that includes passive
material in the detector and its surroundings, true energy thresholds and
energy and position measurement accuracy, as well as a roughly accurate S/C
bus mass and position are crucial to the modeling. In particular, care was
taken to include all passive materials close to the Si and CsI(Tl) detectors.

####  IV.2.1 Background model

Figure 17: Geant4/MEGAlib mass model of the e-ASTROGAM telescope, with a
simulated pair event (in blue) produced by a 30-MeV photon. The vertical green
lines represent the carbon fiber spacers placed between the DSSD layers to
increase the stiffness of the Tracker (see Sect. IV.1.1).

For best environmental conditions, e-ASTROGAM should be launched into a quasi-
equatorial (inclination $i<2.5^{\circ}$) LEO at a typical altitude of 550 km.
The background environment in such an orbit is now well-known (Figure 18),
thanks to the Beppo-SAX mission, which measured the radiation environment on a
low-inclination ($i\sim 4^{\circ}$), 500 – 600 km altitude orbit almost
uninterruptedly during 1996 – 2002 cam14  and the on-going AGILE mission,
which scans the gamma-ray sky since 2007 from a quasi-equatorial orbit at an
average altitude of 535 km tav09 . The dominant sources of background for the
e-ASTROGAM telescope in the MeV domain are the cosmic diffuse gamma-ray
background, the atmospheric gamma-ray emission, the reactions induced by
albedo neutrons, and the background produced by the radioactivity of the
satellite materials activated by fast protons and alpha particles. All these
components were modeled in detail using the MEGAlib environment tools. In the
pair domain above 10 MeV, the background is mainly induced by fast particles
(mainly leptons) impinging the spacecraft, as well as by the cosmic diffuse
radiation and the atmospheric gamma-ray emission.

Figure 18: Background environment of e-ASTROGAM on its orbit. The satellite
will be exposed to Galactic CR (mainly protons and electrons) modulated by the
geomagnetic field, semi-trapped secondary protons and leptons, as well as to
albedo neutrons and atmospheric gamma rays. The cosmic diffuse X- and gamma-
ray radiation (in green) is the dominant background component below a few
hundred keV, but it is also a fundamental science topic for e-ASTROGAM above a
few MeV.

####  IV.2.2 Angular and spectral resolution

e-ASTROGAM will image the Universe with substantially improved angular
resolution both in the MeV domain and above a few hundreds of MeV, i.e.
improving the angular resolution of the CGRO/COMPTEL telescope and that of the
Fermi/LAT instrument by a factor of $\sim$4 at 1 MeV and 1 GeV, respectively.

In the pair production domain, the PSF improvement over Fermi/LAT is due to
(i) the absence of heavy converters in the Tracker, (ii) the light mechanical
structure of this detector minimizing the amount of passive material within
the detection volume and thus enabling a better tracking of the secondary
electrons and positrons, and (iii) the analog readout of the DSSD signals