[a,b,c]M. Nievas Rosillo,

# A model-driven search for extreme BL Lacs among LAT BCU

G. Chiaro  A. Dominguez  and G. La Murag on behalf of the Fermi Large Area
Telescope Collaboration

###### Abstract

The emission of very-high-energy photons (VHE, E$>$100 GeV) in active galactic
nuclei (AGN) is closely connected with the production of ultra-relativistic
particles. Among AGN, the subclass of extreme BL Lacertae are of particular
interest because they challenge state-of-art models on how these cosmic
particle accelerators operate. By cross-matching two gamma-ray catalogs (this
is, 4FGL-DR2 and 2BIGB), we identified 23 high-synchrotron-peaked (HSP) blazar
candidates with photometric or spectroscopic redshifts, good multi-wavelength
coverage, that are possibly detectable by VHE instruments. We performed a new
analysis of Fermi Large Area Telescope data including the effects of
attenuation from the extragalactic background light and complemented these
results by collecting multiwavelength data from optical, radio and X-ray
archival observations. Their broadband spectral energy distributions were
interpreted in terms of synchrotron-self-Compton models with external-Compton
components and compared with the properties of prototypical extreme HSP
blazars. Finally, we test their detectability with imaging atmospheric
Cherenkov telescopes (IACTs) and propose a new method for selecting these
extreme targets for these ground-based telescopes.

##  1 Introduction

The extra-galactic $\gamma$-ray sky is dominated by blazars [1], a peculiar
class of radio-loud Active Galactic Nuclei (AGN) that host a collimated jet of
relativistic plasma in close alignment with our line of sight. Due to the
effects of relativistic beaming, the jet radiation dominates the source
emission at almost every frequency and it produces a characteristic two-hump
spectral energy distribution (SED), that extends from the radio domain all the
way up to $\gamma$-ray energies. The low energy part of the SED, lying between
radio and X-ray frequencies, is interpreted as Synchrotron radiation by
relativistic charged particles in the magnetic field of the jet. The high
energy part, which instead covers the $\gamma$-ray domain and may reach up to
Very High Energies (VHE, $E>100\,$GeV), is due to inverse Compton (IC)
scattering of low energy photons over the emitting particles.

Depending on their optical properties, blazars are generally divided in BL Lac
type objects, whose spectra are dominated by the synchrotron continuum and
appear nearly featureless, showing at most weak emission or absorption lines,
and Flat Spectrum Radio Quasars (FSRQ), which, instead, are characterised by
prominent emission lines and a continuum with a relevant thermal contribution,
suggestive of the presence of a radiatively efficient hot plasma accretion
disk. In addition to the optical properties, another classification scheme is
generally defined through the characteristics of the SED and, in particular,
the position of the synchrotron peak frequency emission. In this way, blazars
can be distinguished in Low Synchrotron Peaked (LSP,
$\nu^{S}_{\mathrm{peak}}\leq 10^{14}\,$Hz), Intermediate Synchrotron Peaked
(ISP, $10^{14}\,\mathrm{Hz}<\nu^{S}_{\mathrm{peak}}\leq 10^{15}\,$Hz) and High
Synchrotron Peaked (HSP, $\nu^{S}_{\mathrm{peak}}>10^{15}\,$Hz) sources. While
LSP and ISP sources can be either BL Lacs or FSRQs, HSP objects are almost
exclusively BL Lacs and are sometimes referred to as HBL. There is a growing
evidence that a further class of extremely high synchrotron peaked BL Lac
objects (EHSP, $\nu^{S}_{\mathrm{peak}}>10^{17}\,$Hz) may exist. HSP and EHSP
objects are of particular interest, because they represent the most frequent
class of extra-galactic VHE sources.

Thanks to its all-sky monitoring observational strategy, the Fermi Large Area
Telescope (Fermi-LAT [2]) has detected $\gamma$-ray emission from thousands of
objects that were subsequently identified as blazars. A fraction of these
sources, however, have not yet been firmly classified, though showing blazar-
like SED characteristics, and they are therefore referred to as blazar
candidates of uncertain class (BCU). In this contribution we present the first
results of a technique based on catalog matching and SED modelling that we
developed to select EHSP source candidates from the Fermi-LAT BCUs.

##  2 Materials and Methods

The sample analyzed in this study derives from a cross-match of the Fermi-LAT
BCUs listed in the second release of the Fourth LAT AGN Catalog (4LAC-DR2,
[3]) with the Second Brazil-ICRANet Gamma-ray Blazars catalog (2BIGB, [4]),
which includes sources whose infrared photometric properties are similar to
those of HBL sources. We limited our selection to objects that, according to
the 2BIGB records, have a broad-band photometric coverage and for which we
could obtain at least a redshift estimate (using either spectroscopic
redshifts provided by [3] and by [5], or the 2BIGB’s photometric estimates).
We further introduced a figure of merit (FOM) parameter, defined as the ratio
among the flux at synchrotron peak of any given source and the synchrotron
peak flux of the faintest blazar presently detected at TeV energies, thereby
considering only objects whose FOM is larger than 0.7. The result of this
operation was the selection of 23 EHBL source candidates, listed in Table 1.

4FGL Name | RAJ2000 | DEJ2000 | $z$ | TS | FOM | Index | VarIndex | FracVar
---|---|---|---|---|---|---|---|---
J0132.7$-$0804 | 23.183 | -8.074 | 0.148 | 88 | 0.8 | 1.9 | 5.54 |
J0212.2$-$0219 | 33.066 | -2.319 | 0.250 | 61 | 0.8 | 2.2 | 16.70 | $0.47\pm 0.29$
J0350.4$-$5144 | 57.613 | -51.743 |  $\sim$0.32 | 98 | 0.8 | 1.8 | 11.62 | $0.32\pm 0.36$
J0515.5$-$0125 | 78.891 | -1.419 |  $\sim$0.25 | 55 | 0.8 | 2.1 | 13.04 | $0.32\pm 0.31$
J0526.7$-$1519 | 81.692 | -15.321 |  $\sim$0.21 | 218 | 1.6 | 2.0 | 8.21 |
J0529.1$+$0935 | 82.297 | 9.597 |  $\sim$0.30 | 86 | 1.3 | 2.1 | 12.81 | $0.23\pm 0.26$
J0557.3$-$0615 | 89.344 | -6.265 |  $\sim$0.29 | 53 | 1.6 | 2.0 | 7.08 |
J0606.5$-$4730 | 91.642 | -47.504 | 0.030 | 137 | 1.0 | 2.0 | 17.90 | $0.39\pm 0.20$
J0647.0$-$5138 | 101.773 | -51.638 |  $\sim$0.22 | 81 | 2.5 | 1.8 | 17.03 | $0.32\pm 0.40$
J0733.4$+$5152 | 113.362 | 51.880 | 0.065 | 162 | 2.5 | 1.8 | 12.43 | $0.26\pm 0.30$
J0847.0$-$2336 | 131.757 | -23.614 | 0.059 | 921 | 0.8 | 2.0 | 14.72 | $0.14\pm 0.09$
J0953.4$-$7659 | 148.367 | -76.993 |  $\sim$0.25 | 104 | 0.8 | 2.0 | 5.88 |
J0958.1$-$6753 | 149.534 | -67.894 |  $\sim$0.21 | 29 | 1.0 | 2.2 | 12.73 | $0.46\pm 0.51$
J1132.2$-$4736 | 173.056 | -47.613 |  $\sim$0.21 | 129 | 1.0 | 2.0 | 11.47 | $0.26\pm 0.22$
J1447.0$-$2657 | 221.765 | -26.962 |  $\sim$0.32 | 46 | 2.0 | 2.0 | 6.81 |
J1714.0$-$2029 | 258.522 | -20.486 |  $\sim$0.09 | 110 | 2.0 | 1.6 | 24.95 | $0.59\pm 0.28$
J1824.5$+$4311 | 276.126 | 43.196 | 0.487 | 99 | 0.8 | 1.8 | 8.17 |
J1934.3$-$2419 | 293.582 | -24.326 |  $\sim$0.23 | 63 | 1.6 | 1.8 | 11.21 |
J1944.4$-$4523 | 296.101 | -45.393 |  $\sim$0.21 | 164 | 1.0 | 1.7 | 14.32 | $0.22\pm 0.32$
J2001.9$-$5737 | 300.491 | -57.631 |  $\sim$0.26 | 123 | 0.8 | 2.1 | 2.94 |
J2142.4$+$3659 | 325.602 | 36.986 |  $\sim$0.24 | 110 | 1.3 | 2.0 | 13.35 | $0.25\pm 0.29$
J2246.7$-$5207 | 341.682 | -52.126 | 0.194 | 95 | 2.5 | 1.7 | 19.10 | $0.57\pm 0.30$
J2251.7$-$3208 | 342.944 | -32.140 | 0.246 | 52 | 2.0 | 1.8 | 11.20 | $0.28\pm 0.49$

Table 1: EHBL candidates considered in this work and main properties extracted
from the 4FGL, 4LAC and 2BIGB catalogs.

In order to optimize the coverage of the high energy emission, we repeated the
$gamma$-ray analysis of the $Fermi$-LAT data using the same observation time
interval of 4LAC-DR2, following the Cicerone analysis recommendations and
taking into account the interaction between $\gamma$-ray photons and the
Extragalactic Background Light radiation (EBL) at the modelling stage [6, 7].
We subsequently built the multi-wavelength SEDs of our sources by searching
for counterparts at different frequencies through the SSDC’s SED builder
tool.111https://tools.ssdc.asi.it/SED/ We looked for archival data covering
from radio to X-rays that, together with our $\gamma$-ray spectra, resulted in
a collection of SEDs that we modelled using the jetset emission code [8].

As a final step, we investigated the detectability of our sources in the VHE
band by generating and anylising samples of simulated VHE $\gamma$-ray
observations, based on the SED modeling previously introduced and using CTA’s
Prod3b response functions for an exposure time of $5\,$hr. Taking into account
the estimated performance of currently existing and planned VHE observatories,
some of which are illustrated as an example in Fig. 1, this approach is
expected to approximately represent also the performance of currently
operating Imaging Atmospheric Cherenkov Telescopes (IACT) in a cumulative
exposure of $50\,$hr, thus illustrating the estimated detection possibilities
of the selected targets with currently operating and future observing
facilities.

Figure 1: Sensitivities of various $\gamma$-ray instruments in operation and
planned.

##  3 Properties of the selected BCU sample

\begin{overpic}[percent,width=238.49231pt]{{Fig_4LAC_SpecIndex_vs_logFlux_poster}.pdf}
\put(45.0,25.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=216.81pt]{{Fig_4LAC_blazarsequence_poster}.pdf}
\put(45.0,25.0){\color[rgb]{1,0,0}\large Preliminary} \end{overpic}

Figure 2: Left: Spectral index vs. Flux in the Fermi-LAT band for: LAC-DR2
sources (density color coded brown tones), 2BIGB BL Lacs (blue) and our
targets (green stars); right: K-corrected $\gamma$-ray luminosity vs. redshift
for FSRQs (red crosses), BL Lacs (blue diamonds) and other extragalactic
objects (open orange circles).

In order to test the ability of our selection procedure to identify candidate
HSP and EHSP sources, we compared the properties of the selected sources with
those of already classified objects. Since we know that BL Lacs have
statistically harder $\gamma$-ray spectra and are less luminous than FSRQs, we
created diagnostic plots to illustrate the distribution of our sources with
respect to the different blazar classes detected by Fermi-LAT. We can see from
Fig. 2 that our targets tend to have low to intermediate redshifts
($0.05\lesssim z\lesssim 0.30$), hard spectra, like those of BL Lacs, but with
rather low luminosities ($L_{\gamma}\lesssim 10^{45}\,\mathrm{erg/s}$).

##  4 Broadband SED modelling

A more precise identification of the most likely EHBL source candidates can be
obtained by the construction of a set of SED models that reproduces the
available observations. Most of the sources are well represented by the
inclusion of some standard components. Specifically, these include:

Non-thermal low-energy component: a one-zone leptonic model with emission
originating in a spherical plasma region of radius $R$ embedded in the blazar
jet. Its distance to the nucleus is $R_{H}$ and its magnetic field strength
$B$. Electrons in the plasma are assumed to have a broken power-law spectrum
with indices $p_{1}$ between Lorentz factors $\gamma_{\mathrm{min}}$ and
$\gamma_{\mathrm{br}}$ and high-energy index $p_{2}$ between
$\gamma_{\mathrm{br}}$ and $\gamma_{\mathrm{max}}$.

High energy emission: Due to inverse Compton scattering from the same
population of electrons on both synchrotron radiation and, if present, the
infrared radiation from a dusty torus.

Thermal radiation: A clear excess in the optical band is visible for all the
sources, and assumed to be host emission from a giant elliptical galaxy. We
model it as a black body with effective temperature $\mathrm{T_{eff,Host}}$
and luminosity $\mathrm{L_{Host}}$. For sources with a dusty torus, we assume
a characteristic size $R_{DT}$, opacity $\tau_{DT}$ and effective temperature
$T_{DT}$.

The spectral fitting is calculated through the jetset code. Due the limitation
of non-simultaneous input data and to the strong degeneracies existing between
different properties, we fixed some parameters to a set of typical values.
Specifically, we adopted a minimum particle Lorentz factor $\gamma_{min}=1$, a
bulk jet Lorentz factor $\Gamma=20$, a scale radius of the emitting region
$R=1\times 10^{16}\,\mathrm{cm}$, and a distance of the emitting region from
the center $R_{H}=2\times 10^{18}\,\mathrm{cm}$. The resulting SEDs for 8
sources out of the 23 included in our sample are shown as an example in Fig.
3.

\begin{overpic}[percent,width=151.76964pt]{{figures/SED_0132m0804.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=151.76964pt]{{figures/SED_0350m5144.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=151.76964pt]{{figures/SED_0606m4729.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=151.76964pt]{{figures/SED_0733p5153.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=151.76964pt]{{figures/SED_0847m2337.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=151.76964pt]{{figures/SED_1446m2656.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=151.76964pt]{{figures/SED_1714m2027.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary}
\end{overpic}\begin{overpic}[percent,width=151.76964pt]{{figures/SED_2251m3206.txt_All_Model}.pdf}
\put(50.0,75.0){\color[rgb]{1,0,0}\large Preliminary} \end{overpic}

Figure 3: SEDs for 8 of the 23 sources considered in this work. Green open
square markers represent archival data, purple and blue open circles show the
Fermi-LAT analysis results (using a LogParabola and PowerLaw with exponential
absorption shapes respectively, both absorbed by EBL) and red stars show the
simulated $5\,h$ exposure observations with CTA, as a proxy for observations
with existing IACTs in $50\,h$. Finally, the different curves show the
different components of the resulting model.

##  5 Conclusions

In this study, we explored the possibility of using a cross-match of catalogs
of photometric data to identify potential HSP and EHSP emitters within the
sample of Fermi-LAT BCU sources and to reconstruct their SEDs. Subsequent
modelling of their broadband emission confirmed that 16 objects, out of the
original sample of 23 sources, can be classified as EHSP, while the remaining
ones have an HSP SED. Interestingly, we note that one-zone leptonic models are
able to provide good fit to all the spectra. We also observe thermal
components, most frequently as a strong host galaxy contribution, which is
commonly expected in EHSP, but with hints of unexpected torus-like IR excesses
in few cases. Three sources, namely 4FGL J0733.4+5152, 4FGL J1447.0-2657 and
4FGL J2251.7-3208m exhibit extreme synchrotron peak frequencies
($\nu^{S}_{\mathrm{peak}}>10^{18}\,$Hz). Only the first and the last have
enough X-ray data to clearly constrain the peak position, while for 4FGL
J1447.0-2657 it was constrained mainly from the Fermi-LAT spectrum (therefore
the peak frequency should be considered a lower limit). The VHE visibility
tests resulted in the identification of only 4FGL J0847.0-2336 and 4FGL
J1714.0-2029 as promising VHE candidates, while the remaining sources have too
low predicted fluxes or soft spectra to be considered possibly interesting
targets. Both these sources have, according to the proposed radiative model,
very low magnetization ($B\sim 10^{-4}\,\mathrm{G}$), strong host emission and
no IR torus.

Based on these results, we conclude that the SED of HSP candidates can be well
reproduced within a Synchrotron-Self Compton scenario (SSC), based on a rather
standard set of spectral components, even with the limitations imposed by non
simultaneous observations. Clearly, the availability of more data,
particularly in the X-ray domain, would help us to better constraint the
physical parameters of the models, but their ability to reproduce the overall
appearance of the SED provides an extremely useful tool to test the HSP nature
candidate sources identified by means of multi-wavelength catalog searches.

## Acknowledgements

The Fermi-LAT Collaboration acknowledges support for LAT development,
operation and data analysis from NASA and DOE (United States), CEA/Irfu and
IN2P3/CNRS (France), ASI and INFN (Italy), MEXT, KEK, and JAXA (Japan), and
the K.A. Wallenberg Foundation, the Swedish Research Council and the National
Space Board (Sweden). Science analysis support in the operations phase from
INAF (Italy) and CNES (France) is also gratefully acknowledged. This work
performed in part under DOE Contract DE-AC02-76SF00515.

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