###### Abstract

Introduced in 1998 to attempt a first unified view of the broad-band emission
properties of blazars, the blazar sequence has been extensively used in the
past 25 years to guide observations as well as physical interpretation of the
overall emission from these galaxies. In this review, we describe the
evolution of the sequence along with the tremendous advances in the
observational field, in particular in the gamma-ray band. A new version of the
sequence built on TeV-detected objects is also presented. Two extreme classes
of objects (MeV and hard-TeV blazars) are included in the discussion, given
their relevance for future observatories. Finally, the current physical
understanding at the base of the sequence is presented along with the major
criticisms to the blazar sequence.

###### keywords:

blazars; astrophysics; TeV astronomy

1 1 0 25 January 2022 https://doi.org/ The Blazar sequence and its Physical
Understanding The Blazar Sequence and its Physical Understanding Elisa
Prandini 1 and Gabriele Ghisellini 2 Elisa Prandini and Gabriele Ghisellini
Prandini, E.; Ghisellini, G. Correspondence<EMAIL_ADDRESS><EMAIL_ADDRESS>
##  1 Introduction: the blazar paradigm

In retrospect, we can identify the discovery of the first quasar, 3C 273
(Schmidt, 1963), as also the discovery of the first blazar. 3C 273 is in fact
a variable radio source with an optical counterpart whose detailed
spectroscopic analysis revealed an extragalactic nature ($z=0.158$). It was
1963, and tremendous advancements in the multiwavelength and, more recently,
multimessenger observations of the Universe have been done since then. Almost
sixty years after this first identification, thousands of blazars now populate
the extragalactic sky. Blazars are by far the most numerous class of
persistent extragalactic sources in the gamma-ray band (see e.g., Abdollahi et
al., 2020). A blazar is also the first extragalactic source associated as a
likely counterpart for high-energy neutrino emission (IceCube Collaboration,
2018).

In the past decades, the increase of the number and quality of observations
from the radio to the gamma-ray band along with improved theoretical models
and numerical simulations allowed to establish a paradigm explaining the
blazar and in general the AGN structure and many of the observed properties
(Antonucci, 1993; Urry and Padovani, 1995; Heckman and Best, 2014). Still, a
number of unknowns remain on the AGN phenomenon. The ones most relevant to the
blazar case are: what is the physical driver of the relativistic jet and how
it connects with the BH mass and the accretion rate; what is the rate of
cosmic rays and neutrinos emitted in a blazar’s jet and how does it connect
with the gamma-ray emission mechanisms; what is the origin of the observed
variability and how it is connected with the overall multi-wavelength and
multi-messenger emission (see e.g., Padovani et al., 2017, for a recent
review).

Blazars are jetted active galactic nuclei (AGNs) with a jet pointing at a
small angle ($<10^{\circ}$) toward the observer. Roughly 10% of jetted-AGNs
are blazars (Padovani et al., 2017). Blazars come in two flavours: Flat
Spectrum Radio Quasars (FSRQs) and BL Lac objects, depending on the width of
their emission lines. FSRQs exhibit lines with an equivalent width $>5$ Å. In
BL Lac objects, the lines are fainter or even absent.

The current paradigm, see the scheme in Fig. 1, foresees that blazars, and
more in general jetted-AGNs, are composed of:

  * •

A supermassive black hole with a mass between 107 to 1010 $M_{\odot}$, located
in the center of an elliptical galaxy.

  * •

A flow of mass feeding the black hole, named accretion disk.

  * •

Two highly collimated jets arising from the proximity of the central object
and extending to several kpcs. Within the jet, there are regions where
particles are accelerated to ultra-relativistic energies.

  * •

In FSRQs, an obscuring torus of dust that surrounds the accretion disc.

Figure 1: Jetted AGN simplified scheme from Urry and Padovani (1995).

In FSRQs, the analysis of optical-to- X-ray data has shown that the accretion
occurs through a thin and hot accretion disc. This disc is surrounded by a hot
thermal corona and by two regions filled with clouds of gas, a narrow line
region (NLR) and a broad line region (BLR). In BL Lac objects, the absence of
broad emission lines and of any sign of thermal emission produced by the
accretion disk suggests that the two properties are related. In other words,
the weakness of the accretion disk radiation may be due to the accretion
occurring in the Advection Dominated Accretion Flow (ADAF) regime (see e.g.,
Narayan et al., 2000; Blandford and Begelman, 1999; Narayan et al., 1997). The
ADAF regime is radiatively inefficient and characterized by a dramatic
decrease of ionizing UV photons, that are therefore unable to photo–ionize the
clouds of gas responsible for the broad lines (see e.g., Mahadevan, 1997).
This coarse division into BL Lac objects and FSRQs, however, is in some cases
arbitrary as transitional objects displaying mixed properties exist (see,
e.g., MAGIC Collaboration et al., 2018).

From the observational point of view, blazar emission covers more than 15
orders of magnitude in frequency, from radio to $\gamma$-rays. Blazars are in
general highly variable objects, with variations ranging from months to minute
timescales (Rieger, 2019). The complexity of the spectral energy distribution
(SED) depends on the class of objects considered, with FSRQs showing in
general a more articulated SED, with several superimposed contributions. As an
example, the multi-epoch SEDs of 2 blazars, the FSRQ 3C 454.3 and the BL Lac
object Mkn 501, are represented in Figure 2. The SED of 3C 454.3 is
interpreted as the superposition of the jet emission (two broad bumps peaking
at optical and gamma-ray energies), the IR torus, and the hot corona. Mkn 501
SED is instead fully characterized by the jet emission and a contribution of
the host galaxy in the sub-optical spectral domain.

As usual in astronomy, classification based on experimental evidences helps
identifying key features and pinpointing the physical mechanisms responsible
for the broadband emission registered. Blazars make no exception to this rule.
This review is devoted to the blazar sequence, a phenomenological sequence
proposed in Fossati et al. (1998) to describe the average properties of the
SED of blazars. Since this first article, great advancements in the
observations, in particular in the $\gamma$-ray band, have questioned and
tested the sequence, which was updated accordingly. § 2 is devoted to an
overview of the up-to-date observational properties of blazars complemented by
a description of the state-of-the-art emission models. In § 3, we present the
blazar sequence and its evolution along the years. We will also propose a new
sequence based on TeV data. In § 4 we discuss the extremes of the sequence,
namely, MeV blazars and extreme blazars. In § 5, we outline the physical
interpretation of the sequence, while § 6 is devoted to the main criticisms
against the sequence. A discussion on future observations concludes this
manuscript in § 7.

Figure 2: Examples of spectral energy distributions (SEDs) of a FSRQ (right,
3C 454.3) and a BL Lac object (left, Mkn 501) from multi-epoch observations.
In the SED of 3C 454.3, in addition to the jet emission, we can identify an
accretion disc component, the X-ray corona contribution and the IR emission
from the torus (dashed black lines), from Bonnoli et al. (2011). In Mkn 501,
instead, only the contributions from the jet and host galaxy are evident, from
Tavecchio et al. (2001), ©AAS. Reproduced with permission.

##  2 Blazar Emission

Given their broadband and variable SED, coordinated, multi-wavelength
observations are the key to accessing a reliable characterization of blazar
emissions. From the observational point of view, the multi-wavelength approach
became accessible only in the ’90ies, when COMPTEL and EGRET in gamma rays
(Schoenfelder et al., 1993; Thompson et al., 1993) flanked several X-ray
satellites (see Giacconi, 2009, for a review) and opened the high-energy
window. A few years later this window was extended to the very high energy
domain (VHE, E>100 GeV) by ground-based Cherenkov telescopes (see Chadwick,
2021, for a review). And starting from 2008, the Fermi satellite (Atwood et
al., 2009), together with the second generation of ground-based Cherenkov
telescopes (H.E.S.S. (Bernlöhr et al., 2003), MAGIC (Aleksić et al., 2016),
VERITAS (Holder et al., 2006)) have literally revolutionized our view and
knowledge of the $\gamma$-ray universe.

One of the challenges that the blazar, and in general the high-energy
community is facing is that of collecting simultaneous, possibly strictly
contemporaneous data in a broad range of frequencies. This often requires a
daily-scale human intervention (for target selection, request of observation,
and coordination). An effort in this sense, focussed on transient phenomena,
is the Astrophysical Multimessenger Observatory Network (AMON) (Smith et al.,
2013) and, under development, a number of broker alert systems designed for
handling the Vera C. Rubin Observatory Legacy Survey of Space and Time alert
stream, such as the one discussed in Möller et al. (2021).

###  2.1 Emission Models

The most popular models to interpret the blazar emission differ in particles
and are:

  * -

One zone, leptonic (Ghisellini et al., 1985), that considers that most (but
not all!) luminosity is produced in a well-defined zone at some distance
$R_{\rm diss}$ from the central engine by relativistic electrons;

  * -

One zone, hadronic (e.g., Böttcher et al., 2013), that assumes that the
relativistic protons are responsible for the emission, even if not directly
(except for the proton–synchrotron model). Proton–proton collisions or more
likely, photo-hadronic interactions can produce electron positron relativistic
pairs that can then radiate;

  * -

Multi zone, either leptonic or hadronic (e.g., Potter and Cotter, 2013),
assumes that the particles are accelerated and radiate all along the jet in a
more or less continuous way. These models then consider that the density of
the emitting particles and the magnetic field are a (power–law) function of
the distance from the black hole. In these models, the jet geometry
(paraboloidal or conical) plays a crucial role.

Radiative processes: In all cases the main radiation processes of the jet are
the synchrotron mechanism for the low energy part and the Inverse Compton (IC)
for the high energy part. If the seed photons for the IC scattering are the
synchrotron photons produced by the same electrons producing the high energy
Compton component, the process is called Synchrotron–Self–Compton (SSC), while
if the main seeds are produced externally to the jet (disk radiation, BLR
lines, torus…) the process is called external Compton (EC).

Spine–layer structure: Observational evidence (e.g., Giroletti et al., 2004)
and important $\gamma$–ray emission from misaligned sources (e.g., Grandi et
al., 2012) led to the suggestion that the blazar jet is structured. Ghisellini
et al. (2005) suggest that a high velocity spine is surrounded by a slower
layer ($\Gamma\sim 13$ and $\Gamma\sim 3$, respectively). The spine sees the
layer radiation beamed and this enhances its Compton emission. On the other
hand, also the layer sees the radiation from the spine as beamed, and thus
also the Compton emission of the layer is enhanced.

Thermal emission: BL Lacs. In BL Lacs we see no sign of thermal emission,
leading to the suggestion that:

  1. 1.

The accretion regime is not radiatively efficient;

  2. 2.

This corresponds to a paucity of ionizing radiation, corresponding to the
absence of broad emission lines;

  3. 3.

There is no molecular torus, as first suggested by Chiaberge et al. (1999);

  4. 4.

All these properties can be understood if the accretion luminosity, in units
of the Eddington one, is smaller than some critical value ($L_{\rm
disk}/L_{\rm Edd}\leq 10^{-2}-10^{-3}$).

Thermal emission: FSRQs. In powerful FSRQ, instead, we do see directly the
accretion disk radiation, besides the broad emission lines and the IR torus
component. This therefore suggests that the high energy emission of FSRQs is
likely due to the EC process, while in BL Lacs is SSC (but possibly accounting
for the spine–layer structure).

###  2.2 Energy budget

The first duty of the jet is to bring mechanical energy to the extended
structures (hot spots and radio lobes). Radiation instead corresponds to
$\sim$10% of the total jet energy (Ghisellini et al., 2014). It is not easy to
calculate the jet power directly from observations, because the radiation is
beamed. Nevertheless, if we know (i.e., from superluminal motion) the bulk
Lorenrz factor $\Gamma$ then we can set a firm lower limit to the jet power,
that cannot be smaller than $L_{obs}/\Gamma^{2}$ (Ghisellini et al., 2014).
Therefore, assuming 10% efficiency (e.g., Nemmen et al., 2012) we have that
the total jet power is of the same order, but on average larger, than the
accretion disk luminosity (Ghisellini et al., 2014). Blazar’s jets are
therefore the most efficient persistent engines in nature.

Figure 3: Dependance of the synchrotron peak frequency with the radio
luminosity (left), spectral indices $\alpha_{RO}$ and $\alpha_{RX}$ (central),
and $\gamma$-ray dominance (right). Figures from Fossati et al. (1998)

Figure 4: Left: The original blazar sequence, from Fossati et al. (1998).
Right: the updated sequence, from Donato et al. (2001) , reproduced with
permission ©ESO, built with additional [2–10 keV] average spectral indices.

###  2.3 Key observational features

The key features of blazar emission in different bands can be interpreted in
light of the above-mentioned emission models as follows:

Radio:

The radio emission of blazars is dominated by the beamed jet emission, and
only at sub-GHz frequency there is the emergence of radiation produced in the
extended structures, such as hot spots and lobes. The radio spectrum is flat,
i.e., $F(\nu)\propto\nu^{-\alpha}$ with the spectral index $\alpha$ around
zero. This is due to the superposition of different jet zones self-absorbing
at different frequencies. This was thought to be a "cosmic conspiracy" in the
old days (Cotton et al., 1980), but now it can be understood on the bases of
simple conservation laws: the conservation of emitting particles along the jet
demands that their density $n\propto R^{-2}$ (where $R$ is the distance from
the start of the jet) while conservation of the Poynting flux demands that the
magnetic field $B\propto R^{-1}$. With these scalings, one derives a flat
synchrotron radio spectrum and a self–absorption frequency $\nu_{\rm t}\propto
R^{-1}$. This implies that at smaller frequencies the emitting region is
larger and the flux is less rapidly variable.

IR:

In the IR band, we can have the contribution of the jet and, for FSRQs, of the
molecular torus. For FSRQs, the sub–mm band is where the synchrotron peaks,
and this corresponds to the self–absorption frequency of the innermost
emitting region.

Optical:

In the optical band we have the contribution of the jet continuum and, for
FSRQs, of the low frequency part of the accretion disk. This component usually
dilutes the polarization of the synchrotron component. For low power BL Lacs
we also have the contribution of the host galaxy.

UV:

For FSRQs, in the UV band we have the contribution of the steep tail of the
synchrotron component and of the accretion disk, that becomes increasingly
dominant as the total power increases. For BL Lacs we only have the
synchrotron emission: if the spectrum is rising (in $\nu F_{\nu}$) we call
these objects High frequency peak BL Lacs (HBLs) or "blue" BL Lacs, if the
spectrum is decreasing we have a Low frequency peak BL Lac (LBL) or "red" BL
Lac. We note, however, that both FSRQ and LBL classes hold transitional
objects with mixed properties whose classification is somehow arbitrary.

X-ray:

In FSRQs the X–ray spectrum is increasing (in $\nu F_{\nu}$) with a spectral
index $\alpha$ around 0.5, generally flatter than what expected for a thermal
X–ray corona (for which $\alpha\sim 0.7-1$). For BL Lacs we can have a rising
spectrum for LBLs, while in the case of HBLs the spectrum can peak in the
X–ray band.

Gamma rays:

In the $\gamma$-ray band we have the contribution of only the non–thermal
beamed component of the jet. In the sub-TeV band the FSRQs usually show a
steep (i.e. $\alpha>1)$ spectrum, while HBLs can have their high energy peak
there, or even at larger energies. In the latter case, we call these objects
"extreme" BL Lacs. At these energies, the Extra–galactic IR and optical
Background Light (EBL) can absorb (through photon–photon collisions producing
electron–positron pairs (see e.g., Stecker et al., 1992)) high energy photons,
making the observed spectrum decreasing almost exponentially. Since the level
of the EBL is still uncertain (Franceschini and Rodighiero, 2017), detailed
observations of blazars can help to fix it.

Neutrinos:

neutrinos are the smoking-gun signature of hadronic interactions in blazars
(Stecker et al., 1991). A single neutrino of energy 2 PeV detected by the
IceCube Observatory has been associated so far with a blazar (IceCube
Collaboration, 2018), the BL Lac object TXS 0506$+$056.

##  3 The blazar sequence: observational approach

###  3.1 The original blazar sequence

The original blazar sequence was proposed in Fossati et al. (1998) as an
attempt to identify observational properties in the SED of blazars. The
sequence was built using multi-wavelength data from 126 sources. It was the
first systematic study of the SED of blazars including $\gamma$-ray data above
30 MeV from EGRET (Hughes et al., 1980). For the compilation of the sequence,
the sources were selected from the following samples:

  * •

X-ray selected BL Lacs from the Einstein Slew survey (Elvis et al., 1992).

  * •

Radio-selected BL Lacs from the catalogue of extragalactic sources with
$F_{5\,\rm GHz}\geq 1$ Jy (Kuehr et al., 1981). For the selection, additional
requirements were adopted based on radio flatness ($\alpha_{R}\leq 0.5$),
optical brightness ($m_{v}\leq 20)$, and the weakness of optical emission
lines ($EW_{\lambda}\leq 5\text{\AA})$.

  * •

Radio-selected FSRQs proposed by Padovani and Urry (1992) from the 2-Jy sample
(Wall and Peacock, 1985).

Table 1 summarizes the number of sources adopted from each catalog. We have
also outlined the corresponding number of $\gamma$-ray detected sources both
in the MeV-GeV and in the TeV range. Once considered that 4 sources were
present in both BL Lac samples and 1 source of the X-ray sample was detected
only at TeV, the net number of blazars selected for the study becomes 126.

Once defined the source sample, to build the sequence, the fluxes at seven
well-sampled frequencies were collected from the NASA Extragalactic Database
(NED111http://ned.ipac.caltech.edu/). When multiple observations were
available, they were averaged logarithmically (magnitudes), to account for the
large variability. The seven frequencies considered for the original blazar
sequence are: radio at 5 GHz, millimeter at 230 GHz, far-infrared at 60 and 25
$\mu$m, near-infrared ($K$ band) at 2.2 $\mu$m, optical ($V$ band) at 5500 Å,
and soft X-ray at 1 keV. Additionally, information on $\gamma$-ray spectral
properties were used when available. In particular, the EGRET detections were
32 (12 BL Lacs and 20 FSRQs) plus only one source TeV-detected (Mkn 501)
belonging to the X-ray selected sample. Out of the 32 EGRET detected sources,
28 spectral determinations were accessible at the time of this study.

Catalog | Sample | MeV-GeV | TeV
---|---|---|---
|  | detected | detected
X-ray BL Lac | 48 | 7 | 2
Radio BL Lac | 34 | 9 | 1
Radio FSRQ | 50 | 20 | 0
Table 1: Sample of sources adopted for the original blazar sequence (Fossati
et al., 1998). The last two columns highlight the number of sources also
detected in $\gamma$-rays, at MeV-GeV and TeV energies, respectively.

Before building the sequence, a number of checks were performed by the authors
aimed at inspecting the differences between $\gamma$-ray detected and
undetected source samples. They compared the redshift, luminosity, and broad-
band spectral indices. No difference emerged between $\gamma$-ray detected and
undetected radio-selected BL Lacs and FSRQs. For the X-ray selected sample,
the analysis showed a tendency for $\gamma$-ray loud sources to have a larger
radio luminosity and steeper radio and X-ray spectral indices. Indeed, the
limited sensitivity of the EGRET instrument implies that only the $\gamma$-ray
brightest sources were detected. Fossati et al. (1998) underline that the
average $\gamma$-ray luminosities reported in the sequence were necessarily
overestimated. 20 years later, the study of the properties of thousands of
blazars detected at $\gamma$-rays with the Fermi satellite would have
confirmed this limitation (§ 3.2).

As a second step toward a comprehensive characterization of the broadband
emission from the sample, the authors determined with a simple fitting
procedure in the $\nu L_{\nu}$-$\nu$ plane the synchrotron peak frequency
$\nu_{peak,sync}$. They then studied the relation between this frequency and
several quantities, namely: luminosity in different bands, optical/radio and
X-ray/radio flux ratios $\alpha_{RO}$ and $\alpha_{RX}$, $\gamma$-dominance,
i.e., the ratio between the luminosity in $\gamma$-ray and that in
correspondence to the synchrotron peak. The main results shown in Fig. 3,
representing the seeds of the blazar sequence, are:

  * -

The radio luminosity strongly correlates with $\nu_{peak,sync}$. In this
study, the authors ruled out a possible bias due to $z$ by performing detailed
tests.

  * -

$\nu_{peak,sync}$ correlates with both $\alpha_{RO}$ and $\alpha_{RX}$,
meaning that the knowledge of either of the two indices allows a first
estimate of $\nu_{peak,sync}$, at least in the range 1014–1016Hz.

  * -

$\nu_{peak,sync}$ also correlates strongly with the $\gamma$-ray dominance.
This indicates that sources with a synchrotron peak at small frequencies are
brighter $\gamma$-ray emitters.

A clear picture emerged from this study: a smooth transition between different
properties of the blazar SED appears when considering the synchrotron peak
frequency. This becomes even more evident when five averaged broadband SEDs
are built adopting as bin criterion the radio luminosity. This means that,
independently from the original classification of a blazar as FSRQ or
radio/X-ray selected BL Lac, the average SEDs were built taking into account
only the luminosity of the source at 5 GHz, available for all objects. The
result is the blazar sequence, displayed in Fig. 4 (left). The radio-bin
log-(L${}_{5}\,GHz$) intervals are: <42; 42-43; 43-44; 44-45; >45\. A set of
lines is superimposed on the data, with the low-frequency part composed by a
power law connected to a parabolic branch (synchrotron peak and successive
steepening). A second power law connects this first peak to a second peak,
approximated with another parabolic branch and built following some basic
assumptions on the peak position and luminosities in different frequencies.

The analysis of the average data and their trends reveals that all SEDs
present a two-peak structure. The frequency of the first peak is
anticorrelated with the luminosity and moves from 1013-14 Hz for brighter
sources, to 1016-17 Hz for fainter sources. A similar effect emerges in the
high-energy peak, despite the poor sampling, and suggests that the frequencies
of the two peaks are correlated. Another evidence is that sources with higher
frequency peaks have a smaller $\gamma$-ray dominance (now called Compton
dominance).

Three years after the compilation of the original sequence, Donato et al.
(2001) updated it by adding the [2–10 keV] average spectral indices and fluxes
for the same source sample introduced in (Fossati et al., 1998). The revised
sequence is shown in Fig. 4 (right), where only minor modifications were
introduced in the parametrization of the SEDs. These modifications were
physically driven and reflected the fact that for low luminosity sources there
were growing evidences supporting the SSC scenario for BL Lacs. The work
confirmed, on a statistical basis, that more powerful blazars emit the X–rays
by the inverse Compton process, while in less powerful objects the dominant
mechanism is the synchrotron. Thus, the shift in both peak positions proposed
by the original sequence was confirmed.

The emergence of the blazar sequence was of uttermost importance in the
experimental, phenomenological, and theoretical panorama. First, it suggested
that a classification based on the synchrotron peak position, in particular
for BL Lac objects classified into low-, intermediate-, and high-synchrotron
peaked sources, must have a physical base, given the smooth evolution observed
between the blazar classes. This explanation should also account for the
Compton dominance behaviour emerging at the highest energies. Moreover, it
became evident that the common observational properties between FSRQ and BL
Lacs call for a common structure and underlying physical mechanisms.

From the experimental perspective, the sequence represented a challenge for
the emerging VHE $\gamma$-ray instruments, whose low-energy detection
threshold was around 1026 Hz. It suggested that the brightest $\gamma$-ray
objects had the bulk of their emission at frequencies well below the VHE
$\gamma$-ray range.

However, this sequence was built with just a hundred sources and, in case of
the $\gamma$-ray band, only $\sim$30 sources were adopted to build the average
SEDs. Moreover, the average SEDs were built not taking into account the
simultaneity of the data in different bands. While the former criticism was
overcome a few years ago, § 3.2, the latter is still a matter of debate, § 6.

Figure 5: Luminosity in the 0.1–100 GeV band as a function of redshift for the
Fermi sequence sample, from Ghisellini et al. (2017). The solid lines
represent EGRET sensitivity limit and Fermi-LAT sensitivity after 4 years of
observations. Horizontal lines delimit the luminosity bins adopted in the
study.

###  3.2 The Fermi blazar sequence

Almost 20 years after the publication of the blazar sequence paper and
motivated by the detection, in the $\gamma$-ray band, of about 1500 blazars by
the Fermi-LAT satellite as reported in the 3LAC catalog (M. et al., 2015),
Ghisellini et al. (2017) proposed a revised sequence, named Fermi sequence.
This time, the authors explored the existence of a SED sequence on a sample of
$\gamma$-ray selected blazars binned according to the luminosity in the MeV-
GeV energy range.

The source sample was extracted from the 3LAC catalog, a complete, flux-
limited sample built with 4 years of Fermi-LAT observations. The full catalog
includes 1563 sources identified with AGNs at high galactic latitudes, with
98% blazar associations. After removing sources with no clear association, no
redshift determination, and/or not associated with blazars, the authors built
a sample of 747 sources (299 classified as BL Lacs and 448 as FSRQs).

The 0.1–100 GeV band K-corrected luminosity as a function of redshift is
represented in Fig. 5. The redshift distribution reaches $z\sim 3$ and the
luminosity range spans several orders of magnitude: from $10^{42}$ to almost
$10^{50}$ erg/s. In general, BL Lac objects are located at smaller $z$, mostly
below $z\sim 1$, and exhibit lower average luminosity. FSRQs are instead
located at intermediate and large $z$ and exhibit larger average luminosity.

For the construction of the revised sequence, the sample was divided into six
luminosity bins, delimited by horizontal lines in the figure. In analogy with
the original sequence, each luminosity bin spans a decade in gamma-ray
luminosity. This allows for an easy comparison between the two sequences.
Table 2 lists the luminosity bins adopted in the study. In the table, the main
phenomenological parameters describing the average SED are reported, namely,
the synchrotron peak position ($\nu_{S}$), the inverse Compton peak position
($\nu_{IC}$), and the Compton dominance value (CD).

The last three columns of the table report the total number of objects in each
bin, the fraction of FSRQs, and that of BL Lac objects, respectively. Above
$10^{45}$ erg/s the sample is dominated by FSRQs, while at low luminosity BL
Lac objects are dominant. Only 2 BL Lac objects display an average luminosity
>$10^{48}$ erg/s. On the other hand, 9 FSRQs belong to the <$10^{44}$ erg/s
luminosity bin, as can be seen in Fig 5. The Fermi sequence is illustrated in
Fig. 6: in the bottom panel FSRQs and BL Lacs are shown together, while in the
top and central panel they are shown separately. This distinction allows for a
comparison of the two source classes.

Figure 6: The Fermi blazar sequence, from Ghisellini et al. (2017). FSRQs (top
panel) of different luminosity classes do show an increasing Compton dominance
for increasing luminosity, but not the trend of decreasing peak frequencies.
This behaviour, instead is clearly evident when considering BL Lac objects
(mid panel) and also when considering all blazars, without dividing them into
FSRQs and BL Lacs.

When divided into luminosity bins, the Fermi-selected blazars form a sequence
with the same average properties of the original sequence. We notice a shift
in both synchrotron and inverse Compton peak position with decreasing
luminosity. A change in CD is also evident. When considering BL Lac objects
and FSRQs separately, however, some additional features emerged in the study.
The peak shift is mostly affecting BL Lac objects and not FSRQs. On the other
hand, the increase in CD with increasing luminosity is only evident in the
FSRQ subsample.

The more complex scenario emerged in this study with respect to the original
one might be in part explained by the presence, in this new sequence, of
blazars with a wide range of black hole masses. While very likely the original
sequence concerned only BL Lacs and FSRQs with black holes with large masses,
the improved sensitivity at all frequencies allowed to detect also objects of
smaller masses. An effect evident in particular in the low-luminosity bins,
where nearby intermediate mass FSRQs coexist with nearby BL Lacs with higher
masses. The net result is a large dispersion of the points in the broadband
SEDs of Fig. 6.

log $L_{\gamma}$ [log(erg/s)] |  $\nu_{S}$ [Hz] |  $\nu_{IC}$ [Hz] | CD | NAll | NFSRQ | N${}_{BL\,Lac}$
---|---|---|---|---|---|---
>48 | 2.5e12 | 9e20 | 15 | 49 | 47 (96%) | 2
47–48 | 2.5e12 | 2e21 | 4.8 | 202 | 177 (88%) | 25
46–47 | 5e12 | 2e21 | 2 | 182 | 144 (79%) | 38
45–46 | 5e12 | 1e22 | 0.6 | 174 | 52 (30%) | 122
44–45 | 1e16 | 8e24 | 0.35 | 111 | 19 (17%) | 92
<44 | 4e16 | 3e25 | 0.25 | 29 | 9 (31%) | 20
Table 2: Main parameters characterizing the Fermi blazar sequence (Ghisellini
et al., 2017).

###  3.3 The TeV blazar sequence

In recent years, the rapid development of Cherenkov telescopes allowed the
detection of a significant number of blazars in the TeV energy range. In the
TeVCat catalogue222http://tevcat2.uchicago.edu/ there are now 81 blazars, and
67 of these have a measured redshift. Using the SSDC Sky Explorer facility
333https://tools.ssdc.asi.it/ we have constructed the SED for each of these
sources. We have then divided FSRQs and BL Lacs, and using SSDC data we have
divided the latter into the same $\gamma$-ray luminosity bins adopted for the
Fermi blazar sequence, to allow for a direct comparison.

The result is shown in Fig. 7, where the source data are compared with the
1-$\sigma$ stripes corresponding to the average SED derived in Ghisellini et
al. (2017) for the same luminosity bin. We can see that there are no strong
differences between the SED of the blazars detected by Fermi and the ones that
have also been detected in the TeV band. However, there is indeed an
indication, for the brightest TeV BL Lacs, that their X-ray luminosity is on
average larger than the average of the Fermi blazars with the same luminosity
in the Fermi energy range.

A possible explanation of this effect is that some of the blazars in this
luminosity bin are blue quasars. These sources would not contradict the
proposed explanation of the sequence based on radiative cooling (§5), because
they could be FSRQs that produce most of their power beyond the typical
distance of the BLR. The deficit of seed photons for scattering in these
sources implies reduced radiative losses and hence a large $\gamma_{\rm
peak}$, resulting in a blue spectrum (Ghisellini and Tavecchio, 2008).
Alternatively, this could be simply due to a selection effect and they might
represent the few bright existing HBLs. Since the TeV sample is strongly
biased toward sources with high X-ray luminosity (one of the criteria used for
the identification of the best targets), it is natural that the first sources
detected in these energy bins are the brightest in the X-ray range.

A third plausible option is that these are HBLs with a wrong (too large)
redshift determination, hence their bolometric luminosity is overestimated.
Indeed, many of the redshifts determined for HBLs are debated due to the
difficulty to detect the host galaxy (Paiano et al., 2017). We will
investigate this issue in a future paper (Prandini et al. in prep.).

‘

Figure 7: The TeV blazar sequence. The SED of all TeV detected blazars with
known redshift have been collected and divided into the luminosity bins
adopted in the Fermi blazar sequence (Ghisellini et al., 2017). The stripes
are the 1$\sigma$ dispersion from (Ghisellini et al., 2017).

##  4 At the extremes of the blazar sequence

The most recent sequence proposed in Ghisellini et al. (2017) includes 747
sources, a sample almost six times larger than the original one used in
Fossati et al. (1998). Despite the large number of sources considered,
however, two subclasses of sources might still elude the study. These are the
MeV blazars and the extreme-TeV blazars. The former have a powerful high
energy peak up to a few MeV, and therefore could remain undetected by the
Fermi-LAT that observes in the energy range beyond 100 MeV. Extreme-TeV
blazars instead peak at a few TeV and are expected to have a steady but faint
emission in the GeV-TeV range. Thus, the fainter or more distant objects might
easily remain below the sensitivity limit of Fermi. These subclasses represent
the two extremes of the sequence and their detection by future experiments in
the MeV and TeV range is of great importance for testing the current
undestanding of the blazar sequence.

Figure 8: Extremes of the blazar sequence: SED of a MeV blazar (left) and of a
TeV blazar (right). The MeV blazar is the FSRQ 2149–306, located at $z=2.354$,
from Tagliaferri et al. (2015), ©AAS. Reproduced with permission. The TeV
blazar is, instead, 1ES 0229+200, at $z=0.14$. Adapted from Biteau et al.
(2020).

###  4.1 Extreme TeV blazars

Out of the several tens of VHE gamma-ray emitting blazars discovered so far,
roughly 1/4 are classified as extreme blazars (see Biteau et al., 2020, and
references therein). These extreme blazars come in two flavours: extreme
synchrotron and extreme TeV sources. The former are characterized by a
synchrotron peak beyond 1 keV, while the latter exhibit a $\gamma$-ray peak
energy above 1 TeV. The prototypical extreme TeV blazar is 1ES 0229+200, Fig.
8 (right). Located at redshift 0.14, this blazar holds the record for the
highest $\gamma$-ray peak frequency, exceeding 10 TeV. As reported in Biteau
et al. (2020), all 14 known extreme TeV blazars but one are also extreme
synchrotron sources. This suggests that reaching $>$TeV peak energy requires
also $>$keV synchrotron peak frequencies, hence an efficient electron
acceleration up to high energies. Another key characteristic of this class of
sources is their faint luminosity. For the known extreme TeV blazars, the
luminosity measured barely exceeds the Fermi-LAT sensitivity limit in the
gamma-ray range. This might suggest that the extreme TeV blazars identified so
far are only the tip of the (faint) iceberg of the extreme TeV blazar
population. The bias is due to the selection effect in the VHE energy range,
where the observations in the last 15 years have been driven by the Fermi-LAT
properties reported in the published catalogs.

In the context of the Fermi blazar sequence, these still missing extreme TeV
blazars would populate the lowest luminosity bin. Future gamma-ray instruments
like LHAASO (Bai et al., 2019), SWGO (Albert et al., 2019), ASTRI (Pintore et
al., 2020), and CTA (Cherenkov Telescope Array Consortium, 2019), will be the
key to probe this expected trend. Wide field-of view instruments, like LHAASO
and SWGO, will have the advantage of a wide field of view, but a limited
sensitivity at sub-TeV energies, limiting the survey to nearby objects ($z\leq
0.2$). Future IACTs, instead, will access a parameter space (GeV-TeV energy,
meaning large distance $z>1$, and low luminosity) still not explored by the
current generation of gamma-ray instruments with a limited field of view.
Therefore, targeted observations on promising objects (e.g., Costamante, 2020)
will be performed in this case.

###  4.2 MeV blazars

If we now focus the attention on the opposite side of the sequence, we might
wonder if there are missing objects peaking at low frequencies. Following the
sequence main trend, these would be most likely very powerful FSRQs peaking in
the $\sim$meV and MeV energies, respectively. The Fermi-LAT non-detection is
due to the fact that the high energy emission does not reach the LAT energy
band.

Interestingly, a number of recent studies in the hard X-ray band have
consolidated the existence of ultra-luminous FSRQs, located at a large
redshift ($z>2$). In addition to the large distance, these objects are
characterized by an extremely massive central black hole, estimated to be
among the largest known (see e.g., Sbarrato et al., 2021). The main factor
limiting Fermi-LAT detection of this subclass of objects is their distance,
that affects the observed flux. An example of a powerful MeV blazar is
2149–306 whose SED is shown in Fig. 8 (left). The source is located at
redshift 2.354. de Angelis et al. (2018) show that if this blazar were located
at $z=7$, it would be out of reach for current generation of $\gamma$-ray
telescopes. Still, it would be detectable in the MeV range by future
generations of MeV satellites such as $eASTROGAM$ (de Angelis et al., 2018),
$AMEGO$ (McEnery et al., 2019), or COSI (Tomsick et al., 2019).

##  5 The blazar sequence: theoretical approach

The original blazar sequence was soon (Ghisellini et al., 1998) interpreted as
a sequence of radiative cooling. The main idea is that the emitting electrons
cannot achieve high energies if their radiative cooling (by the synchrotron,
Self Compton and External Compton processes) is severe. According to this
idea, in FSRQs the broad line region and the molecular torus are essential
sources of seed photons for the External Compton process, and this implies
that the random Lorentz factor of the electrons emitting at the peaks of the
SED, $\gamma_{\rm peak}$, is relatively small. Therefore, the peak of their
synchrotron emission occurs in the sub-mm band, while their high energy peak
is in the MeV band. Another important consequence is that the inverse Compton
process becomes dominant with respect to the synchrotron one, implying a large
Compton dominance. In BL Lacs, the absence of an important thermal source
implies the paucity of seed photons for Compton scattering. The radiative
cooling is less severe, allowing electrons to achieve large $\gamma_{\rm
peak}$. Thus, the synchrotron peaks in the UV–X–ray band, and the high energy
hump peaks in the GeV and sometimes even in the TeV band. The lack of ambient
seed photons implies that only the SSC process contributes to the high energy
emission. As a result, in BL Lacs the high energy component is usually not
dominant.

Figure 9: The energy $\gamma_{\rm peak}$ of the electrons emitting at the
peaks of the SED as a function of the energy density (magnetic plus radiative)
as seen in the comoving frame. Grey symbols refer to blazars studied
previously in Celotti and Ghisellini (2008); Ghisellini et al. (2010). The
stars are objects with weak and very weak broad emission lines that can be
classified as BL Lacs or transitional objects. Adapted from Ghisellini and
Tavecchio (2015).

###  5.1 Physical parameters

This idea can be checked by applying a leptonic one-zone model, that allows to
infer the physical parameters of the different classes of sources. Fig. 9
shows the random Lorentz factor $\gamma_{\rm peak}$ of the electrons
contributing to the peaks of the SED as a function of the total (magnetic plus
radiative) energy density as measured in the comoving frame (Ghisellini and
Tavecchio, 2015; Celotti and Ghisellini, 2008; Ghisellini et al., 2010). There
is a clear trend: the largest electron energies are possible only for small
values of $U^{\prime}_{B}+U^{\prime}{\rm{}_{r}}$.

We also know that the presence or absence of broad emitting lines is
associated with the luminosity of the accretion disk and its accretion regime.
Furthermore, there is a relation between the jet power and the disk
luminosity. Therefore, the blazar sequence can be thought also as a sequence
whose main parameter is the jet power or, equivalently, the disk luminosity.
On the other hand, we must consider also the mass of the central black hole,
since the accretion regime changes when the disk luminosity is of the order of
$10^{-3}$–$10^{-2}$ of the Eddington one.

The fact that, for FSRQs, there is no trend between luminosity and location of
the peak frequencies can be readily explained if the emission occurs within
the distance of influence of the BLR and/or the molecular torus. In fact, the
BLR radius is proportional to $L_{\rm disk}^{1/2}$: this implies that the
radiation energy density of the line photons ($U_{\rm BLR}\sim 0.1L_{\rm
disk}/(4\pi R^{2}_{\rm BLR}c$) is constant as long as the emission site is
within $R_{\rm BLR}$: $U_{\rm BLR}\sim 1/(12\pi)$. A similar conclusion holds
also for the energy density of the torus radiation, once the torus is
approximated as a portion of a sphere of radius $R_{\rm T}$ intercepting and
re-emitting a fraction $f$ of $L_{\rm disk}$: $U_{\rm T}\sim 0.07f/(12\pi)$
(Ghisellini and Tavecchio, 2015). Both $U_{\rm BLR}$ and $U_{\rm T}$ are seen
boosted by a $\Gamma^{2}$ factor in the comoving frame.

We note that Costamante et al. (2018) argued that the fingerprint of the
$\gamma$–$\gamma$ $\to$ $e^{+}e^{-}$ process due to the presence of emission
line photons is not visible in the Fermi-LAT spectra of blazars (but see
Poutanen and Stern, 2010). To the aim of explaining the sequence in terms of
different radiative cooling, the fact that the location of the emitting region
is inside the BLR sphere or outside it, but within the torus, is not very
important. Within the BLR the corresponding radiation energy density is
larger, and timescales are shorter. Within the torus the external radiation
energy density is smaller, but the size of the source is probably larger, so
that the ratio between the dynamical and the cooling timescales in the two
zones are not very different.

For BL Lacs, the absence of these structures implies that the radiation energy
density is of synchrotron origin only, therefore associated to the jet power
and luminosity, and we do see a trend of larger $\gamma_{\rm peak}$ for
smaller $(U^{\prime}_{B}+U^{\prime}{\rm{}_{r})}$.

##  6 Criticisms to the blazar sequence

Since its birth in 1998, the blazar sequence was and is a matter of active
debate. We can divide the main criticisms roughly into two categories: in the
first the validity of the observed sequence is accepted, but the
interpretation is different from the original one (that assumes that what
drives the sequence is the difference in radiative cooling); for the second
category, instead, the sequence is not real, it is only the results of
observational biases.

Into the first category there is the paper by Fan et al. (2017), extending the
work of Nieppola et al. (2008). They explain the observed blazar sequence as a
sequence of Doppler factors, that boosts the intrinsic luminosity and the
intrinsic peak frequencies. They claim that in the comoving frame the sequence
should have just the opposite trend. Consider that the method to derive the
beaming factor uses the brightness temperature compared with the equipartition
value of $5\times 10^{10}$ K (Readhead, 1994), and the minimum variability
timescale observed in the radio. This implies that sources that are not
varying during the observing periods are bound to have very small Doppler
factors, and in fact they range from $\sim 1$ to $\sim 30$. It is thus
possible that some sources are under-corrected. Consider also that the beaming
in the radio band may be different from the beaming of more compact and
innermost regions producing the bulk of the emission.

Alternatively, the sequence has been interpreted by Potter and Cotter (2015),
in their inhomogeneous jet model, as a consequence of the different location
of the dissipation region: in BL Lacs is closer to the central engine than in
FSRQs, resulting in stronger magnetic fields, smaller Compton dominance, and
larger peak frequencies with respect to FSRQs.

In the second category we find, among others, the study of Keenan et al.
(2021), considering more than a thousand blazars of both the FSRQ and BL Lac
type. They conclude that FSRQs, while spanning a large range of observed
luminosity, are all low peaked blazars. More importantly, there is no
luminosity - peak frequency trend in BL Lacs. In their Fig. 4 that illustrates
this point, one notices indeed a very large spread of peak frequency for any
luminosity for BL Lacs, but there is still a trend of larger synchrotron peak
for smaller luminosities.

A detailed proposal questioning the reality of the sequence was put forward by
Giommi et al. (2012) proposing what they call a ‘simplified scenario’ for
blazars. According to these authors, the blazar sequence is a result of
selection effects with no physical link between the luminosity and the overall
blazar SED. Both at high and low luminosities, we have both red and blue
blazars. In this scenario, the distribution of $\gamma_{\rm peak}$ is assumed
to peak at $\sim 10^{3}$, and it is asymmetric, with a high-energy tail
broader than the low-energy one (see Fig. 4 of (Giommi et al., 2012)).
Furthermore, the radio luminosity function is $N(L)\propto L^{-3}$, the
magnetic field is assumed to be $B=0.15$ G for all blazars, and the mean
Doppler factor distribution is $<\delta>=15\pm 2$. The immediate consequence
is that there exist many BL Lacs of low power with a “red" SED. These are the
ones with $\gamma_{\rm peak}\sim 10^{3}$ or smaller. We do not see them yet
because, being at low luminosity, we do not have a survey deep enough to let
them emerge. These objects should have a high energy, synchrotron self-Compton
spectrum peaking in the 1–10 keV band (see Fig. 11 of (Ghisellini et al.,
2017) for illustration of this point). According to this idea, there should be
BL Lacs already detected in the radio, at moderate (but still unknown)
redshifts, with no X-ray detection yet. On the other hand, with the advent of
$eROSITA$ (Merloni et al., 2012), and further in time $Athena$ these objects
would be detected in X-rays, helping to probe their BL Lac nature.

##  7 Future perspectives

After more than 20 years from its proposition, the blazar sequence is still
alive and lively discussed. It has been deeply investigated from both the
experimental and theoretical point of view. The original trend is confirmed by
recent, extensive data in $\gamma$ rays, even if BL Lac objects and FSRQs seem
to behave differently, but consistently with the theoretical expectation. The
emerging sample of TeV-detected blazars, despite a strong selection bias,
offers a new interesting perspective that will be further investigated.

Interestingly, Rodrigues et al. (2018) studied the neutrino and ultra-high
energy cosmic ray (UHECR) emission based on the model (Murase et al., 2014) in
the context of the blazar sequence. A strong connection is reported, with HBLs
tending to produce UHECRs, while FSRQs are more efficient neutrino emitters.
In (Inoue and Totani, 2009) the blazar sequence is used to estimate the gamma-
ray background.

From the theoretical side, the sequence has been interpreted as an effect of
the efficiency of the cooling mechanism on accelerated electrons. In FSRQs,
the cooling is efficient, hence preventing electrons to reach high energies
and causing a large Compton dominance. In BL Lacs, instead, the cooling is
less effective allowing electrons to reach the highest energies. Remarkably,
this efficiency turned out to be intimately connected with the accretion
regime. In this perspective, the blazar sequence is the result of a change in
the accretion regime: efficient accretion (high disk luminosity) is the cause
of efficient cooling. Alternative theoretical interpretations have been
proposed, invoking changes in the Doppler factor (Nieppola et al., 2008; Fan
et al., 2017) or in the location of the dissipation region (Potter and Cotter,
2015) as possible explanations to the observed sequence.

The very existence of the sequence is the target of criticisms, mostly based
on selection effect arguments (Giommi et al., 2012; Keenan et al., 2021).
Recent and near-future observatories represent a unique opportunity to solve
the dispute and probe (or reject) the validity of the sequence. Moreover,
future observations will contribute to solve the main open points about the
sequence, namely: i) evaluate the existence of a population of MeV blazars
and, on the opposite side, that of extreme blazars; ii) probe the properties
of the accretion disk, in particular in the BL Lac object case (ADAF?); iii)
evaluate the effect of cosmic evolution.

The advent of X-ray and $\gamma$-ray surveys with $eROSITA$, LHAASO, SWGO in
the current decade and, later on, that of $Athena$ (Nandra et al., 2013), will
provide an unbiased census of blazars. This will complement very deep
observations in other bands (e.g., SKA and JWST (Braun et al., 2015; Gardner
et al., 2006)). Finally, the recently launched $IXPE$ mission (Weisskopf et
al., 2016) is going to characterize for the first time the X-ray polarized
emission from some bright selected sources, improving significantly our
understanding of the blazar phenomena. Despite all these exciting advancements
foreseen in the near future, we point out that the lack of Fermi-LAT full sky
monitoring successor and that of Swift-XRT and UVOT fast reactions to
transients and ToOs will strongly affect these observations.

A complementary approach to the study of the sequence that might become
accessible thanks to the improved sensitivity of future instruments is the
study of misaligned blazars, i.e., radio galaxies. However, the emerging
evidence of composite jets in blazars (internal, fast spine, and external,
slow layer) represents a non-trivial aspect to be taken into account when
trying to connect blazars and radio galaxies.

E.P. acknowledges funding from Italian Ministry of Education, University and
Research (MIUR) through the "Dipartimenti di eccellenza” project Science of
the Universe.

For the study presented in § 3.3, we have used the following public archives:

  * -

TeVCAT online catalog for TeV Astronomy, http://tevcat2.uchicago.edu/

  * -

SSDC Sky Explorer facility, https://tools.ssdc.asi.it/

###### Acknowledgements.

We would like to thank L. Costamante for re-adapting Fig. 8 for this
manuscript. We are grateful to the reviewers for their constructive feedback
on the manuscript. The authors declare no conflict of interest. References

## References

  * Schmidt (1963) Schmidt, M.  3C 273 : A Star-Like Object with Large Red-Shift.  Nature 1963, 197, 1040.  doi:black10.1038/1971040a0.
  * Abdollahi et al. (2020) Abdollahi, S.; Acero, F.; Ackermann, M.; et al.  Fermi Large Area Telescope Fourth Source Catalog.  Astrophysical Journal, Supplement 2020, 247, 33, [arXiv:astro-ph.HE/1902.10045].  doi:black10.3847/1538-4365/ab6bcb.
  * IceCube Collaboration (2018) IceCube Collaboration.  Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A.  Science 2018, 361, eaat1378, [arXiv:astro-ph.HE/1807.08816].  doi:black10.1126/science.aat1378.
  * Antonucci (1993) Antonucci, R.  Unified models for active galactic nuclei and quasars.  Annual Review of Astron and Astrophys 1993, 31, 473–521.  doi:black10.1146/annurev.aa.31.090193.002353.
  * Urry and Padovani (1995) Urry, C.M.; Padovani, P.  Unified Schemes for Radio-Loud Active Galactic Nuclei.  Publications of the ASP 1995, 107, 803, [arXiv:astro-ph/astro-ph/9506063].  doi:black10.1086/133630.
  * Heckman and Best (2014) Heckman, T.M.; Best, P.N.  The Coevolution of Galaxies and Supermassive Black Holes: Insights from Surveys of the Contemporary Universe.  Annual Review of Astron and Astrophys 2014, 52, 589–660, [arXiv:astro-ph.GA/1403.4620].  doi:black10.1146/annurev-astro-081913-035722.
  * Padovani et al. (2017) Padovani, P.; Alexander, D.M.; Assef, R.J.; et al.  Active galactic nuclei: what’s in a name?  Astronomy and Astrophysics Reviews 2017, 25, 2, [arXiv:astro-ph.GA/1707.07134].  doi:black10.1007/s00159-017-0102-9.
  * Narayan et al. (2000) Narayan, R.; Igumenshchev, I.V.; Abramowicz, M.A.  Self-similar Accretion Flows with Convection.  Astrophysical Journal 2000, 539, 798–808, [arXiv:astro-ph/astro-ph/9912449].  doi:black10.1086/309268.
  * Blandford and Begelman (1999) Blandford, R.D.; Begelman, M.C.  On the fate of gas accreting at a low rate on to a black hole.  Monthly Notices of the RAS 1999, 303, L1–L5, [arXiv:astro-ph/astro-ph/9809083].  doi:black10.1046/j.1365-8711.1999.02358.x.
  * Narayan et al. (1997) Narayan, R.; Garcia, M.R.; McClintock, J.E.  Advection-dominated Accretion and Black Hole Event Horizons.  Astrophysical Journal, Letters 1997, 478, L79–L82, [arXiv:astro-ph/astro-ph/9701139].  doi:black10.1086/310554.
  * Mahadevan (1997) Mahadevan, R.  Scaling Laws for Advection-dominated Flows: Applications to Low-Luminosity Galactic Nuclei.  Astrophysical Journal 1997, 477, 585–601, [arXiv:astro-ph/astro-ph/9609107].  doi:black10.1086/303727.
  * MAGIC Collaboration et al. (2018) MAGIC Collaboration.; Ahnen, M.L.; Ansoldi, S.; Antonelli, L.A.; Arcaro, C.; Baack, D.; Babić, A.; Banerjee, B.; Bangale, P.; Barres de Almeida, U.; et al.  Detection of the blazar S4 0954+65 at very-high-energy with the MAGIC telescopes during an exceptionally high optical state.  Astronomy and Astrophysics 2018, 617, A30, [arXiv:astro-ph.HE/1801.04138].  doi:black10.1051/0004-6361/201832624.
  * Rieger (2019) Rieger, F.  Gamma-Ray Astrophysics in the Time Domain.  Galaxies 2019, 7, 28, [arXiv:astro-ph.HE/1901.10216].  doi:black10.3390/galaxies7010028.
  * Fossati et al. (1998) Fossati, G.; Maraschi, L.; Celotti, A.; Comastri, A.; Ghisellini, G.  A unifying view of the spectral energy distributions of blazars.  Monthly Notices of the RAS 1998, 299, 433–448, [arXiv:astro-ph/astro-ph/9804103].  doi:black10.1046/j.1365-8711.1998.01828.x.
  * Bonnoli et al. (2011) Bonnoli, G.; Ghisellini, G.; Foschini, L.; Tavecchio, F.; Ghirlanda, G.  The $\gamma$-ray brightest days of the blazar 3C 454.3.  Monthly Notices of the RAS 2011, 410, 368–380, [arXiv:astro-ph.CO/1003.3476].  doi:black10.1111/j.1365-2966.2010.17450.x.
  * Tavecchio et al. (2001) Tavecchio, F.; Maraschi, L.; Pian, E.; et al.  Theoretical Implications from the Spectral Evolution of Markarian 501 Observed with BeppoSAX.  Astrophysical Journal 2001, 554, 725–733.  doi:black10.1086/321394.
  * Schoenfelder et al. (1993) Schoenfelder, V.; Aarts, H.; Bennett, K.; et al.  Instrument Description and Performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory.  Astrophysical Journal, Supplement 1993, 86, 657.  doi:black10.1086/191794.
  * Thompson et al. (1993) Thompson, D.J.; Bertsch, D.L.; Fichtel, C.E.; et al.  Calibration of the Energetic Gamma-Ray Experiment Telescope (EGRET) for the Compton Gamma-Ray Observatory.  Astrophysical Journal, Supplement 1993, 86, 629.  doi:black10.1086/191793.
  * Giacconi (2009) Giacconi, R.  History of X-ray telescopes and astronomy.  Experimental Astronomy 2009, 25, 143–156.  doi:black10.1007/s10686-009-9139-8.
  * Chadwick (2021) Chadwick, P.  35 Years of Ground-Based Gamma-ray Astronomy.  Universe 2021, 7.  doi:black10.3390/universe7110432.
  * Atwood et al. (2009) Atwood, W.B.; Abdo, A.A.; Ackermann, M.; Althouse, W.; Anderson, B.; et al.  The Large Area Telescope on the Fermi Gamma-Ray Space Telescope Mission.  Astrophysical Journal 2009, 697, 1071–1102, [arXiv:astro-ph.IM/0902.1089].  doi:black10.1088/0004-637X/697/2/1071.
  * Bernlöhr et al. (2003) Bernlöhr, K.; Carrol, O.; Cornils, R.; et al.  The optical system of the H.E.S.S. imaging atmospheric Cherenkov telescopes. Part I: layout and components of the system.  Astroparticle Physics 2003, 20, 111–128, [arXiv:astro-ph/astro-ph/0308246].  doi:black10.1016/S0927-6505(03)00171-3.
  * Aleksić et al. (2016) Aleksić, J.; Ansoldi, S.; Antonelli, L.A.; et al.  The major upgrade of the MAGIC telescopes, Part I: The hardware improvements and the commissioning of the system.  Astroparticle Physics 2016, 72, 61–75, [arXiv:astro-ph.IM/1409.6073].  doi:black10.1016/j.astropartphys.2015.04.004.
  * Holder et al. (2006) Holder, J.; Atkins, R.W.; Badran, H.M.; et al.  The first VERITAS telescope.  Astroparticle Physics 2006, 25, 391–401, [arXiv:astro-ph/astro-ph/0604119].  doi:black10.1016/j.astropartphys.2006.04.002.
  * Smith et al. (2013) Smith, M.W.E.; Fox, D.B.; Cowen, D.F.; Mészáros, P.; Tešić, G.; Fixelle, J.; Bartos, I.; Sommers, P.; Ashtekar, A.; Jogesh Babu, G.; et al.  The Astrophysical Multimessenger Observatory Network (AMON).  Astroparticle Physics 2013, 45, 56–70, [arXiv:astro-ph.HE/1211.5602].  doi:black10.1016/j.astropartphys.2013.03.003.
  * Möller et al. (2021) Möller, A.; Peloton, J.; Ishida, E.E.O.; et al.  FINK, a new generation of broker for the LSST community.  Monthly Notices of the RAS 2021, 501, 3272–3288, [arXiv:astro-ph.IM/2009.10185].  doi:black10.1093/mnras/staa3602.
  * Ghisellini et al. (1985) Ghisellini, G.; Maraschi, L.; Treves, A.  Inhomogeneous synchrotron-self-compton models and the problem of relativistic beaming of BL Lac objects.  Astronomy and Astrophysics 1985, 146, 204–212.
  * Böttcher et al. (2013) Böttcher, M.; Reimer, A.; Sweeney, K.; Prakash, A.  Leptonic and Hadronic Modeling of Fermi-detected Blazars.  Astrophysical Journal 2013, 768, 54, [arXiv:astro-ph.HE/1304.0605].  doi:black10.1088/0004-637X/768/1/54.
  * Potter and Cotter (2013) Potter, W.J.; Cotter, G.  Synchrotron and inverse-Compton emission from blazar jets - IV. BL Lac type blazars and the physical basis for the blazar sequence.  Monthly Notices of the RAS 2013, 436, 304–314, [arXiv:astro-ph.HE/1310.0462].  doi:black10.1093/mnras/stt1569.
  * Giroletti et al. (2004) Giroletti, M.; Giovannini, G.; Feretti, L.; et al.  Parsec-Scale Properties of Markarian 501.  Astrophysical Journal 2004, 600, 127–140, [arXiv:astro-ph/astro-ph/0309285].  doi:black10.1086/379663.
  * Grandi et al. (2012) Grandi, P.; Torresi, E.; Stanghellini, C.  The $\gamma$-Ray Emission Region in the Fanaroff-Riley II Radio Galaxy 3C 111.  Astrophysical Journal, Letters 2012, 751, L3, [arXiv:astro-ph.HE/1204.2268].  doi:black10.1088/2041-8205/751/1/L3.
  * Ghisellini et al. (2005) Ghisellini, G.; Tavecchio, F.; Chiaberge, M.  Structured jets in TeV BL Lac objects and radiogalaxies. Implications for the observed properties.  Astronomy and Astrophysics 2005, 432, 401–410, [arXiv:astro-ph/astro-ph/0406093].  doi:black10.1051/0004-6361:20041404.
  * Chiaberge et al. (1999) Chiaberge, M.; Capetti, A.; Celotti, A.  The HST view of FR I radio galaxies: evidence for non-thermal nuclear sources.  Astronomy and Astrophysics 1999, 349, 77–87, [arXiv:astro-ph/astro-ph/9907064].
  * Ghisellini et al. (2014) Ghisellini, G.; Tavecchio, F.; Maraschi, L.; Celotti, A.; Sbarrato, T.  The power of relativistic jets is larger than the luminosity of their accretion disks.  Nature 2014, 515, 376–378, [arXiv:astro-ph.HE/1411.5368].  doi:black10.1038/nature13856.
  * Nemmen et al. (2012) Nemmen, R.S.; Georganopoulos, M.; Guiriec, S.; Meyer, E.T.; Gehrels, N.; Sambruna, R.M.  A Universal Scaling for the Energetics of Relativistic Jets from Black Hole Systems.  Science 2012, 338, 1445, [arXiv:astro-ph.HE/1212.3343].  doi:black10.1126/science.1227416.
  * Donato et al. (2001) Donato, D.; Ghisellini, G.; Tagliaferri, G.; Fossati, G.  Hard X-ray properties of blazars.  Astronomy and Astrophysics 2001, 375, 739–751, [arXiv:astro-ph/astro-ph/0105203].  doi:black10.1051/0004-6361:20010675.
  * Cotton et al. (1980) Cotton, W.D.; Wittels, J.J.; Shapiro, I.I.; Marcaide, J.; et al.  The very flat radio spectrum of 0735 plus 178 - A cosmic conspiracy.  Astrophysical Journal, Letters 1980, 238, L123–L128.  doi:black10.1086/183271.
  * Stecker et al. (1992) Stecker, F.W.; de Jager, O.C.; Salamon, M.H.  TeV Gamma Rays from 3C 279: A Possible Probe of Origin and Intergalactic Infrared Radiation Fields.  Astrophysical Journal, Letters 1992, 390, L49.  doi:black10.1086/186369.
  * Franceschini and Rodighiero (2017) Franceschini, A.; Rodighiero, G.  The extragalactic background light revisited and the cosmic photon-photon opacity.  Astronomy and Astrophysics 2017, 603, A34, [arXiv:astro-ph.HE/1705.10256].  doi:black10.1051/0004-6361/201629684.
  * Stecker et al. (1991) Stecker, F.W.; Done, C.; Salamon, M.H.; Sommers, P.  High-energy neutrinos from active galactic nuclei.  Physical Review Letters 1991, 66, 2697–2700.  doi:black10.1103/PhysRevLett.66.2697.
  * Hughes et al. (1980) Hughes, E.B.; Hofstadter, R.; Johansson, A.; et al.  Characteristics of the telescope for high energy gamma -ray astronomy selected for definition studies on the Gamma Ray Observatory.  IEEE Transactions on Nuclear Science 1980, 27, 364–369.
  * Elvis et al. (1992) Elvis, M.; Plummer, D.; Schachter, J.; Fabbiano, G.  The Einstein Slew Survey.  Astrophysical Journal, Supplement 1992, 80, 257.  doi:black10.1086/191665.
  * Kuehr et al. (1981) Kuehr, H.; Witzel, A.; Pauliny-Toth, I.I.K.; Nauber, U.  A catalogue of extragalactic radio sources having flux densities greater than 1 Jy at 5 GHz.  Astrophysical Journal, Supplement 1981, 45, 367–430.
  * Padovani and Urry (1992) Padovani, P.; Urry, C.M.  Luminosity Functions, Relativistic Beaming, and Unified Theories of High-Luminosity Radio Sources.  Astrophysical Journal 1992, 387, 449.  doi:black10.1086/171098.
  * Wall and Peacock (1985) Wall, J.V.; Peacock, J.A.  Bright extragalactic radio sources at 2.7 GHz- III. The all-sky catalogue.  Monthly Notices of the RAS 1985, 216, 173–192.  doi:black10.1093/mnras/216.2.173.
  * Ghisellini et al. (2017) Ghisellini, G.; Righi, C.; Costamante, L.; Tavecchio, F.  The Fermi blazar sequence.  Monthly Notices of the RAS 2017, 469, 255–266, [arXiv:astro-ph.HE/1702.02571].  doi:black10.1093/mnras/stx806.
  * M. et al. (2015) M., A.; Ajello, M.; Atwood, W.B.; et al.  The Third Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope.  Astrophysical Journal 2015, 810, 14, [arXiv:astro-ph.HE/1501.06054].  doi:black10.1088/0004-637X/810/1/14.
  * Ghisellini and Tavecchio (2008) Ghisellini, G.; Tavecchio, F.  The blazar sequence: a new perspective.  Monthly Notices of the RAS 2008, 387, 1669–1680, [arXiv:astro-ph/0802.1918].  doi:black10.1111/j.1365-2966.2008.13360.x.
  * Paiano et al. (2017) Paiano, S.; Landoni, M.; Falomo, R.; Treves, A.; Scarpa, R.; Righi, C.  On the Redshift of TeV BL Lac Objects.  Astrophysical Journal 2017, 837, 144, [arXiv:astro-ph.GA/1701.04305].  doi:black10.3847/1538-4357/837/2/144.
  * Tagliaferri et al. (2015) Tagliaferri, G.; Ghisellini, G.; Perri, M.; et al.  NuSTAR and Multifrequency Study of the Two High-redshift Blazars S5 0836+710 and PKS 2149-306.  Astrophysical Journal 2015, 807, 167, [arXiv:astro-ph.HE/1503.04848].  doi:black10.1088/0004-637X/807/2/167.
  * Biteau et al. (2020) Biteau, J.; Prandini, E.; Costamante, L.; et al.  Progress in unveiling extreme particle acceleration in persistent astrophysical jets.  Nature Astronomy 2020, 4, 124–131, [arXiv:astro-ph.HE/2001.09222].  doi:black10.1038/s41550-019-0988-4.
  * Bai et al. (2019) Bai, X.; Bi, B.Y.; Bi, X.J.; et al.  The Large High Altitude Air Shower Observatory (LHAASO) Science White Paper – 2021 Edition.  arXiv e-prints 2019, p. arXiv:1905.02773, [arXiv:astro-ph.HE/1905.02773].
  * Albert et al. (2019) Albert, A.; Alfaro, R.; Ashkar, H.; et al.  Science Case for a Wide Field-of-View Very-High-Energy Gamma-Ray Observatory in the Southern Hemisphere.  arXiv e-prints 2019, p. arXiv:1902.08429, [arXiv:astro-ph.HE/1902.08429].
  * Pintore et al. (2020) Pintore, F.; Giuliani, A.; Belfiore, A.; et al.  Scientific prospects for a mini-array of ASTRI telescopes: A $\gamma$-ray TeV data challenge.  Journal of High Energy Astrophysics 2020, 26, 83–94, [arXiv:astro-ph.HE/2003.10982].  doi:black10.1016/j.jheap.2020.03.002.
  * Cherenkov Telescope Array Consortium (2019) Cherenkov Telescope Array Consortium.  Science with the Cherenkov Telescope Array; WORLD SCIENTIFIC, 2019; [https://www.worldscientific.com/doi/pdf/10.1142/10986].  doi:black10.1142/10986.
  * Costamante (2020) Costamante, L.  TeV-peaked candidate BL Lac objects.  Monthly Notices of the RAS 2020, 491, 2771–2778, [arXiv:astro-ph.HE/1911.05027].  doi:black10.1093/mnras/stz3018.
  * Sbarrato et al. (2021) Sbarrato, T.; Ghisellini, G.; Giovannini, G.; Giroletti, M.  Jetted radio-quiet quasars at z>5\.  arXiv e-prints 2021, p. arXiv:2109.08156, [arXiv:astro-ph.HE/2109.08156].
  * de Angelis et al. (2018) de Angelis, A.; Tatischeff, V.; Grenier, I.; et al.  Science with e-ASTROGAM. A space mission for MeV-GeV gamma-ray astrophysics.  Journal of High Energy Astrophysics 2018, 19, 1–106, [arXiv:astro-ph.HE/1711.01265].  doi:black10.1016/j.jheap.2018.07.001.
  * McEnery et al. (2019) McEnery, J.; van der Horst, A.; Dominguez, A.; et al.  All-sky Medium Energy Gamma-ray Observatory: Exploring the Extreme Multimessenger Universe.  Bulletin of the American Astronomical Society, 2019, Vol. 51, p. 245, [arXiv:astro-ph.IM/1907.07558].
  * Tomsick et al. (2019) Tomsick, J.; Zoglauer, A.; Sleator, C.; Lazar, H.; Beechert, J.; Boggs, S.; Roberts, J.; Siegert, T.; Lowell, A.; Wulf, E.; et al.  The Compton Spectrometer and Imager.  Bulletin of the American Astronomical Society, 2019, Vol. 51, p. 98, [arXiv:astro-ph.IM/1908.04334].
  * Ghisellini et al. (1998) Ghisellini, G.; Celotti, A.; Fossati, G.; Maraschi, L.; Comastri, A.  A theoretical unifying scheme for gamma-ray bright blazars.  Monthly Notices of the RAS 1998, 301, 451–468, [arXiv:astro-ph/astro-ph/9807317].  doi:black10.1046/j.1365-8711.1998.02032.x.
  * Celotti and Ghisellini (2008) Celotti, A.; Ghisellini, G.  The power of blazar jets.  Monthly Notices of the RAS 2008, 385, 283–300, [arXiv:astro-ph/0711.4112].  doi:black10.1111/j.1365-2966.2007.12758.x.
  * Ghisellini et al. (2010) Ghisellini, G.; Tavecchio, F.; Foschini, L.; Ghirlanda, G.; Maraschi, L.; Celotti, A.  General physical properties of bright Fermi blazars.  Monthly Notices of the RAS 2010, 402, 497–518, [arXiv:astro-ph.CO/0909.0932].  doi:black10.1111/j.1365-2966.2009.15898.x.
  * Ghisellini and Tavecchio (2015) Ghisellini, G.; Tavecchio, F.  Fermi/LAT broad emission line blazars.  Monthly Notices of the RAS 2015, 448, 1060–1077, [arXiv:astro-ph.HE/1501.03504].  doi:black10.1093/mnras/stv055.
  * Costamante et al. (2018) Costamante, L.; Cutini, S.; Tosti, G.; Antolini, E.; Tramacere, A.  On the origin of gamma-rays in Fermi blazars: beyond the broad-line region.  Monthly Notices of the RAS 2018, 477, 4749–4767, [arXiv:astro-ph.HE/1804.02408].  doi:black10.1093/mnras/sty887.
  * Poutanen and Stern (2010) Poutanen, J.; Stern, B.  GeV Breaks in Blazars as a Result of Gamma-ray Absorption Within the Broad-line Region.  Astrophysical Journal, Letters 2010, 717, L118–L121, [arXiv:astro-ph.HE/1005.3792].  doi:black10.1088/2041-8205/717/2/L118.
  * Fan et al. (2017) Fan, J.H.; Yang, J.H.; Xiao, H.B.; Lin, C.; Constantin, D.; Luo, G.Y.; Pei, Z.Y.; Hao, J.M.; Mao, Y.W.  Intrinsic Correlations for Flaring Blazars Detected by Fermi.  Astrophysical Journal, Letters 2017, 835, L38, [arXiv:astro-ph.HE/1702.05692].  doi:black10.3847/2041-8213/835/2/L38.
  * Nieppola et al. (2008) Nieppola, E.; Valtaoja, E.; Tornikoski, M.; Hovatta, T.; Kotiranta, M.  Blazar sequence - an artefact of Doppler boosting.  Astronomy and Astrophysics 2008, 488, 867–872, [arXiv:astro-ph/0803.0654].  doi:black10.1051/0004-6361:200809716.
  * Readhead (1994) Readhead, A.C.S.  Equipartition Brightness Temperature and the Inverse Compton Catastrophe.  Astrophysical Journal 1994, 426, 51.  doi:black10.1086/174038.
  * Potter and Cotter (2015) Potter, W.J.; Cotter, G.  New constraints on the structure and dynamics of black hole jets.  Monthly Notices of the RAS 2015, 453, 4070–4088, [arXiv:astro-ph.HE/1508.00567].  doi:black10.1093/mnras/stv1657.
  * Keenan et al. (2021) Keenan, M.; Meyer, E.T.; Georganopoulos, M.; Reddy, K.; French, O.J.  The relativistic jet dichotomy and the end of the blazar sequence.  Monthly Notices of the RAS 2021, 505, 4726–4745, [arXiv:astro-ph.GA/2007.12661].  doi:black10.1093/mnras/stab1182.
  * Giommi et al. (2012) Giommi, P.; Padovani, P.; Polenta, G.; Turriziani, S.; D’Elia, V.; Piranomonte, S.  A simplified view of blazars: clearing the fog around long-standing selection effects.  Monthly Notices of the RAS 2012, 420, 2899–2911, [arXiv:astro-ph.CO/1110.4706].  doi:black10.1111/j.1365-2966.2011.20044.x.
  * Merloni et al. (2012) Merloni, A.; Predehl, P.; Becker, W.; et al.  eROSITA Science Book: Mapping the Structure of the Energetic Universe.  arXiv e-prints 2012, p. arXiv:1209.3114, [arXiv:astro-ph.HE/1209.3114].
  * Rodrigues et al. (2018) Rodrigues, X.; Fedynitch, A.; Gao, S.; Boncioli, D.; Winter, W.  Neutrinos and Ultra-high-energy Cosmic-ray Nuclei from Blazars.  Astrophysical Journal 2018, 854, 54, [arXiv:astro-ph.HE/1711.02091].  doi:black10.3847/1538-4357/aaa7ee.
  * Murase et al. (2014) Murase, K.; Inoue, Y.; Dermer, C.D.  Diffuse neutrino intensity from the inner jets of active galactic nuclei: Impacts of external photon fields and the blazar sequence.  Physical Review D 2014, 90, 023007, [arXiv:astro-ph.HE/1403.4089].  doi:black10.1103/PhysRevD.90.023007.
  * Inoue and Totani (2009) Inoue, Y.; Totani, T.  The Blazar Sequence and the Cosmic Gamma-ray Background Radiation in the Fermi Era.  Astrophysical Journal 2009, 702, 523–536, [arXiv:astro-ph/0810.3580].  doi:black10.1088/0004-637X/702/1/523.
  * Nandra et al. (2013) Nandra, K.; Barret, D.; Barcons, X.; et al.  The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission.  arXiv e-prints 2013, p. arXiv:1306.2307, [arXiv:astro-ph.HE/1306.2307].
  * Braun et al. (2015) Braun, R.; Bourke, T.; Green, J.A.; Keane, E.; Wagg, J.  Advancing Astrophysics with the Square Kilometre Array.  Advancing Astrophysics with the Square Kilometre Array (AASKA14), 2015, p. 174.
  * Gardner et al. (2006) Gardner, J.P.; Mather, J.C.; Clampin, M.; et al.  The James Webb Space Telescope.  Space Science Reviews 2006, 123, 485–606, [arXiv:astro-ph/astro-ph/0606175].  doi:black10.1007/s11214-006-8315-7.
  * Weisskopf et al. (2016) Weisskopf, M.C.; Ramsey, B.; O’Dell, S.; Tennant, A.; et al.  The Imaging X-ray Polarimetry Explorer (IXPE).  Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray; den Herder, J.W.A.; Takahashi, T.; Bautz, M., Eds., 2016, Vol. 9905, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, p. 990517.  doi:black10.1117/12.2235240.