# The strange case of the transient HBL blazar 4FGL J1544.3-0649

N. Sahakyan,1,2, P. Giommi,2,3,4,5
1ICRANet-Armenia, Marshall Baghramian Avenue 24a, Yerevan 0019, Armenia
2ICRANet, P.zza della Repubblica 10, 65122 Pescara, Italy
3 Institute for Advanced Study, Technische Universität München,
Lichtenbergstrasse 2a, D-85748 Garching bei München, Germany
4 Excellence Cluster ORIGINS, Boltzmannstrasse 2, D-85748 Garching bei
München, Germany
$5$ Associated to Italian Space Agency, ASI, via del Politecnico snc, 00133
Roma, Italy E-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present a multifrequency study of the transient $\gamma$-ray source 4FGL
J1544.3-0649, a blazar that exhibited a remarkable behaviour raising from the
state of an anonymous mid-intensity radio source, never detected at high
energies, to that of one of the brightest extreme blazars in the X-ray and
$\gamma$-ray sky. Our analysis shows that the averaged $\gamma$-ray spectrum
is well described by a powerlaw with a photon index of $1.87\pm 0.04$, while
the flux above 100 MeV is $(8.0\pm 0.9)\times 10^{-9}\>{\rm
photon\>cm^{-2}\>s^{-1}}$, which increases during the active state of the
source. The X-ray flux and spectral slope are both highly variable, with the
highest 2-10 keV flux reaching $(1.28\pm 0.05)\times 10^{-10}\>{\rm
erg\>cm^{-2}\>s^{-1}}$. On several observations the X-ray spectrum hardened to
the point implying as SED peak moving to energies larger than 10 keV. As in
many extreme blazars the broadband spectral energy distribution can be
described by a homogeneous one-zone synchrotron-self-Compton leptonic model.
We briefly discuss the potential implications for high-energy multi-messenger
astrophysics in case the dual behaviour shown by 4FGL J1544.3-0649 does not
represent an isolated case, but rather a manifestation of a so far unnoticed
relatively common phenomenon.

###### keywords:

radiation mechanisms: non-thermal – BL Lacertae objects: individual: 4FGL
J1544.3-0649 – gamma-rays: galaxies – X-rays: galaxies

††pubyear: 2020††pagerange: The strange case of the transient HBL blazar 4FGL
J1544.3-0649–The strange case of the transient HBL blazar 4FGL J1544.3-0649

##  1 Introduction

Blazars are a special type of active galactic nuclei (AGN) hosting a
relativistic jet that happens to be pointing very close to the observer’s line
of sight (e.g. Blandford & Rees, 1978; Padovani et al., 2017). The emission in
these sources is dominated by non-thermal radiation across the electromagnetic
spectrum, often exhibiting large variations on timescales ranging from years
to a few hundred seconds. Blazars are usually grouped into BL Lacertae (BL
Lac) objects and Flat Spectrum Radio Quasars (FSRQs), based on their
properties in the optical band: the optical spectrum of FSRQs shows broad
emission lines while that of BL Lacs is featureless, shows narrow lines or
features of the host galaxy (Urry & Padovani, 1995).
Blazars are the most luminous and energetic persistent emitters in the known
Universe, dominating in the extragalactic $\gamma$-ray sky. Among the total
$\sim 5,800$ sources in the Fermi Large Area Telescope (Fermi-LAT) fourth
source catalog of $\gamma$-ray sources (4FGL-DR2 ; Abdollahi et al., 2020;
Ajello et al., 2020a; Ballet et al., 2020; Lott et al., 2020) $\sim 3,100$ are
blazars, 47 are radio galaxies, and 19 are other types of AGNs. Interestingly,
$\gamma$-ray emission has been detected also from blazars at very high
redshifts (e.g., Ackermann et al., 2017; Sahakyan et al., 2020) which enables
the study of the evolution of the most luminous relativistic jets over cosmic
time.
The broadband spectral energy distribution (SED) of blazars has a
characteristic double hump shape. The low energy peak (radio to UV/X-ray) is
usually explained by synchrotron emission of relativistic electrons
propagating along the jet, although recent models developed to explain the
emission of high-energy neutrinos in blazars assume that this component may be
due to proton synchrotron radiation, at least during flares (Mastichiadis &
Petropoulou, 2020). The second SED component extends to High Energies (HEs;
$>100$ MeV), its origin being debatable. In the leptonic scenarios, this
component is interpreted as inverse Compton upscattering of synchrotron
photons (Synchrotron self Compton [SSC] Maraschi et al. (1992); Bloom &
Marscher (1996); Ghisellini et al. (1985)) or of external photons (external
inverse Compton) (Sikora et al., 1994; Dermer & Schlickeiser, 1994). On the
other hand, in hadronic models the HE component is mainly due to proton
synchrotron emission (Aharonian, 2000; Mücke & Protheroe, 2001) or pion decay
(Mannheim, 1993; Mannheim & Biermann, 1989; Mücke & Protheroe, 2001; Mücke et
al., 2003; Böttcher et al., 2013). In the latter case, blazars are also
sources of Very High Energy (VHE; $>100$ GeV) neutrinos (IceCube Collaboration
et al., 2018a, b; Ansoldi et al., 2018; Keivani et al., 2018; Murase et al.,
2018; Padovani et al., 2018; Sahakyan, 2018; Righi et al., 2019; Cerruti et
al., 2019; Gao et al., 2019).
Depending on the peak of the low energy component ($\nu_{\rm peak-s}$) blazars
can be further grouped into three subcategories, namely low-energy peaked
blazars (LBLs; $\nu_{\rm peak-s}<10^{14}$ Hz), intermediate peaked blazars
(IBLs; $10^{14}\>{\rm Hz}<\nu_{\rm peak-s}<10^{15}$ Hz) and high-energy peaked
blazars (HBLs; $\nu_{\rm peak-s}>10^{15}$ Hz) (Padovani & Giommi, 1995; Abdo
et al., 2010). Observations in the X-ray band reveal that sometimes the
synchrotron peak of HBLs can reach energies of $\sim$1 keV, ($\sim 2\times
10^{17}$ Hz) or even larger, showing an extreme behaviour (extreme HBLs, e.g.
Giommi et al., 1999; Costamante et al., 2001; Biteau et al., 2020). For
example, during the flares of Mkn 501 the synchrotron peak reached $\sim 100$
keV (Pian et al., 1998) or during the flares of 1ES 1218+304 the X-ray
spectral index hardened to $\Gamma\leq 1.80$ shifting the peak towards higher
energies (Sahakyan, 2020). The third catalog of high energy peaked blazars
(3HSP, Chang et al., 2019) includes several objects in this category. Recent
observations in the HE and VHE $\gamma$-ray bands have revealed an additional
class of BL Lac objects VHE $\gamma$-ray spectrum of which is characterized by
a hard intrinsic photon index of up to 1 TeV after correction for the
extragalactic background light (EBL) absorption effects (BL Lacs extreme in
$\gamma$-rays; Bonnoli et al. (2015); Tavecchio et al. (2011)).
The BL Lacs having featureless optical spectra have been traditionally
discovered in radio or X-ray surveys (e.g., Giommi et al., 2005). Currently,
the $\gamma$-ray data from Fermi-LAT observations combined with the data at
the lower energy bands (e.g., infrared, IR, Massaro et al., 2012) are a
powerful additional method; nearly 38% of detected blazars are BL Lacs in the
fourth catalog of AGNs detected by Fermi-LAT (Chang et al., 2019; Ajello et
al., 2020b). Nearly all these $\gamma$-ray emitting BL Lacs have detected
radio counterparts, which makes radio observations crucial also for the
understanding of their physics. In this regard of a particular interest is the
identification of radio-weak BL Lac objects, i.e., BL Lacs from which radio
emission is detected in follow-up observations but which do not have
counterparts in the major radio surveys. A handful observations of radio-weak
BL Lac objects will have an impact on our understanding of AGN unification as
well as on BL Lac associations based on the radio surveys (Massaro et al.,
2017; Chang et al., 2019).
In May 2017 Fermi-LAT detected a new $\gamma$-ray source (4FGL J1544.3-0649)
not associated with any previously known $\gamma$-ray object; it was bright in
the $\gamma$-ray band in two consecutive weeks after the discovery (Ciprini et
al., 2017). The X-ray follow-up observations by Neil Gehrels Swift Observatory
(Gehrels et al., 2004), (hereafter Swift) found a new bright X-ray source at a
position corresponding to the optical transient ASASSN-17gs detected at
$V=17.3$ mag on May 25 (Ciprini et al., 2017). The MAXI team (Kawase et al.,
2017) also reported a detection of this source on May 12-13 at a flux level
similar to that observed by Swift, with a flux increase of a factor $\sim$2 in
the period May 21 to May 25. The spectroscopic redshift of the host galaxy,
2MASX J15441967-0649156, was estimated to be $z=0.171$ using the MDM 2.4m
Hiltner telescope (Chornock & Margutti, 2017). The source has a counterpart in
the radio band with flux densities of 46.6 mJy at $1.4$ GHz (NRAO VLA Sky
Survey; Condon et al., 1998), 35.8 mJy at 3 GHz (Very Large Array Sky Survey,
epoch 1; Gordon et al., 2020) and 67 mJy at 150 MHz (TIFR GMRT Sky Survey;
Intema et al., 2017). In addition, Bruni et al. (2018) monitored 4FGL
J1544.3-0649 with the Effelsberg-100 m single dish radio telescope for a four-
month follow-up after the brightening in the $\gamma$-ray band. These data
imply a flat radio spectrum very similar to that of most blazars, and a radio
luminosity of $\sim 5\times 10^{40}$ erg s-1, at 1.4 GHz, more than a factor
10 larger than that of Mrk 501 and Mrk 421. Considering the post-burst data at
5 GHz, the radio to X-ray flux ratio is log (F${}_{\rm 1.4\>GHz}$/F${}_{\rm
1\>kev}$) = -4.3 typical of HBL Blazars. Follow up XMM-Newtown (XMM)
observations showed that the source exhibits strong variability both in the
X-ray flux and spectral shape on timescales of $\sim 10$ ks and the X-ray
spectrum is described by a broken powerlaw (Ursini et al., 2019).
The transient nature of 4FGL J1544.3-0649 makes it a unique object and an
interesting target for broadband studies. Understanding of the changes in the
jet of 4FGL J1544.3-0649 which led to its brightening in the X-ray and
$\gamma$-ray bands can shed light on the physics at work in the BL Lac jets.
In this regard, the many monitoring observations of 4FGL J1544.3-0649 with
Swift provide unprecedented data both in the optical/UV and X-ray bands while
that of Fermi-LAT in the 100 MeV-500 GeV band allowing to shape the low and
high energy components in the SED of 4FGL J1544.3-0649 and investigate its
evolution in time. Also, this broadband data can be used for theoretical
modeling allowing to derive the main parameters characterizing the jet of 4FGL
J1544.3-0649.
The purpose of this paper is to investigate the origin of the broadband
emission from 4FGL J1544.3-0649 and the astrophysical multi-messenger
implications of the dual behaviour of this source. The paper is organized as
follows. The Fermi-LAT and Swift data extraction and analyses are described in
Section 2. In Section 3 the origin of the multiwavelength emission is
investigated, and the discussion and conclusion are given in Section 4.

##  2 Data analyses

###  2.1 Fermi-LAT observations of 4FGL J1544.3-0649

The $\gamma$-ray data used in this work was collected by Fermi-LAT from August
2008 to July 2020 (from MJD 54683 to MJD 59058). The Pass 8 SOURCE data in the
energy range from 100 MeV to 600 GeV have been downloaded and analyzed using
the standard reduction methodology suggested by the LAT collaboration. The
events classified as evclass=128 and evtype=3 within a circular region of
$12^{\circ}$ centered on the $\gamma$-ray position of 4FGL J1544.3-0649 (R.A.,
decl.=236.078, -6.825) are analyzed using Fermi ScienceTools (1.2.1) with
P8R3_ SOURCE_ V2 instrument response functions. The good time intervals were
selected with the filter "DATA_ QUAL>0" and "LAT_ CONFIG==1" in the gtmktime
tool whereas the Earth limb photons were removed by applying a maximum zenith
angle cut of $90^{\circ}$. The data are divided into 38 logarithmically equal
energy bins and binned into the $16.9^{\circ}\times 16.9^{\circ}$ square
region of interest. Then, binned likelihood analysis implemented in gtlike
tool was used to estimate the $\gamma$-ray flux and photon index of the
sources. The model file is generated from the Fermi-LAT fourth source catalog
of $\gamma$-ray sources (4FGL) using the 4FGL-DR2 version of the 4FGL which is
based on 10 years of survey; it includes all point like sources within
$12^{\circ}+5^{\circ}$ around 4FGL J1544.3-0649 together with the Galactic and
extragalactic diffuse $\gamma$-ray models (gll_iem_v07 and
iso_P8R3_SOURCE_V2_v1, respectively). The spectral shapes of all sources are
adopted from the 4FGL catalog and all spectral parameters of the sources
falling between $12^{\circ}$ and $12^{\circ}+5^{\circ}$ are kept fixed. The
$\gamma$-rayspectrum of 4FGL J1544.3-0649is modeled with a power-law function.
The significance of the $\gamma$-ray signal is evaluated by the test statistic
($TS$) (Mattox et al., 1996) defined as $TS=2\times(logL_{1}-logL_{0})$ where
$L_{1}$ and $L_{0}$ are the likelihood of the data with and without a point
source at the position of the source under investigation.
The $\gamma$-ray light curves were calculated by performing an unbinned
maximum likelihood analysis with the appropriate quality cuts as described
above. Initially the light curve was computed by dividing the entire
observation time into 21-day intervals and selecting events in the energy
range from 500 MeV to 300 GeV. The lower limit was selected 500 MeV, since due
to the hard $\gamma$-ray photon index of 4FGL J1544.3-0649 the source is
undetected below $\sim 500$ MeV. Also, the normalization of both diffuse
components was fixed to the values obtained for the whole time period, as no
variability is expected from the background. Next, the $\gamma$-ray
variability of the 4FGL J1544.3-0649 was investigated by generating the light
curves with the help of an adaptive binning method (Lott et al., 2012). This
produces light curves with flexible time bins, requiring a fixed uncertainty
for the flux estimation in each time interval. The fluxes are computed above
the optimal energy, which is $E_{\rm p}=801.6$ MeV in this case. This allows a
detailed investigation of the $\gamma$-ray flux variation and is a powerful
method for identifying flaring periods (if any).

Figure 1: Multiwavelength light curves for 4FGL J1544.3-0649.

The analysis of entire $\sim 11.9$ years data yielded the source detection
with ${\rm TS=778.7\>(27.9\>\sigma)}$ and with a hard $\gamma$-ray spectrum
($\sim E^{-1.87\pm 0.04}$) consistent with the value given in 4FGL. The
integral flux estimated above 100 MeV is $(8.0\pm 0.9)\times 10^{-9}\>{\rm
photon\>cm^{-2}\>s^{-1}}$. Panels a) and b) of Fig. 1 show the $\gamma$-ray
flux and photon index evolution in time calculated for 21-day (red) bins where
only the intervals when $TS>12$ are presented. The source was in the bright
$\gamma$-ray flaring state around MJD 58000 with the highest $\gamma$-ray flux
of $(2.59\pm 0.43)\times 10^{-8}\>{\rm photon\>cm^{-2}\>s^{-1}}$ observed on
MJD $57997.5\pm 10.5$ with a detection significance of $15.0\>\sigma$. Yet,
the adaptively binned light curve illustrates the changes in the $\gamma$-ray
flux better; the source being mostly undetected in the $\gamma$-ray band shows
significant ($>5\sigma$) $\gamma$-ray emission with a flux of $(7.75\pm
2.96)\times 10^{-10}\>{\rm photon\>cm^{-2}\>s^{-1}}$ above $E_{\rm p}=801.6$
MeV starting from MJD $57484.7$. However, the $\gamma$-ray flux of the source
substantially increased during MJD 57869.3-58052.0; the highest $\gamma$-ray
flux of $(2.35\pm 0.55)\times 10^{-8}\>{\rm photon\>cm^{-2}\>s^{-1}}$ above
$E_{\rm p}=801.6$ MeV was observed on MJD $57998.9\pm 4.60$. The flux was
above $10^{-8}\>{\rm photon\>cm^{-2}\>s^{-1}}$ in other eight periods (see
Fig. 1). Despite the increase in the $\gamma$-ray flux, the photon index does
not vary significantly (panel b in Fig. 1); even though there are periods when
the spectrum hardens, no definite conclusion can be drawn because of large
uncertainties. For example, in the adaptively binned intervals the hardest
indexes of $1.43\pm 0.19$ and $1.44\pm 0.18$ were observed on MJD $57955.64\pm
4.49$ and MJD $58241.06\pm 8.70$ which within the uncertainty agree with the
averaged photon index estimated during $\sim 11.9$ years. Interestingly, in
the 21-day binned light curve, when the highest $\gamma$-ray flux was observed
on MJD $57997.5\pm 10.5$, the $\gamma$-ray spectrum is hard with a photon
index of $1.57\pm 0.11$.
The distribution of HE photons arrival time (computed using the gtsrcprob
tool) is shown in panel f) of Fig. 1. The highest energy event with
$E_{\gamma}=174.20$ GeV has been observed on MJD 57951.92 within a circle of
$0.054^{\circ}$ around 4FGL J1544.3-0649, with the probability of $0.99428$
being associated with it. There are four additional events from the innermost
region around 4FGL J1544.3-0649 with an energy exceeding 100 GeV; the 107.4,
106.0, 147.4 and 117.8 GeV events were observed on MJD 58060.2, 57780.9,
57960.1 and 58579.9, respectively.

###  2.2 Swift XRT observations

4FGL J1544.3-0649 was pointed by Swift 51 times, starting shortly after the
discovery of the $\gamma$-ray and optical transient in May 2017 to mid 2020.
All the XRT data was processed using Swift$\\_$xrtproc, a data reduction,
spectral and imaging analysis tool, which automatically downloads the data
from one of the Swift archives, generates high-level calibrated data products,
and performs a comprehensive spectral and imaging analysis of a source, using
the XSPEC12.11 and XIMAGE4.5.1 packages, released as part of the HEASoft-6.28
distribution. Swift$\\_$xrtproc was developed within the Open Universe
initiative 111https://openuniverse.asi.it (Giommi et al., 2020a), in
collaboration with the ASI Space Science Data Center (SSDC)
222https://www.ssdc.asi.it, to analyse all the XRT observations of a large
sample of blazars and the data of all the Gamma Ray Bursts (GRBs) observed by
Swift-XRT (Giommi et al. 2020 in preparation).

The 2-10 keV X-ray flux and photon index (0.3-10 keV) are shown in Fig. 1 c)
and d), respectively. The source was in a high X-ray state contemporaneously
with the $\gamma$-ray flare around MJD 58000 with the highest X-ray flux of
$(1.28\pm 0.05)\times 10^{-10}\>{\rm erg\>cm^{-2}\>s^{-1}}$ observed on MJD
57991.40. Then, the source is in a quiescent state with the flux of the order
of $(1.0-5.0)\times 10^{-12}\>{\rm erg\>cm^{-2}\>s^{-1}}$. The X-ray photon
index shows an interesting evolution in time; the X-ray spectrum is hard
($<2.0$) during the bright X-ray state around MJD 58000 which then softens
during the moderately bright or quiescent states. The hardest X-ray photon
index of $1.57\pm 0.07$ was observed on MJD 58009.92; there are additional 12
periods with an X-ray photon index of $\leq 1.9$. This hardening clearly
shifts the peak of synchrotron component to higher energies. The relation
between the X-ray flux and the photon index was further investigated by
considering XRT observations in each orbit. The X-ray flux (2-10 keV) versus
the photon index is shown in Fig. 2. The spectrum clearly tends to harden as
the source gets brighter. The linear-Pearson correlation test yields $r_{\rm
d}=-0.71$ which suggests a negative correlation between the flux and photon
index, i.e., when the photon index decreases (hardens) the flux increases.

Figure 2: The X-ray flux versus photon index.

###  2.3 Swift UVOT

Along with the XRT observations, the source was also targeted with Swift UVOT
in the ultraviolet UVW1, UVM2, and UVW2 and optical V, B and U bands. For each
observation, the aperture photometry analysis was performed using the standard
UVOT software distributed within the HEASoft package (v6.26.1) and the
calibration included in the latest release of CALDB. Source counts were
extracted from a circular region with $5^{\prime\prime}$ radius, and the
background ones from a source-free region with $20^{\prime\prime}$ radius.
Each individual observation was checked to validate that in any filter the
background was not contaminated by the nearby objects. The counts were
extracted and converted to fluxes using UVOTSOURCE tool and the conversion
factors from Poole et al. (2008). The flux values were then de-reddened using
the value of E (B - V ) = 0.138 (Schlafly & Finkbeiner, 2011).
The optical/UV light curve is shown in Fig. 1 e). The source appears to be in
a quiescent state in the optical/UV bands when it was bright in the X-ray and
$\gamma$-ray bands (MJD 58000). However, the optical/UV flux substantially
increased around MJD $\sim 58250$. The highest flux of $(1.87\pm 0.12)\times
10^{-11}\>{\rm erg\>cm^{-2}\>s^{-1}}$ was observed on MJD 58191.08 in UVM2
filter. In this period the flux increased in other UVOT filters as well, being
at the level of $(1.06-1.47)\times 10^{-11}\>{\rm erg\>cm^{-2}\>s^{-1}}$.

##  3 The origin of multiwavelength emission

Multiwavelength observations of 4FGL J1544.3-0649 allow us to build the SEDs
with simultaneous and archival data as well as to investigate the time
evolution of SEDs. The $\gamma$-ray data for each adaptively binned interval
has been combined with the Swift optical/UV and X-ray data, with the
Effelsberg-100 m observations of 4FGL J1544.3-0649 (in the frequencies from 5
to 15 GHz Bruni et al., 2018) and with archival data from the VOU-Blazar
(Chang et al., 2020) and the SSDC333https://tools.ssdc.asi.it/SED SED tools,
to form the SED/light curve animation that is available here
youtu.be/9feaNWi1RDs. Moving over each $\gamma$-ray light curve interval, the
spectrum observed in different bands evolves showing the time evolution of the
SED. The months before the large $\gamma$-ray flare on MJD 58000 (May 2017)
the source is barely detected in the $\gamma$-ray band, i.e., the SED on MJD
57274.73 contain upper limits and large uncertainties in the flux estimation.
Then the $\gamma$-ray emission substantially increases which is sometimes
accompanied by increases in the other bands. The $\gamma$-ray emission from
the source is still detectable with a high significance until MJD 58865.87,
but in the last interval (MJD $58947.23\pm 81.36$) the detection significance
drops to $TS=17$. To investigate the nature of the multiwavelength emission
from 4FGL J1544.3-0649, data from the following periods are considered:

  * Period 1 (P1): averaged $\gamma$-ray emission state on MJD $57938.35\pm 12.80$ with contemporaneous Swift UVOT/XRT (Obsid: 10145008) and radio data.

  * Period 2 (P2): from MJD 57984.00 to MJD 57994.33 corresponding to the period when the highest X-ray flux was observed (Obsid: 10145018).

  * Period 3 (P3): 105.06-days interval centered on MJD $58571.12$ when the source was in a low state in the $\gamma$-ray, X-ray and optical/UV bands (Obsid: 10145057).

Figure 3: Modeling of multiwavelength SEDs of 4FGL J1544.3-0649 during the MJD $57938.35\pm 12.80$ (left panel) and MJD $57989.16\pm 5.16$ and MJD $58571.12\pm 52.53$ (right panel; squares and circles, respectively). The model is presented in the text and the parameters are given in Table 1. Table 1: Parameters of modeling of the broadband SED of 4FGL J1544.3-0649 within a simple one zone synchrotron/SSC scenario. Parameter | P1 | P2 | P3
---|---|---|---
$\delta$ | 25.6 | 25.6 | 25.6
$\alpha$ | 2.25 | 2.19 | 1.90
$\gamma^{\prime}_{\rm min}$ | 481.1 | 481.1 | 481.1
$\gamma^{\prime}_{\rm cut}\times$106 | 1.16 | 6.82 | 0.11
B [G] $10^{-3}$ | 7.82 | 8.54 | 13.71
Le [$\times 10^{44}$ erg cm-3] | 1.60 | 2.47 | 0.50
LB [$\times 10^{40}$ erg cm-3] | 5.73 | 6.83 | 17.61

The broadband SED of 4FGL J1544.3-0649 shows the usual double humped shape
that can be interpreted within a common one-zone synchrotron/SSC model (Fig.
3). It is assumed that the broadband emission is produced by relativistic
electrons in a spherical blob of radius $R$ with a uniform magnetic field $B$.
The emitting region is seen in the direction close to the line of sight which
moves with the bulk Lorentz factor of $\Gamma\approx\delta$. It is assumed
that the emitting electrons have commonly used powerlaw with an exponential
cut-off energy distribution in the form of $N^{\prime}_{\rm
e}(\gamma^{\prime}_{\rm
e})=N^{\prime}_{0}\>\gamma^{\prime-\alpha}_{e}\>Exp[-\gamma^{\prime}_{\rm
e}/\gamma^{\prime}_{\rm cut}]$ for $\gamma^{\prime}_{\rm
e}\geq\gamma^{\prime}_{\rm min}$ where $\gamma^{\prime}_{\rm
min}=E^{\prime}_{e}/m_{e}\>c^{2}$ is the minimum electron energy. These
electrons interacting with the magnetic field radiate via the synchrotron
mechanism which explains the low energy component in the SED while the HE hump
is described by up-scattering of the synchrotron photons by the same
electrons.
The model includes seven free parameters to describe the emitting electrons
($\alpha$, $N^{\prime}_{0}$, $\gamma^{\prime}_{\rm min}$ and
$\gamma^{\prime}_{\rm cut}$) and specify the emitting region ($\delta$, $B$
and $R$). In order to reduce the number of free parameters, we assume the
emission is produced in a compact region with $R=5\times 10^{16}\>{\rm cm}$
which corresponds to flux changes on days time scales, as it has been found in
the $\gamma$-ray band (adaptive bins in Fig. 1). In order to constrain
$\delta$ and $\gamma^{\prime}_{\rm min}$ initially the SED on MJD $57938.35\pm
12.8$ (P1) with (quasi) simultaneous data ranging from radio to HE
$\gamma$-ray bands is modeled, resulting in $\delta=25.6$ and
$\gamma^{\prime}_{\rm min}=481.1$ which are typical values usually obtained
for BL Lacs. These were fixed parameters when modeling the SEDs observed on
MJD 57989.16 (P2) and MJD 58571.12 (P3) for which the low energy data are
lacking. In order to account for the contribution of the host galaxy (see the
excess in the IR/optical part of the SED on MJD $57938.35\pm 12.8$), in
addition to synchrotron/SSC component also we adopted a template of giant
elliptical galaxy. The fitting is performed with the open source package
JetSet (Massaro et al., 2006; Tramacere et al., 2011, 2009).
The SED modeling results are shown in Fig. 3 and the corresponding parameters
presented in Table 1. In the left panel of Fig. 3, the Swift UVOT and ASAS-SN
data (downloaded from the ASAS-SN Sky Patrol web site Kochanek et al.,
2017)444https://asas-sn.osu.edu/) allow to estimate or at least constrain the
contribution of the host galaxy; the peak of the host galaxy component is at
$6.13\times 10^{-12}\>{\rm erg\>cm^{-2}\>s^{-1}\>}$. When the SED in the
quiescent state is considered (P3), the data can be reproduced when
$\alpha=1.96$ and $\gamma^{\prime}_{\rm cut}=1.13\times 10^{5}$ and the
magnetic field is $B=1.37\times 10^{-2}\>{\rm G}$. In this case low energy
component peak is at $\sim 2.42\times 10^{16}\>{\rm Hz}$ which, however,
shifts to higher frequencies when the active states of the source are
considered; i.e., during MJD $57938.35\pm 12.8$ the X-ray data defines the
peak to be at $\sim 10^{17}\>{\rm Hz}$ whereas during the hardest X-ray state
the peak is shifted well beyond $\sim 10^{18}\>{\rm Hz}$. This shift changes
also the maximum energy up to which the particles are effectively accelerated;
$\gamma^{\prime}_{\rm cut}$ increases to $1.16\times 10^{6}$ and $6.82\times
10^{6}$ when the SEDs on P1 and P2 are considered, respectively. The modeling
resulted in a similar powerlaw index and magnetic field for these active X-ray
periods, namely $\alpha=2.25$ and $7.82\times 10^{-3}\>{\rm G}$ for P1 and
$\alpha=2.19$ and $8.54\times 10^{-3}\>{\rm G}$ for P2. In all the considered
periods the jet is particle dominated with $U_{\rm e}/U_{\rm B}>3000$ for P1
and P2 and $U_{\rm e}/U_{\rm B}\simeq 350$ for P3. However, $U_{\rm e}$
strongly depends on $\gamma^{\prime}_{\rm min}$ which is not well constrained,
and when allowing $\gamma^{\prime}_{\rm min}$ to vary $U_{\rm e}/U_{\rm B}$
ratio can change. The jet power in the form of magnetic field and electron
kinetic energy are calculated by $L_{B}=\pi cR^{2}\Gamma^{2}U_{B}$ and
$L_{e}=\pi cR^{2}\Gamma^{2}U_{e}$, respectively, and are given in Table 1.

##  4 Discussion and Conclusion

Figure 4: The $\gamma$-ray (@ 1 GeV) and the reconstructed X-ray (@ 1 keV)
light curve of 4FGL J1544.3-0649 (see text for details). Figure 5: The SED of
4FGL J1544.3-0649. Green points are from Swift UVOT, Swift XRT and Fermi-LAT;
grey points are archival multi-frequency data retrieved with VOU-Blazars. For
comparison the average SED of Mrk 501, scaled to flux level of 4FGL
J1544.3-0649, in the radio/infrared bands, is plotted as orange points. In the
X-ray band instead of the average spectrum we plot the the spectra observed in
Mrk 501 when it was at maximum and minimum intensity.

We have presented the results of the analysis of optical/UV, X-ray and
$\gamma$-ray observations of 4FGL J1544.3-0649, a remarkable blazar showing
transient-like high-energy multiwavelength emission, a behaviour that was
never observed before in AGNs. This source, in fact, remained below the
sensitivity limits of all the high-energy surveys and observations until May
2017, when it brightened to such a level to be detectable in short exposures
by Fermi-LAT and by the MAXI X-ray sky monitor (Kawase et al., 2017).
During the brightening, the 2-10 keV X-ray flux strongly varies increasing
from $(1.0-5.0)\times 10^{-12}\>{\rm erg\>cm^{-2}\>s^{-1}}$ to $(1.28\pm
0.05)\times 10^{-10}\>{\rm erg\>cm^{-2}\>s^{-1}}$. This variability, which is
even larger when the flux is estimated in each Swift orbit, is clearly
associated to a harder-when-brighter behavior, as also noted by Ursini et al.
(2019) based on XMM observations. Such flux and photon index correlation is
well known in HBL blazars (e.g. Giommi et al., 1990), and is usually
interpreted by the relation between the acceleration and cooling processes. As
shown in Kirk et al. (1998), when the injection decreases and particles are
loosing energy by synchrotron cooling or any other cooling process that is
faster at higher energies, the spectrum becomes steeper.
During the $\gamma$-ray brightening of the source, the X-ray spectrum modifies
as well. The available optical/UV and X-ray data defines the peak of the low
energy component to be within $\nu_{\rm peak-s}=10^{15}-10^{17}$ Hz, implying
4FGL J1544.3-0649 is a classical HBL. However, occasionally (e.g., on MJD
57898.70, 57989.16, 57998.92, 58009.40 and 58021.40, see the SED/Light curve
animation) the source shows a synchrotron component peak above $10^{18}$ Hz
which is more typical for extreme synchrotron BL Lacs. Yet, in all these
periods the $\gamma$-ray photon index hardens as well, e.g., $1.74\pm 0.20$ on
MJD 57898.70, $1.57\pm 0.18$ on MJD 57989.16 or $1.61\pm 0.19$ on MJD
58021.40, shifting the HE peak to VHEs. The peaks in the SED are determined by
$\gamma^{\prime}_{\rm cut}\simeq\sqrt{\frac{\nu_{\rm c,\>peak}}{\nu_{\rm
s,\>peak}}}$ implying in P1 and P2 when $\nu_{\rm peak}\geq 10^{18}$ Hz the
second peak is around $2\times 10^{26}-2\times 10^{27}$ Hz ($1-10$ TeV) (solid
line in Fig. 3). However, this peak and the flux is suppressed by Klein
Nishina effects and EBL absorption, which for the distance of 4FGL
J1544.3-0649 starts at $\sim 5\times 10^{25}$ Hz ($\sim 200$ GeV; dot dashed
line in Fig. 3). Despite this reduction, during the bright $\gamma$-ray and
X-ray states the source flux at 1 TeV is expected to be $\simeq 10^{-11}\>{\rm
erg\>cm^{-2}\>s^{-1}}$, well above the sensitivity of the current ground-based
Cherenkov telescopes. In addition, the hard $\gamma$-ray photon index observed
in the MeV/GeV band and the applied SSC model predict also a very hard photon
index in TeV band after EBL correction. So, 4FGL J1544.3-0649 mimics
properties similar to those of extreme BL Lacs in $\gamma$-rays (Bonnoli et
al., 2015; Tavecchio et al., 2011), which are an interesting sub-class not
only because of their possible association with VHE neutrinos and cosmic rays
(Padovani et al., 2016; Resconi et al., 2017) but also because they pose major
difficulties for the understanding of particle acceleration and emission
processes in blazar jets.
The multiwavelength light curve of Fig. 1, shows that the optical/UV flux
increase is delayed compared to that in the X-ray and $\gamma$-ray bands. In
general, lags in different energy bands are expected either from changes in
the global parameters (magnetic field, Lorentz factor, etc.) or from the
temporal evolution of emitting particles. For example, Bednarek & Wagner
(2008) proposed that if the low energy component is produced when the blob is
closer to the base of the jet and the HE photons are emitted at larger
distances from the jet base, at which the blob has accelerated to a higher
Lorentz factor, this could result in delays between the low energy and HE
flares. Or, in complex jet geometry if the flare is caused by the collision
between a relativistic shock wave and a stationary feature in the jet, it
results in a complex variation of fluxes in different bands (Sokolov et al.,
2004). However, a simple consideration of particle acceleration and cooling
can naturally explain the observed delay in the multiwavelength light curve of
4FGL J1544.3-0649. The SED in different epochs is well reproduced by a one-
zone synchrotron/SSC model with physically reasonable parameters. Within this
model, the flare and modification of the spectra can be interpreted by an
increase in the luminosity of electrons and $\gamma^{\prime}_{\rm cut}$;
during the bright X-ray period the luminosity increased $\sim 4.9$ times
whereas $\gamma^{\prime}_{\rm cut}$ $\sim 62$ times. This implies that the
X-ray and $\gamma$-ray flare was most likely caused by injection of new
freshly accelerated electrons. The cooling time of these electrons is

$t_{\rm cooling}=\frac{3}{4}\>\frac{m_{\rm e}\>c}{\sigma_{\rm T}(u_{\rm
B}+u_{\rm syn})\>\gamma}=\frac{6\>\pi\>m_{\rm e}\>c}{\sigma_{\rm
T}\>\gamma\>B^{2}\>(1+u_{\rm syn}/u_{\rm B})}.$ (1)

where $u_{\rm syn}$ is the synchrotron photon energy density in the jet frame.
Considering the peak frequency of synchrotron emission is $\nu=1.2\times
10^{6}\>B\>\delta\>\gamma^{2}$, the cooling time of the electron emitting
photons with frequency $\nu_{15}=\nu/10^{15}\>{\rm Hz}$ is $t_{\rm
cooling}=2.7\times 10^{4}\>(\delta/1+z)^{0.5}\>B^{-1.5}\>\nu_{15}^{-0.5}$ s.
Considering the parameters obtained for the P1, the delay between X-ray
emission ($10^{18}$ Hz) and optical/UV ($5\times 10^{14}$ Hz) is of the order
of $\sim 114$ days which is very similar to that observed. However, in order
to investigate in details the time lags and thus to infer the underlying
physics, much denser monitoring of 4FGL J1544.3-0649 in optical/UV band is
needed.
Figure 4 plots the high-energy light-curves of 4FGL J1544.3-0649 spanning the
entire operational period of the Fermi mission. The upper panel, reporting the
$\gamma$-ray data, illustrate the transient nature of this source, which was
not detected by the LAT instrument until late 2015, peaked in 2017-2018 and
then faded. The lower panel reconstructs the long-term light-curve of 4FGL
J1544.3-0649 in the X-ray band, combining the Swift-XRT measurements (blue
points), with the XMM data from Ursini et al. (2019) (red points) and the
expected X-ray flux level estimated from the flux in the $\gamma$-ray band,
based on the average x-ray/$\gamma$-ray flux ratio (open grey circles). An
upper limit derived from the non-detection of the source in the RASS X-ray sky
survey, which was carried out in 1990-1991 (Voges et al., 1999), is also shown
in the leftmost part of the plot. The reconstructed X-ray light-curve implies
a rise time of approximately two years, and a decay of approximately three
years.

The SED of 4FGL J1544.3-0649 (Fig. 5) assembled with Swift-XRT, Swift-UVOT,
Fermi-LAT (green points) and archival data (grey points), shows a highly
variable X-ray spectrum, changing its intensity by large factors and often
peaking in the X-ray band. For comparison the average SED of the well known
HBL blazar Mrk501, rescaled to match the radio emission of 4FGL J1544.3-0649,
together with its X-ray spectra observed during its minimum and maximum
intensity state, is plotted as light orange points in the same figure. The
nearly identical shape of the radio to optical SEDs and the range of observed
X-ray fluxes, confirms that 4FGL J1544.3-0649 during the active phase is a
typical HBL blazar, with X-ray and $\gamma$-ray emission that are even
stronger than that of Mrk 501. The upper limit from the RASS all sky survey
(red arrow) sets the magnitude of the intensity variations in the soft X-ray
band, which ranged from below $\approx 10^{-13}\>{\rm erg\>cm^{-2}\>s^{-1}}$
in 1990 to such a high value to place 4FGL J1544.3-0649 amongst the 20
brightest X-ray blazars.

The existence of transient (HBL) blazars was never considered in the
literature before the discovery of 4FGL J1544.3-0649 and, consequently, never
taken into account in population studies. However, if this dual behaviour is
relatively common, it might have implications in blazar research, especially
in high-energy and multi-messenger astrophysics, depending on how numerous
transient blazars are and how frequently they enter a bright phase. We don’t
know if 4FGL J1544.3-0649 represents the tip of the iceberg, or it is simply
an isolated event. The discovery of 4FGL J1544.3-0649 is a clear selection
effect. Similar transient sources would probably go unnoticed in case their
top flux was only a factor of a few lower than that of 4FGL J1544.3-0649, or
with a softer $\gamma$-ray spectrum, or even in cases where the $\gamma$-ray
intensity remains low during strong X-ray flares associated to neutrino
emission as in scenarios similar to that presented by Mastichiadis &
Petropoulou (2020). The identification of similar objects would add crucial
information to our knowledge of the population of extragalactic sources,
especially in the high-energy domain. Areas of research that might be affected
would be e.g. the determination of the LogN-LogS, luminosity function and
cosmological evolution of HBL blazars, the identification of still
unassociated $\gamma$-ray and VHE sources, and the contribution of discrete
sources to the $\gamma$-ray and possibly high-energy neutrino cosmic
backgrounds. A transient nature of some electromagnetic counterparts of
astrophysical neutrinos would on one side increase the level of difficulty in
the process of the association, but on the other it would strengthen the
association in case X-ray or $\gamma$-ray flares are detected in association
with the neutrino arrival. A recent study (Giommi et al., 2020b) reported a
3.23 $\sigma$ post trial excess of HBL Blazars in the error regions of a
sample of 70 Ice Cube astrophysical neutrinos with good positional accuracy.
If HBL blazars are indeed counterparts of high-energy neutrinos, some of them
could be transient sources somewhat fainter than 4FGL J1544.3-0649 that would
difficult to locate since they are not listed in catalogues and no full
coverage of the still large Ice Cube neutrino positional uncertainty can be
achieved with current X-ray imaging instruments. A full census of these so far
elusive objects would be possible only when sensitive all-sky X-ray (or
$\gamma$-ray) monitor detectors become available. Meanwhile a careful search
for weak transients in the Fermi-LAT data on the positions of flat-spectrum
radio sources with radio to infrared flux ratio similar to that of HBL objects
could lead to the discovery of additional transient blazars and set limits to
their duty cycle and space density.

## Acknowledgements

We acknowledge the use of data, analysis tools and services from the Open
Universe platform, the ASI Space Science Data Center (SSDC), the Astrophysics
Science Archive Research Center (HEASARC), the Fermi Science Tools, the All-
Sky Automated Survey for Supernovae (ASAS-SN), the Astrophysics Data System
(ADS), and the National Extra-galactic Database (NED).
PG acknowledges the support of the Technische Universität München - Institute
for Advanced Studies, funded by the German Excellence Initiative (and the
European Union Seventh Framework Programme under grant agreement no. 291763)
and support by the DFG Cluster of Excellence "Origin and Structure of the
Universe".
NS acknowledges the support by the Science Committee of RA, in the frames of
the research project No 20TTCG-1C015.
This work used resources from the EGI infrastructure with the dedicated
support of CESGA (Spain).
We are grateful to M. Petropoulou, P. Padovani and R. Middei for useful
comments.

## Data availability

All the Swift-XRT spectra, best fit values and other imaging analysis results
will be made available as part of a publication presenting the analysis of XRT
data of all the blazars frequently observed by Swift (Giommi et. al. 2020, in
preparation), in the framework of the Open Universe for blazars program. The
remaining data underlying this article will be shared on reasonable request to
the corresponding author.

## References

  * Abdo et al. (2010) Abdo A. A., et al., 2010, ApJ, 716, 30
  * Abdollahi et al. (2020) Abdollahi S., et al., 2020, ApJS, 247, 33
  * Ackermann et al. (2017) Ackermann M., et al., 2017, ApJ, 837, L5
  * Aharonian (2000) Aharonian F. A., 2000, New Astron., 5, 377
  * Ajello et al. (2020a) Ajello M., et al., 2020a, ApJ, 892, 105
  * Ajello et al. (2020b) Ajello M., et al., 2020b, ApJ, 892, 105
  * Ansoldi et al. (2018) Ansoldi S., et al., 2018, ApJ, 863, L10
  * Ballet et al. (2020) Ballet J., Burnett T. H., Digel S. W., Lott B., 2020, arXiv e-prints, p. arXiv:2005.11208
  * Bednarek & Wagner (2008) Bednarek W., Wagner R. M., 2008, A&A, 486, 679
  * Biteau et al. (2020) Biteau J., et al., 2020, Nature Astronomy, 4, 124
  * Blandford & Rees (1978) Blandford R., Rees M., 1978, in Wolfe A. M., ed., Pittsburgh Conference on BL Lac Objects. University of Pittsburgh press, Pittsburgh, pp 341–347
  * Bloom & Marscher (1996) Bloom S. D., Marscher A. P., 1996, ApJ, 461, 657
  * Bonnoli et al. (2015) Bonnoli G., Tavecchio F., Ghisellini G., Sbarrato T., 2015, MNRAS, 451, 611
  * Böttcher et al. (2013) Böttcher M., Reimer A., Sweeney K., Prakash A., 2013, ApJ, 768, 54
  * Bruni et al. (2018) Bruni G., et al., 2018, ApJ, 854, L23
  * Cerruti et al. (2019) Cerruti M., Zech A., Boisson C., Emery G., Inoue S., Lenain J.-P., 2019, MNRAS, 483, L12
  * Chang et al. (2019) Chang Y. L., Arsioli B., Giommi P., Padovani P., Brandt C. H., 2019, A&A, 632, A77
  * Chang et al. (2020) Chang Y. L., Brandt C. H., Giommi P., 2020, Astronomy and Computing, 30, 100350
  * Chornock & Margutti (2017) Chornock R., Margutti R., 2017, The Astronomer’s Telegram, 10491, 1
  * Ciprini et al. (2017) Ciprini S., Cheung C. C., Kocevski D., Chiang J., Fermi Large Area Telescope Collaboration Shore S. N., 2017, The Astronomer’s Telegram, 10482, 1
  * Condon et al. (1998) Condon J. J., Cotton W. D., Greisen E. W., Yin Q. F., Perley R. A., Taylor G. B., Broderick J. J., 1998, AJ, 115, 1693
  * Costamante et al. (2001) Costamante L., et al., 2001, A&A, 371, 512
  * Dermer & Schlickeiser (1994) Dermer C. D., Schlickeiser R., 1994, ApJS, 90, 945
  * Gao et al. (2019) Gao S., Fedynitch A., Winter W., Pohl M., 2019, Nature Astronomy, 3, 88
  * Gehrels et al. (2004) Gehrels N., et al., 2004, ApJ, 611, 1005
  * Ghisellini et al. (1985) Ghisellini G., Maraschi L., Treves A., 1985, A&A, 146, 204
  * Giommi et al. (1990) Giommi P., Barr P., Garilli B., Maccagni D., Pollock A. M. T., 1990, ApJ, 356, 432
  * Giommi et al. (1999) Giommi P., Menna M. T., Padovani P., 1999, MNRAS, 310, 465
  * Giommi et al. (2005) Giommi P., Piranomonte S., Perri M., Padovani P., 2005, A&A, 434, 385
  * Giommi et al. (2020a) Giommi P., et al., 2020a, in S. F., ed., Space Capacity Building in the XXI Century. Studies in Space Policy. Springer, pp 377–386 (arXiv:1805.08505)
  * Giommi et al. (2020b) Giommi P., Glauch T., Padovani P., Resconi E., Turcati A., Chang Y. L., 2020b, MNRAS, 497, 865
  * Gordon et al. (2020) Gordon Y. A., et al., 2020, Research Notes of the American Astronomical Society, 4, 175
  * IceCube Collaboration et al. (2018a) IceCube Collaboration et al., 2018a, Science, 361, 147
  * IceCube Collaboration et al. (2018b) IceCube Collaboration et al., 2018b, Science, 361, eaat1378
  * Intema et al. (2017) Intema H. T., Jagannathan P., Mooley K. P., Frail D. A., 2017, A&A, 598, A78
  * Kawase et al. (2017) Kawase T., et al., 2017, The Astronomer’s Telegram, 10495, 1
  * Keivani et al. (2018) Keivani A., et al., 2018, ApJ, 864, 84
  * Kirk et al. (1998) Kirk J. G., Rieger F. M., Mastichiadis A., 1998, A&A, 333, 452
  * Kochanek et al. (2017) Kochanek C. S., et al., 2017, PASP, 129, 104502
  * Lott et al. (2012) Lott B., Escande L., Larsson S., Ballet J., 2012, A&A, 544, A6
  * Lott et al. (2020) Lott B., Gasparrini D., Ciprini S., 2020, arXiv e-prints, p. arXiv:2010.08406
  * Mannheim (1993) Mannheim K., 1993, A&A, 269, 67
  * Mannheim & Biermann (1989) Mannheim K., Biermann P. L., 1989, A&A, 221, 211
  * Maraschi et al. (1992) Maraschi L., Ghisellini G., Celotti A., 1992, ApJ, 397, L5
  * Massaro et al. (2006) Massaro E., Tramacere A., Perri M., Giommi P., Tosti G., 2006, A&A, 448, 861
  * Massaro et al. (2012) Massaro F., D’Abrusco R., Tosti G., Ajello M., Paggi A., Gasparrini D., 2012, ApJ, 752, 61
  * Massaro et al. (2017) Massaro F., Marchesini E. J., D’Abrusco R., Masetti N., Andruchow I., Smith H. A., 2017, ApJ, 834, 113
  * Mastichiadis & Petropoulou (2020) Mastichiadis A., Petropoulou M., 2020, arXiv e-prints, p. arXiv:2009.12158
  * Mattox et al. (1996) Mattox J. R., et al., 1996, ApJ, 461, 396
  * Mücke & Protheroe (2001) Mücke A., Protheroe R. J., 2001, Astroparticle Physics, 15, 121
  * Mücke et al. (2003) Mücke A., Protheroe R. J., Engel R., Rachen J. P., Stanev T., 2003, Astroparticle Physics, 18, 593
  * Murase et al. (2018) Murase K., Oikonomou F., Petropoulou M., 2018, ApJ, 865, 124
  * Padovani & Giommi (1995) Padovani P., Giommi P., 1995, ApJ, 444, 567
  * Padovani et al. (2016) Padovani P., Resconi E., Giommi P., Arsioli B., Chang Y. L., 2016, MNRAS, 457, 3582
  * Padovani et al. (2017) Padovani P., et al., 2017, A&ARv, 25, 2
  * Padovani et al. (2018) Padovani P., Giommi P., Resconi E., Glauch T., Arsioli B., Sahakyan N., Huber M., 2018, MNRAS, 480, 192
  * Pian et al. (1998) Pian E., et al., 1998, ApJ, 492, L17
  * Poole et al. (2008) Poole T. S., et al., 2008, MNRAS, 383, 627
  * Resconi et al. (2017) Resconi E., Coenders S., Padovani P., Giommi P., Caccianiga L., 2017, MNRAS, 468, 597
  * Righi et al. (2019) Righi C., Tavecchio F., Pacciani L., 2019, MNRAS, 484, 2067
  * Sahakyan (2018) Sahakyan N., 2018, ApJ, 866, 109
  * Sahakyan (2020) Sahakyan N., 2020, MNRAS, 496, 5518
  * Sahakyan et al. (2020) Sahakyan N., Israyelyan D., Harutyunyan G., Khachatryan M., Gasparyan S., 2020, MNRAS, 498, 2594
  * Schlafly & Finkbeiner (2011) Schlafly E. F., Finkbeiner D. P., 2011, ApJ, 737, 103
  * Sikora et al. (1994) Sikora M., Begelman M. C., Rees M. J., 1994, ApJ, 421, 153
  * Sokolov et al. (2004) Sokolov A., Marscher A. P., McHardy I. M., 2004, ApJ, 613, 725
  * Tavecchio et al. (2011) Tavecchio F., Ghisellini G., Bonnoli G., Foschini L., 2011, MNRAS, 414, 3566
  * Tramacere et al. (2009) Tramacere A., Giommi P., Perri M., Verrecchia F., Tosti G., 2009, A&A, 501, 879
  * Tramacere et al. (2011) Tramacere A., Massaro E., Taylor A. M., 2011, ApJ, 739, 66
  * Urry & Padovani (1995) Urry C. M., Padovani P., 1995, PASP, 107, 803
  * Ursini et al. (2019) Ursini F., et al., 2019, A&A, 622, A116
  * Voges et al. (1999) Voges W., et al., 1999, A&A, 349, 389