Strong X-ray emission from large scale jets of radio loud quasars still remains an open problem. Models based on inverse Compton scattering off CMB photons by relativistically beamed jets have recently been ruled out, since Fermi LAT observations for 3C 273 and PKS 0637-752 give the upper limit far below the model prediction. Synchrotron emission from a separate electron population with multi-hundred TeV energies remains a possibility although its origin is not well known. We examine a photo-hadronic origin of such high energy electrons/positrons, assuming that protons are accelerated up to $10^{19}$ eV and produce electrons/positrons through Bethe-Heitler process and photo-pion production. These secondary electrons/positrons are injected at sufficiently high energies and produce X-rays and $γ$-rays by synchrotron radiation without conflicting with the Fermi LAT upper limits. We find that the resultant spectrum well reproduces the X-ray observations from PKS 0637-752, if the proton power is at least $10^{49} \text{erg} \, \text{s}^{-1}$, which is highly super-Eddington. It is noted that the X-ray emission originates primarily from leptons through Bethe-Heitler process, while leptons from photo-pion origin lose energy directly through synchrotron emission of multi-TeV photons rather than cascading. To avoid the overproduction of the optical flux, optical emission is primarily due to synchrotron emission of secondary leptons rather than primary electrons, or a mild degree of beaming of the jet is needed, if it is owing to the primary electrons. Proton synchrotron luminosity is a few orders of magnitude smaller.
The high-peaked BL Lac object Pks2155-304 shows high variability at multiwavelengths, i.e. from optical up to TeV energies. A giant flare of around 1 hour at X-ray and TeV energies was observed in 2006. In this context, it is essential to understand the physical processes in terms of the primary spectrum and the radiation emitted, since high-energy emission can arise in both leptonic and hadronic processes. In this contribution, we investigate the possibility of neutrino production in photo-hadronic interactions. In particular, we predict a direct correlation between optical and TeV energies a…
The radiation mechanism of Radio-Loud Narrow-Line Seyfert 1 (RL-NLS1) from X-ray to $γ$-ray bands remains an open question. While the leptonic model has been employed to explain the spectral energy distribution (SED), the hadronic process may potentially account for the high energy radiation of some $γ$-ray loud Narrow-Line Seyfert 1 (NLS1) as well. We study one of such RL-NLS1, PKS 1502+036, comparing the theoretical SEDs predicted by the leptonic model and the lepto-hadronic model to the observed one. For the hadronic processes, we take into account the proton synchrotron radiation and proto…
The models developed to describe the spectral energy distribution (SED) of blazars can be divided into leptonic or hadronic scenarios, according to the particles responsible for the high-energy component. We have developed a new stationary code which computes all the relevant leptonic and hadronic processes, permitting the study of both leptonic and hadronic scenarios in a consistent way. Interestingly, mixed lepto-hadronic scenarios (in which both components contribute to the high energy emission) naturally arise in this framework. We present the first application to the well known BL Lac obj…
We investigate the formation of TeV spectrum of a distant blazar PKS 1424+240 residing at a redshift $z\geq0.6$ in two scenarios in the frame of a lepto-hadronic jet model taking both the uncertainties of the extragalactic background light (EBL) and its redshift into account. In the first scenario, TeV emission is attributed to the synchrotron emission of pair cascades resulting from $pγ$ interaction; in the second scenario, TeV emission is attributed to the proton-synchrotron emission, and an internal absorption due to interaction with the photons around the jet is included. The results show …
X-ray bright knots of kpc-scale jets of several radio loud quasars have been an actively discussed issue. Among various models to explain observations, synchrotron radiation from the electron population different from radio to IR emitting electrons is promising. However, the origin of this electron population has been debated. Recently, we proposed that this electron population is produced by proton-photon collisions (mainly, Bethe-Heitler process), and we applied this model to PKS 0637-752. We found that this model works if the proton power is by an order of magnitude larger than the Eddingto…
The current generation of Cherenkov telescopes, together with Fermi-LAT, have greatly increased our knowledge of $γ$-ray blazars. Among them, the high-frequency-peaked BL Lacertae object (HBL) PKS1424+240 (z $\simeq$ 0.6) is the farthest persistent emitter of very-high-energy (VHE; E $\geq$ 100 GeV) $γ$-ray photons. Current emission models can satisfactorily reproduce typical blazar emission assuming that the dominant emission process is synchrotron-self-Compton (SSC) in HBLs; and external-inverse-Compton (EIC) in LBLs and FSRQs. Hadronic models are also able to correctly reproduce the $γ$-ray…
In July of 2019, the IceCube experiment detected a high-energy neutrino from the direction of the powerful blazar PKS 1502+106. We perform multi-wavelength and multi-messenger modeling of this source, using a fully self-consistent one-zone model that includes the contribution of external radiation fields typical of flat-spectrum radio quasars (FSRQs). We identify three different activity states of the blazar: one quiescent state and two flaring states with hard and soft gamma-ray spectra. We find two hadronic models that can describe the multi-wavelength emission during all three states: a lep…
The broadband spectral energy distribution of candidate neutrino blazars
CHRIST (Deemed to be University)（克里斯特大学）；North-West University（西北大学）；Indian Institute of Astrophysics（印度天体物理研究所）；Bhabha Atomic Research Centre（巴哈原子研究中心）；Homi Bhabha National Institute（霍米·巴巴国立研究所）
We report the Fermi Large Area Telescope (LAT) detection of two very-high-energy (VHE, E>100 GeV) gamma-ray photons from the directional vicinity of the distant (redshift, z = 1.1) blazar PKS 0426-380. The null hypothesis that both the 134 and 122 GeV photons originate from unrelated sources can be rejected at the 5.5 sigma confidence level. We therefore claim that at least one of the two VHE photons is securely associated with the blazar, making PKS 0426-380 the most distant VHE emitter known to date. The results are in agreement with the most recent Fermi-LAT constraints on the Extragalactic…
PKS 1424+240 is a distant very high energy gamma-ray BL Lac object with redshift $z=0.601$. It was found that pure synchrotron self-Compton (SSC) process normally need extreme input parameters (e.g., very low magnetic field intensity and extraordinarily large Doppler factor) to explain its multi-wavelength spectral energy distributions (SEDs). To avoid the extreme model parameters, different models have been proposed (e.g., two-zone SSC model or lepto-hadronic model). In this work, we employ the traditional one-zone leptonic model after including a weak external Compton component to re-explore…
I present a study of the high-energy gamma-ray properties of the flat spectrum radio quasar, PKS 1510-089, based on 3.75 years of observations with the Large Area Telescope (LAT) detector on-board the Fermi gamma-ray Space Telescope. Throughout the observing period, the 0.1 GeV < E < 300 GeV gamma-ray flux was highly variable, undergoing several flaring events where the daily flux exceeded 10^{-5} photons cm^{-2} s^{-1} on 3 separate occasions. The increased photon statistics of these large flares allowed the observations to be re-analysed in 6 and 3 hour intervals, revealing flux doubling tim…
The beamed inverse Compton/cosmic microwave background model has generally been used for the interpretation of X-ray radiation from kiloparsec-scale jets of the core-dominated quasars. Recent \textit{Fermi}-LAT and \textit{HST} observations have brought this model into question. We examine the assumption that X-rays from the kiloparsec-scale jet of the quasar PKS~1127$-$145 are produced by inverse Compton scattering of the central source emission. In this context, we show that both similarity and distinction between the observed radio and X-ray spectral indices for some of the jet knots can be…
The blazar PKS 1510-089 is an example of flat spectrum radio quasars. High-energy emissions from this class of objects are believed to have been produced by inverse Compton radiation with seed photons originating from the broad line region. In such a paradigm, a lack of very high-energy emissions is expected because of the Klein-Nishina effect and strong absorption in the broad line region. Recent detection of at least three such blazars by Cherenkov telescopes has forced a revision of our understanding of these objects. We have aimed to model the observed spectral energy distribution of PKS 1…
Hadronic origin of gamma rays and neutrinos from blazars: Multi-messenger implications and observational constraints
Universidade Estadual de Londrina (UEL)（隆德里纳州立大学）；Universidade Federal de São Carlos (UFSCar)（圣卡洛斯联邦大学）；Universidade Tecnológica Federal do Paraná (UTFPR)（巴拉那联邦技术大学）；Universidade Federal da Integração Latino-Americana（拉丁美洲一体化联邦大学）；Universidade Federal do Espírito Santo（圣埃斯皮里图联邦大学）
We report the $γ$-ray detection of a young radio galaxy, PKS 1718$-$649, belonging to the class of Compact Symmetric Objects (CSOs), with the Large Area Telescope (LAT) on board the {\it Fermi} satellite. The third {\it Fermi} Gamma-ray LAT catalog (3FGL) includes an unassociated $γ$-ray source, 3FGL J1728.0$-$6446, located close to PKS 1718$-$649. Using the latest Pass 8 calibration, we confirm that the best fit $1 σ$ position of the $γ$-ray source is compatible with the radio location of PKS 1718$-$649. Cross-matching of the $γ$-ray source position with the positions of blazar sources from s…
The origin of the multiwavelength emission from PKS 0502+049 neighboring the first cosmic neutrino source TXS 0506+056 is studied using the data observed by Fermi-LAT and Swift UVOT/XRT. This source was in a flaring state in the considered bands before and after the neutrino observations in 2014-2015, characterized by hard emission spectra in the X-ray and $γ$-ray bands, $1.5-1.8$ and $\leq2.0$, respectively. During the neutrino observations, the $γ$-ray spectrum shows a deviation from a simple power-law shape, indicating a spectral cutoff at $E_c =8.50\pm2.06$ GeV. The spectral energy distrib…
PKS 0558-504 is a highly variable, X-ray bright, radio-loud, Narrow-Line Seyfert 1 galaxy with super-Eddington accretion rate and extended jets that do not dominate the emission beyond the radio band. Therefore, this source represents an ideal laboratory to shed some light on the central engine in highly accreting systems and specifically on the link between accretion disc and corona. Here we present the results from a 1.5 year monitoring with Swift XRT and UVOT. The simultaneous coverage at several wavelengths confirms that PKS 0558-504 is highly variable in any band from optical, to UV and X…
We examine the gamma-ray quasi-periodic variability of PKS 2155-304 with the latest publicly available \emph{Fermi}-LAT Pass 8 data which covers the years from 2008 August to 2016 October. We produce the light curves in two ways, i.e., the exposure-weighted aperture photometry and the maximum likelihood optimization. The light curves are then analyzed by using Lomb-Scargle Periodogram (LSP) and Weighted Wavelet Z-transform (WWZ); and the results reveal a significant quasi-periodicity with a period of $1.74\pm0.13$ years and a significance of $\sim$ 4.9 $σ$. The constraint of multifrequencies q…
We carried a detailed spectral and temporal study of blazar, \mbox{PKS\,0903-57} using the \emph{Fermi}-LAT and \emph{Swift}-XRT/UVOT observations, during its brightest flaring period MJD\,58931--58970. During this period, the maximum daily averaged $γ$-ray flux ($\rm F_{0.1-500\,GeV}$) of $\rm9.42\times10^{-6}\,ph\,cm^{-2}\,s^{-1}$ is observed on MJD\,58951.5, the highest $γ$-ray flux detected from \mbox{PKS\,0903-57} till now. Several high-energy (HE) photons ($>\,10$\,GeV) consistent with the source location at high probability (>\,99\%) are detected, and the $γ$-ray lightcurve in the activ…
PKS 1510-089 is one of the most variable blazars. Very high energy gamma ray emission from this source was observed by H.E.S.S. during March-April 2009 and by MAGIC from February 3 to April 3, 2012 quasi-simultaneously with multi-wavelength flares. The spectral energy distributions of these flares have been modeled earlier with the external Compton mechanism which depends on our knowledge of the densities of the seed photons in the broad line region, the dusty infrared torus or a hypothetical slow sheath surrounding the jet around the radio core. Here we show that to explain the multi-waveleng…
We present the $γ$-ray observations of the flat-spectrum radio quasar PKS 1441+25 (z=0.939), using the {\it Fermi} large Area Telescope data accumulated during January - December 2015. A $γ$-ray flare was observed in January 24, when the flux increased up to $(2.22\pm0.38)\times10^{-6}\;{\rm photon\:cm^{-2}\:s^{-1}}$ with the flux-doubling time scale being as short as $\sim1.44$ days. The spectral analysis shows that from April 13 to April 28, 2015 the MeV-to-GeV photon index has hardened and changes in the range of $Γ=(1.73-1.79)$ for most of the time. The hardest photon index of $Γ=1.54\pm0.…
Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; nevertheless, this component is elusive due to a high value of the energy threshold of proton interaction with photon fields inside the source. However, broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons "visible" in $γ$ rays via hadronuclear interactions. In the present paper we study the persistent $γ$-ray emission of the FSRQ PKS…
The axion-like particle (ALP)-photon mixing in the magnetic field around $γ$-ray sources or along the line-of-sight could induce oscillation between photons and ALPs, which then causes irregularities in the $γ$-ray spectra. In this work we try to search for such spectral irregularities in the spectrum of PKS $2155-304$ using 8.6 years of the Fermi Large Area Telescope (Fermi-LAT) data. No significant evidence for the presence of ALP-photon oscillation is obtained, and the parameter space of ALPs is constrained. The exclusion region sensitively depends on the poorly known magnetic field of host…
Gamma-ray emitting blazars have been the targets of many recent or ongoing monitoring programmes. The resulting multi-wavelength data archives are ideal for studying the quiescent and variable behaviour of the blazar PKS 2155-304. Here, observations of the optical polarization of the source between 2009 and 2014 (> 5 years) are presented, together with overlapping radio, photometric and gamma-ray measurements. During this time the source displayed significant variability at all wavelengths, with several prominent flares seen for the optical and gamma-ray intensity. These flares are typically a…
20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. I. The case of PKS 2155-304
Institute of Nuclear Physics, Polish Academy of Sciences（波兰科学院核物理研究所）；Landessternwarte, Universität Heidelberg（海德堡大学州立天文台）；Centre for Space Research, North-West University（西北大学空间研究中心）
The synchrotron peak of the X-ray bright High Energy Peaked Blazar (HBL) PKS 2155$-$304 occurs in the UV-EUV region and hence its X-ray emission (0.6--10 keV) lies mostly in the falling part of the synchrotron hump. We aim to study the X-ray emission of PKS 2155$-$304 during different intensity states in 2009$-$2014 using XMM$-$Newton satellite. We studied the spectral curvature of all of the observations to provide crucial information on the energy distribution of the non-thermal particles. Most of the observations show curvature or deviation from a single power-law and can be well modeled by…
We report a nominally high-confidence $γ$-ray quasi-periodic modulation in the blazar PKS 0301$-$243. For this target, we analyze its \emph{Fermi}-LAT Pass 8 data covering from 2008 August to 2017 May. Two techniques, i.e., the maximum likelihood optimization and the exposure-weighted aperture photometry, are used to build the $γ$-ray light curves. Then both the Lomb-Scargle Periodogram and the Weighted Wavelet Z-transform are applied to the light curves to search for period signals. A quasi-periodicity with a period of $2.1\pm0.3$ yr appears at the significance level of $\sim5σ$, although it …
An analysis is presented of the optical polarimetric and multicolour photometric ($BVRJ$) behaviour of the blazar PKS 2155$-$304 during an outburst in 2010. This flare develops over roughly 117 days, with a flux doubling time $τ\sim 11$ days that increases from blue to red wavelengths. The polarization angle is initially aligned with the jet axis but rotates by roughly $90^\circ$ as the flare grows. Two distinct states are evident at low and high fluxes. Below 18 mJy, the polarization angle takes on a wide range of values, without any clear relation to the flux. In contrast, there is a positiv…
The blazar PKS 0735+178 is possibly associated with multiple neutrino events observed by the IceCube, Baikal, Baksan, and KM3NeT neutrino telescopes while it was flaring in the $γ$-ray, X-ray, ultraviolet and optical bands. We present a detailed study of this peculiar blazar to investigate the temporal and spectral changes in the multi-wavelength emission when the neutrino events were observed. The analysis of Swift-XRT snapshots reveal a flux variability of more than a factor 2 in about $5\times10^3$ seconds during the observation on December 17, 2021. In the $γ$-ray band, the source was in i…
Blazars exhibit flares across the electromagnetic spectrum. Many $γ$-ray flares are highly correlated with flares detected at optical wavelengths; however, a small subset appears to occur in isolation, with little or no variability detected at longer wavelengths. These "orphan" $γ$-ray flares challenge current models of blazar variability, most of which are unable to reproduce this type of behavior. We present numerical calculations of the time-variable emission of a blazar based on a proposal by Marscher et al. (2010) to explain such events. In this model, a plasmoid ("blob") propagates relat…
A study of blazar PKS 1424-418 was carried out using multi waveband data collected by Fermi-LAT, Swift-XRT, Swift-UVOT and SMARTS telescopes between MJD 56000 to MJD 56600 (14 Mar 2012 to 4 Nov 2013). Two flaring episodes were identified by analysing the gamma ray light curve. Simultaneous multi waveband Spectral Energy Distributions (SED) were obtained for those two flaring periods. A cross-correlation analysis of IR-Optical and $γ$-ray data suggested the origin of these emissions from the same region. We have set a lower limit for the Doppler factor using the highest energy photon observed f…
Rapid variability on a time scale much faster than the light-crossing time of the central supermassive black hole has been seen in TeV emission from the blazar PKS 2155-304. The most plausible explanation of this puzzling observation is that the radiating fluid in the relativistic jet is divided into a large number of sub-regions which move in random directions with relativistic speeds. We consider two versions of this "jets in a jet" model. In the first, the "subjets" model, stationary regions in the mean jet frame emit relativistic subjets that produce the observed radiation. The variability…
We search for $γ$-ray and optical periodic modulations in a distant flat spectrum radio quasar (FSRQ) PKS 0426-380 (the redshift $z=1.1$). Using two techniques (i.e., the maximum likelihood optimization and the exposure-weighted aperture photometry), we obtain $γ$-ray light curves from \emph{Fermi}-LAT Pass 8 data covering from 2008 August to 2016 December. We then analyze the light curves with the Lomb-Scargle Periodogram (LSP) and the Weighted Wavelet Z-transform (WWZ). A $γ$-ray quasi-periodicity with a period of 3.35 $\pm$ 0.68 years is found at the significance-level of $\simeq3.6\ σ$. Th…
We investigate the axion like particle (ALP)-photon oscillation effect in the high energy $γ$-ray spectra of PG 1553+113 and PKS 2155-304 measured by Fermi-LAT and H.E.S.S.. The choice of extragalactic background light (EBL) model, which induces the attenuate effect in observed $γ$-ray spectra, would affect the ALP implication. For the ordinary EBL model that prefers a null hypothesis, we set constraint on the ALP-photon coupling constant at 95% C.L. as $g_{aγ}\lesssim 5\times 10^{-11} ~\rm{GeV}^{-1}$ for the ALP mass $\sim 10$ neV. We also consider the CIBER observation of the cosmic infrared…
Flat spectrum radio quasars (FSRQs) can suffer strong absorption above E = 25/(1+z) GeV, due to gamma-gamma interaction if the emitting region is at sub-parsec scale from the super-massive black hole (SMBH). Gamma-ray flares from these astrophysical sources can investigate the location of the high-energy emission region and the physics of the radiating processes. We present a remarkable gamma-ray flaring activity from FSRQ PKS 2023-07 during April 2016, as detected by both AGILE and Fermi satellites. An intensive multi-wavelength campaign, triggered by Swift, covered the entire duration of the…
We propose a scenario for self-consistent interpretation of GeV - TeV spectrum of a distant blazar PKS 1424+240. In this scenario, ultra-high energy (UHE) protons are assumed to exist in the blazar jet and produce gamma rays through synchrotron emission emitted by relativistic protons and pair cascades (resulted from $pγ$ interaction); meanwhile, some of these UHE protons may escape from the jet and are injected into intergalactic space. Therefore, we assume that UHE cosmic rays (CR) originate from relativistic protons in the jet and use energy-independent escape timescale to obtain UHECR inje…
We present the results from the Suzaku X-ray observations of five flat-spectrum radio quasars (FSRQs), namely PKS0208-512, Q0827+243, PKS1127-145, PKS1510-089 and 3C 454.3. All these sources were additionally monitored simultaneously or quasi-simultaneously by the Fermi satellite in gamma-rays and the Swift UVOT in the UV and optical bands, respectively. We constructed their broad-band spectra covering the frequency range from 10^14 Hz up to 10^25 Hz, and those reveal the nature of high-energy emission of luminous blazars in their low-activity states. The analyzed X-ray spectra are well fitted…
We present a model for the broadband radio-to-$γ$-ray spectral energy distribution of the compact radio source, PKS 1718-649. Because of its young age (100 years) and proximity ($z=0.014$), PKS 1718-649 offers a unique opportunity to study nuclear conditions and the jet/host galaxy feedback process at the time of an initial radio jet expansion. PKS 1718-649 is one of a handful of young radio jets with $γ$-ray emission confirmed with the Fermi/LAT detector. We show that this $γ$-ray emission can be successfully explained by Inverse Compton scattering of the ultraviolet photons, presumably from …
Our goal is to limit the redshifts of three blazars PG 1553+113, 3C 66A and PKS 1424+240, through the investigation of their Fermi and VHE (very high energy) $γ$-ray observations. We assume that the intrinsic spectra of PG 1553+113, 3C 66A, and PKS 1424+240 have not any cutoff across the Fermi and VHE $γ$-ray energy ranges. The intrinsic spectra of VHE $γ$-rays are obtained through the extrapolation of Fermi spectra. Comparing the measured and intrinsic VHE spectra due to extragalactic background light (EBL) absorption, we give the redshift upper limits of three blazars assuming a specific EBL…
As part of our comprehensive long-term multi-waveband monitoring of 34 blazars, we followed the activity in the jet of the blazar PKS 1510-089 during major outbursts during the first half of 2009. The most revealing event was a two-month long outburst that featured a number of gamma-ray flares. During the outburst, the position angle of optical linear polarization rotated by about 720 degrees, which implies that a single emission feature was responsible for all of the flares during the outburst. At the end of the rotation, a new superluminal knot (~ 22c) passed through the "core" seen on 43 GH…
PKS 1510--089 is a bright and active $γ$-ray source that showed strong and complex $γ$-ray flares in mid-2015 during which the Major Atmospheric Gamma Imaging Cherenkov telescopes detected variable very high energy (VHE; photon energies $>$100 GeV) emission. We present long-term multi-frequency radio, optical, and $γ$-ray light curves of PKS 1510--089 from 2013 to 2018, and results of an analysis of the jet kinematics and linear polarization using 43 GHz Very Long Baseline Array data observed between late 2015 and mid-2017. We find that a strong radio flare trails the $γ$-ray flares in 2015, s…
We report on temporal and spectral study of a flat spectrum radio quasar, PKS B1222+216, in flare state to get insight into acceleration and emission mechanisms inside the jet. This is one of the brightest and highly active blazar in the MeV-GeV regime. Long term multi-waveband light curves of this object showed a flaring activity in 2014 with two distinct flares. Work presented here includes the study of flux-index variation, flare fitting, hardness ratio and spectral modelling of both X-ray and $γ-$ray data. The flux-index correlation we have found in MeV-GeV regime indicates a 'softer when …
We report the first hard X-ray observations with NuSTAR of the BL Lac type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and gamma-ray data from the Fermi Large Area Telescope, obtained in April 2013 when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5 - 60 keV, is best described by a model consisting of a log-parabola component with curvature beta = 0.3(+0.2,-0.1) and a (local) photon index 3.04 +/- 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 +/- 0.4. The hard X-ray tail can be sm…
We present the temporal and spectral study of blazar PKS 0208-512, using recent flaring activity from November 2019 to May 2020, as detected by the Fermi-LAT observatory. The contemporaneous X-ray, optical/UV observations from Swift-XRT/UVOT are also used. During the activity state, 2-days binned $γ$-ray lightcurve shows multiple peaks indicating sub-flares. To understand the possible physical mechanisms behind flux enhancement, we divided the activity state of the source into several flux-states and carried out detailed temporal and spectral studies. Timing analysis of lightcurves suggest tha…
Here we report on the detailed analysis of the gamma-ray light curve of a luminous blazar PKS1510-089 observed in the GeV range with the Large Area Telescope (LAT) onboard the Fermi satellite during the period 2011 September -- December. By investigating the properties of the detected three major flares with the shortest possible time binning allowed by the photon statistics, we find a variety of temporal characteristics and variability patterns. This includes a clearly asymmetric profile (with a faster flux rise and a slower decay) of the flare resolved on sub-daily timescales, a superpositio…
The broad band optical/UV and X-ray spectra of blazars have been often modeled as synchrotron component arising from a broken power-law distribution of electrons. A broken power-law distribution is expected, since the high energy electrons undergo radiative losses effectively. The change in the energy index should then be approximately 1 and corresponds to a spectral index difference of 0.5. However, one of the long outstanding problems has been that the observed index change is significantly different. On the other hand, recent high quality observations of blazars suggest that their local spe…
The Chandra X-ray observatory has discovered dozens of resolved, kiloparsec-scale jets associated with powerful quasars in which the X-ray fluxes are observed to be much higher than the expected level based on the radio-optical synchrotron spectrum. The most popular explanation for the anomalously high and hard X-ray fluxes is that these jets do not decelerate significantly by the kiloparsec scale, but rather remain highly relativistic (Lorentz factors $Γ\approx$10). By adopting a small angle to the line-of-sight, the X-rays can thus be explained by inverse Compton upscattering of CMB photons …
