The blazar zone in quasars is commonly assumed to be located inside the broad-line region at some hundreds of Schwartzschild radii from the central black hole. Now, the simultaneous Fermi/LAT and MAGIC observations of a strong flare in the FSRQ PKS 1222+21 (4C 21.35, z=0.432) on 2010 June 17 challenge this picture. The spectrum can be described by a single power law with photon index 2.72+/-0.34 between 3 GeV and 400 GeV, and this is consistent with emission from a single component in the jet. The absence of a spectral cutoff constrains the gamma-ray emission region to lie outside of the broad-line region, which would otherwise absorb the VHE gamma-rays. On the other hand, the MAGIC measurement of a doubling time of about 10 minutes indicates an extremely compact emission region, in conflict with the "far dissipation" scenario. This could be a hint for the importance of jet sub-structures, such as filaments, reconnection zones or shear layers for the occurrence of blazar flares.
The gamma-ray emission offers a powerful diagnostic tool to probe jets and their surroundings in flat spectrum radio quasars (FSRQ). In particular, sources emitting at high energies (>10 GeV) give us the strongest constraints. This motivates us to start a systematic study of flares with bright emission above 10 GeV, examining archival data of Fermi-LAT gamma-ray telescope. At the same time, we began to trigger Target of Opportunity observations to the Swift observatory at the occurrence of high-energy flares, obtaining a wide coverage of the spectral energy distributions for several FSRQs duri…
We study the locations of emission regions through modelling the quasi-simultaneous multi-frequency spectral energy distributions of 21 {\it Fermi} flat spectrum radio quasars (FSRQs) in the frame of a multi-component one-zone leptonic model. In our calculations, we take the detailed broad line region (BLR) structure into account and discuss the effect of the uncertainty of the BLR structure on constraining the location of the emission regions for each FSQR, meanwhile we also include both the internal and external absorptions. Our results indicate that (1) the contribution of external Compton-…
Among more than fifty blazars detected in very high energy (VHE, E>100GeV) gamma-rays, only three belong to the subclass of Flat Spectrum Radio Quasars (FSRQs): PKS 1510-089, PKS 1222+216 and 3C 279. The detection of FSRQs in the VHE range is challenging, mainly because of their steep soft spectra in the GeV-TeV regime. MAGIC has observed and detected all FSRQs known to be VHE emitters up to now and found that they exhibit very different behavior. The 2010 discovery of PKS 1222+216 (z = 0.432) with the fast variability observed, challenges simple one-zone emission models and more complicated s…
In the third catalog of active galactic nuclei detected by the $Fermi$ Large Area Telescope Clean (3LAC) sample, there are 402 blazars candidates of uncertain type (BCU). The proposed analysis will help to evaluate the potential optical classification flat spectrum radio quasars (FSRQs) versus BL Lacertae (BL Lacs) objects of BCUs, which can help to understand which is the most elusive class of blazar hidden in the Fermi sample. By studying the 3LAC sample, we found some critical values of $γ$-ray photon spectral index ($Γ_{\rm ph}$), variability index (VI) and radio flux (${\rm F_R}$) of the …
By modeling the broadband spectral energy distributions (SEDs) of a typical flat spectrum radio quasar (FSRQ, 3C 279) and two GeV narrow-line Seyfert 1 galaxies (NLS1s, PMN J0948+0022 and 1H 0323+342) in different flux stages with the one-zone leptonic models, we find a universal correlation between their Doppler factors ($δ$) and peak luminosities ($L_{\rm c}$) of external Compton scattering bumps. Compiling a combined sample of FSRQs and GeV NLS1s, it is found that both FSRQs and GeV NLS1s in different stages and in different sources well follow the same $δ$-$L_{\rm c}$ correlation. This ind…
We try to infer the location of the GeV emission region for 21 flat spectrum radio quasars (FSRQs) with quasi-simultaneous spectral energy distributions (SEDs), in which the SEDs of 21 FSRQs are reproduced by the one-zone leptonic model including the synchrotron-self Compton (SSC) and external Compton (EC) processes. We suggest that the X-ray emission could be produced by the SSC process and the GeV emission could come from the EC process. The EC emission could originate from the inverse Compton (IC) scattering of photons from the broad line region (BLR) and accretion disk or dust torus by the…
We discuss the production of ultra-high-energy cosmic ray (UHECR) nuclei and neutrinos from blazars. We compute the nuclear cascade in the jet for both BL Lac objects and flat-spectrum radio quasars (FSRQs), and in the ambient radiation zones for FSRQs as well. By modeling representative spectral energy distributions along the blazar sequence, two distinct regimes are identified, which we call "nuclear survival" -- typically found in low-luminosity BL Lacs, and "nuclear cascade" -- typically found in high-luminosity FSRQs. We quantify how the neutrino and cosmic-ray (CR) emission efficiencies …
The blazar PKS 1510-089 was the first of the flat spectrum radio quasar type, which had been detected simultaneously by a ground based Cherenkov telescope (H.E.S.S.) and the LAT instrument on board the Fermi satellite. Given the strong broad line region emission defining this blazar class, and the resulting high optical depth for VHE ($E>100\,$GeV) $γ$-rays, it was surprising to detect VHE emission from such an object. In May 2015, PKS 1510-089 exhibited high states throughout the electromagnetic spectrum. Target of Opportunity observations with the H.E.S.S. experiment revealed strong and unpr…
Modelling the Time-variable Broadband Emission and Correlation Study of FSRQ S5 1044+71
University of Kashmir（克什米尔大学）；Central University of Kashmir（克什米尔中央大学）；Bhabha Atomic Research Centre（巴哈原子研究中心）；Homi Bhabha National Institute（霍米·巴巴国家研究所）；National Institute of Technology, Srinagar（斯利那加国立技术学院）；Indian Institute of Astrophysics（印度天体物理研究所）
In this paper, we characterize the first $γ$-ray flaring episode of the FSRQ PKS 0346-27 (z=0.991), as revealed by Fermi-LAT monitoring data, and the concurrent multi-wavelength variability observed from radio through X-rays. The quasi-simultaneous multi-wavelength coverage allowed us to construct time-resolved spectral energy distributions (SEDs). PKS 0346-27 entered an elevated $γ$-ray activity state starting from the beginning of 2018. The high-state continued throughout the year, displaying the highest fluxes in May 2018. We find evidence of short-time scale variability down to $\sim$1.5 h…
During a decade of the $Fermi$-Large Area Telescope (LAT) operation, thousands of blazars have been detected in the $γ$-ray band. However, there are still numbers of blazars that have not been detected in the $γ$-ray band. In this work, we focus on investigating why some flat-spectrum radio quasars (FSRQs) are undetected by $Fermi$-LAT. By cross-matching the Candidate Gamma-ray Blazars Survey catalog with the Fourth Catalog of Active Galactic Nuclei Detected by the $Fermi$-LAT, we select 11 $γ$-ray undetected ($γ$-ray quiet) FSRQs as our sample whose quasi-simultaneous multi-wavelength data ar…
[abridged] We present results of modeling the SED and multiwavelength variability of the bright FSRQ PKS1510-089 with our time-dependent multizone Monte Carlo/Fokker-Planck code (Chen et al. 2001). As primary source of seed photons for inverse Compton scattering, we consider radiation from the broad line region (BLR), from the molecular torus, and the local synchrotron radiation (SSC). Different scenarios are assessed by comparing simulated light curves and SEDs with one of the best flares by PKS1510-089, in March 2009. The time-dependence of our code and its correct handling of light travel t…
In this work, we collect quasi-simultaneous infrared, optical, X-ray and $γ$-ray data of 60 $Fermi$-4LAC flat spectrum radio quasars (FSRQs). In the framework of the conventional one-zone leptonic model, we investigate the physical properties of $Fermi$-4LAC FSRQs' jets by modeling their quasi-simultaneous spectral energy distributions (SEDs). Our main results are summarized as follows. (1) There is a linear correlation between synchrotron peak frequency and curvature of the electron energy distribution. As suggested by previous works, the slope of the best linear fitting equation of this corr…
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…
We present a method of constraining the properties of the $γ$-ray emitting region in flat spectrum radio quasars (FSRQs) in the one-zone proton synchrotron model, where the $γ$-rays are produced by synchrotron radiation of relativistic protons. We show that for low enough values of the Doppler factor $δ$, the emission from the electromagnetic (EM) cascade which is initiated by the internal absorption of high-energy photons from photohadronic interactions may exceed the observed $\sim$GeV flux. We use that effect to derive an absolute lower limit of $δ$; first, an analytical one, in the asympto…
Mildly-relativistic shocks that are embedded in colliding magnetohydrodynamic flows are prime sites for relativistic particle acceleration and production of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars and gamma-ray bursts. The principal energization mechanisms at these shocks are diffusive shock acceleration and shock drift acceleration. In recent work, we had self-consistently coupled shock acceleration and radiation transfer simulations in blazar jets in a basic one-zone scenario. These one-zone models revealed that the observed spectr…
Variable gamma-ray emission has been discovered in five Radio-loud Narrow Line Seyfert 1 (NLSy1) galaxies by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope. This has clearly demonstrated that these NLSy1 galaxies do have relativistic jets similar to two other cases of gamma-ray emitting Active Galactic Nuclei (AGN), namely blazars and radio galaxies. We present here our results on the multi-band analysis of two gamma-ray emitting NLSy1 galaxies namely PKS 1502+036 (z = 0.409) and PKS 2004-447 (z = 0.240) using archival data. We generate multi-band long term light cu…
The $γ-$ray emission from flat-spectrum radio quasars (FSRQs) is thought to be dominated by the inverse Compton scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the reprocessed disk emission. We study the connection between the optical continuum, H$γ$ line, and $γ-$ray emissions from the FSRQ PKS~1222+216, using long-term ($\sim$2011-2018) optical spectroscopic data from Steward Observatory and $γ-$ray observations fro…
We present multiwavelength data for twelve blazars observed from 2008-2010 as part of an ongoing optical-infrared photometric monitoring project. Sources were selected to be bright, southern (dec < 20 deg) blazars observed by the Fermi Gamma-Ray Space Telescope, with daily and weekly gamma-ray fluxes made available from the start of the Fermi mission. Light curves are presented for the twelve blazars in BVRJK at near-daily cadence. We find that optical and infrared fluxes are well correlated in all sources. Gamma-ray bright flat spectrum radio quasars (FSRQs) in our sample have optical/infrare…
The expected level of gamma-gamma absorption in the Broad Line Region (BLR) radiation field of gamma-ray loud Flat Spectrum Radio Quasars (FSRQs)is evaluated as a function of the location of the gamma-ray emission region. This is done self-consistently with parameters inferred from the shape of the spectral energy distribution (SED) in a single-zone leptonic EC-BLR model scenario. We take into account all geometrical effects both in the calculation of the gamma-gamma opacity and the normalization of the BLR radiation energy density. As specific examples, we study the FSRQs 3C279 and PKS 1510-0…
We study the quasi-simultaneous Spectral Energy Distributions (SED) of 48 LBAS blazars, collected within three months of the Fermi LAT Bright AGN Sample (LBAS) data taking period, combining Fermi and Swift data with radio NIR-Optical and hard-X/gamma-ray data. We have used these SEDs to characterize the peak position and intensity of both the low and the high-energy features of blazar spectra. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broad-band colors (i.e. the radio to optical, alpha_ro, and optical to X-ray, alpha_ox, s…
Aiming to delineate the physical framework of blazars, we present an effective method to estimate four important parameters based on the idea proposed by \citet{BK95}, including the upper limit of central black hole mass $M$, the Doppler factor $δ$, the distance along the axis to the site of the $γ$-ray production $d$ (which then can be transformed into the location of $γ$-ray-emitting region $R_γ$) and the propagation angle with respect to the axis of the accretion disk $Φ$. To do so, we adopt an identical sample with 809 {\it Fermi}-LAT-detected blazars which had been compiled in \citet{Pei2…
Fast Optical Variability of the TeV Blazar PKS 1725+123 Observed by SVOM-VT and Insights from Multi-wavelength Follow-up Observations
Beijing Institute of Technology（北京理工大学）；Université Paris Cite, CNRS, Astroparticule et Cosmologie（巴黎西岱大学）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University（广西大学物理科学与技术学院）
Quasi-Simultaneous Broadband Spectral Energy Distributions of a Sample of Fermi Blazars -- I. Correlation Results
Zhejiang Normal University（浙江师范大学）；Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）；University of Science and Technology of China（中国科学技术大学）；Shanghai Normal University（上海师范大学）；Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；Center for Astronomical Mega-Science, Chinese Academy of Sciences（中国科学院国家天文台）；College of Physics and Electronic Engineering, Qilu Normal University（齐鲁师范学院物理与电子工程学院）
We present a multi-wavelength study of the radio-loud narrow line Seyfert 1 galaxy (NLSy1), 1H 0323+342, detected by Fermi Gamma Ray Space Telescope. Multi-band light curves show many orphan X-ray and optical flares having no corresponding γ-ray counterparts. Such anomalous variability behavior can be due to different locations of the emission region from the central source. During a large flare, γ-ray flux doubling time scale as small as $\sim$ 3 hours is noticed. We built spectral energy distribution (SED) during different activity states and modeled them using an one-zone leptonic model. Th…
The detection of several radio-loud narrow-line Seyfert 1 (NLS1) galaxies by the Fermi Gamma-Ray Space Telescope hints at the existence of a rare, new class of gamma-ray emitting active galactic nuclei with low black hole masses. Like flat spectrum radio quasars (FSRQs), their gamma-ray emission is thought to be produced via the external Compton mechanism whereby relativistic jet electrons upscatter a photon field external to the jet, e.g. from the accretion disc, broad line region (BLR) and dusty torus, to higher energies. Here we study the origin of the gamma-ray emission in the lowest-redsh…
Jets in blazars are an excellent forum for studying acceleration at relativistic shocks using the highly-variable emission seen across the electromagnetic spectrum. Our recent work on combining multi-wavelength leptonic emission models with simulated thermal+non-thermal distributions from shock acceleration theory has resulted in new insights into plasma conditions in blazars. This has demonstrated the ability to infer the cyclotron frequency, the plasma density and thus also the Alfven speed, thereby determining the rapidity of particle energization. An important inference was that MHD turbul…
The $γ$-ray loud narrow line Seyfert 1 ($γ$-NLSy1) galaxy PKS 1502+036 ($z=0.409$) exhibited its first $γ$-ray outburst on 2015 December 20. In the energy range of 0.1-300 GeV, the highest flux measured by {\it Fermi}-Large Area Telescope is (3.90 $\pm$ 1.52) $\times$ 10$^{-6}$ ph cm$^{-2}$ s$^{-1}$, which is the highest $γ$-ray flux ever detected from this object. The associated spectral shape is soft ($Γ_{0.1-300~{\rm GeV}}=2.57\pm0.17$) and this corresponds to an isotropic $γ$-ray luminosity of (1.2 $\pm$ 0.6) $\times$ 10$^{48}$ erg s$^{-1}$. We generate the broadband spectral energy distri…
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…
Radio-Gamma-Ray Properties and High-Energy Implications for Fermi Blazars
Guangzhou University（广州大学）；Università di Padova（帕多瓦大学）；Astronomy Science and Technology Research Laboratory of Department of Education of Guangdong Province（广东省教育厅天文科学与技术重点实验室）；Greater Bay Brand Center of the National Astronomical Data Center（国家天文数据中心大湾区品牌中心）
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…
Blazars are highly variable sources over timescales that can be as low as minutes. This is the case of the High Energy Peaked BL Lac (HBL) objects showing strong variability in X-rays, which highly correlate with that of the TeV emission. The degree of this correlation is still debated, particularly when the flaring activity is followed down to very short time scales. This correlation could challenge the synchrotron-self-Compton scenario in which one relativistic electron population dominates the entire radiative output. We argue that the LOFT Large Area Detector (10 m$^2$, LAD), thanks to its…
Blazar spectra are one of the most important windows into the physical processes occurring along jets. The spectrum, composed from the different emitting regions along the jet, allows us to constrain the physical conditions in the jet. I present my work modelling blazar spectra using an extended inhomogeneous jet model with an accelerating, magnetically dominated, parabolic base transitioning to a slowly decelerating, conical section motivated by observations, simulations and theory. We set the inner geometry of our multi-zone model using observations of the jet in M87 which transitions from p…
Blazars are very broadband cosmic sources with spectra spanning over twenty orders of magnitude in frequency, down to the 100 MHz regime in the radio range, up to VHE at several tens of TeV. The modelling of their spectral energy distribution at high energies currently considers two main classes of models, leptonic and lepto-hadronic, which both succeed fairly well in describing the observed spectra for the two populations of blazars, namely BL Lac objects (BL Lacs) and flat spectrum radio quasars (FSRQs). However they are both confronted with difficulties, in particular to reproduce flaring p…
The origin of the high-energy component in spectral energy distributions (SED) of blazars is still a bit of a mystery. While BL Lac objects can be rather successfully modelled within the one-zone synchrotron self-Compton (SSC) scenario, the SED of low peaked Flat Spectrum Radio Quasars (FSRQ) is more difficult to reproduce. Their high-energy component needs the abundance of strong external photon sources, giving rise to stronger cooling via the inverse Compton channel, and thusly to a powerful component in the SED. Recently, we were able to show that such a powerful inverse Compton component c…
Blazars are known to show flux variations over a range of energies from low energy radio to high energy gamma-rays. Cross-correlation analysis of the optical and $γ$-ray light curves in blazars shows that flux variations are generally correlated in both bands, however, there are exceptions. We explored this optical-GeV connection in four flat spectrum radio quasars (FSRQs) by a systematic investigation of their long term optical and $γ$-ray light curves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510$-$089 and CTA 102 we noticed different behaviours between the optical and Ge…
The regular monitoring of flat-spectrum radio quasars (FSRQs) in $γ$-rays by Fermi-LAT since past 12 years indicated six sources who exhibited extreme $γ$-ray outbursts crossing daily flux of $10^{-5}$ photons/cm$^{2}$/s. We obtained nearly-simultaneous multi-wavelength data of these sources in radio to $γ$-ray waveband from OVRO, Steward Observatory, SMARTS, Swift-UVOT, Swift-XRT, and Fermi-LAT. The time-averaged broadband Spectral Energy Distributions (SEDs) of these sources in quiescent states were studied to get an idea about the underlying baseline radiation processes. We modeled the SEDs…
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…
In the work, we explore the role of jet properties in the spectral evolution for a sample of Fermi-LAT bright blazars composed primarily by flat spectrum radio quasars (FSRQs). We introduce a near-equipartition log-parabola (NELP) model to fit the quasi-simultaneous multi-waveband spectral energy distributions (SEDs). The Markov Chain Monte Carlo (MCMC) method is employed to determine the best values of spectral parameters and its errors. The correlations of synchrotron peak luminosity $L_{pk}^*$ and its peak frequency $ν_{pk}^*$, SSC-dominant factor $ξ_s$ and $ν_{pk}^*$, and their implication…
The search for detection of gamma-rays from distant AGNs by Imaging Atmospheric Cherenkov Telescopes (IACTs) is challenging at high redshifts, not only because of lower flux due to the distance of the source, but also due to the consequent absorption of gamma-rays by the extragalactic background light (EBL). Before the MAGIC discoveries reported in this work, the farthest source ever detected in the VHE domain was the blazar PKS 1424+240, at z>0.6. MAGIC, a system of two 17 m of diameter IACTs located in the Canary island of La Palma, has been able to go beyond that limit and push the boundari…
Among more than fifty blazars detected in very high energy (VHE, E>100GeV) gamma-rays, only three belong to the subclass of Flat Spectrum Radio Quasars (FSRQs). MAGIC observed FSRQ PKS 1510-089 in February-April 2012 during a high activity state in the high energy (HE, E>100 MeV) gamma-ray band observed by AGILE and Fermi. MAGIC observations result in the detection of a source with significance of 6.0 sigma. In agreement with the previous VHE observations of the source, we find no statistically significant variability during the MAGIC observations in daily, weekly or monthly time scales. The o…
Thirteen yr observation data of 4FGL J0824.9+3915 with the Large Area Telescope on board the Fermi Gamma Ray Space Telescope (Fermi/LAT) are analyzed for revisiting whether 4C +39.23B, a compact steep-spectrum (CSS) source closed to a flat-spectrum radio quasar (FSRQ) 4C +39.23A in the $γ$-ray emitting region of 4FGL J0824.9+3915, is a $γ$-ray emitter. We find that the time-integrated $γ$-ray emission of 4FGL J0824.9+3915 is overwhelmingly dominated by 4C +39.23A. It shows significant variability at a 6.7$σ$ confidence level and the average $γ$-ray flux in the 0.1--300 GeV energy band is $(1.6…
Determining the location of $γ$-ray emission in blazar jets is a challenging task. Pinpointing the exact location of $γ$-ray production within a relativistic jet can place strong constraints on our understanding of high-energy astrophysics and astroparticle physics. We present a study of the radio- and $γ$-bright flat-spectrum radio quasar (FSRQ) 4C +01.28 (PKS B1055+018) in which we try to pinpoint the emission site of several prominent GeV flares. This source shows prominent high-amplitude broadband variability on time scales ranging from days to years. We combine high-resolution VLBI observ…
Locating the gamma-ray emission sites in blazar jets is a long-standing and highly controversial issue. We investigate jointly several constraints on the distance scale r and Lorentz factor Gamma of the gamma-ray emitting regions in luminous blazars (primarily flat spectrum radio quasars, FSRQs). Working in the framework of one-zone external radiation Comptonization (ERC) models, we perform a parameter space study for several representative cases of actual gamma-ray flares in their multiwavelength context. We find a particularly useful combination of three constraints: from an upper limit on t…
