Within the classical emission model, where the emission region is placed within the broad line region (BLR), flat spectrum radio quasars (FSRQs) were believed not to emit photons with energies above few tens of GeV because of the absorption with the optical-UV photons from the BLR. However, photons with observed energies up to about $300 \, \rm GeV$ have been detected for few FSRQs, whose most iconic example is PKS 1441+25 at redshift $z = 0.94$. The most conservative explanation for these observations is that the emission occurs at distances comparable to the size of the dusty torus. In this case, absorption of high-energy gamma-ray photons for energies above $200-300 \, {\rm GeV}$ is dominated by the interaction with infrared radiation emitted by the torus. We investigate if current observational data about FSRQs in flaring state can give us information about: (i) the importance of the torus absorption and (ii) the properties of the torus i.e. its temperature and its geometry. We find that present data do not arrive at energies where the torus influence is prominent and as a result it is currently hardly possible to infer torus properties from observations. However, with dedicated simulations, we demonstrate that observations with the forthcoming Cherenkov Telescope Array (CTA) will be able to constrain the torus parameters (temperature and geometry).
Flat spectrum radio quasars (FSRQs) have been detected at TeV energies by ground-based atmospheric Cherenkov telescopes mainly during flaring states. VERITAS is carrying out the first systematic and unbiased search for TeV emissions from a set of FSRQs. Fermi-LAT-detected FSRQs with positive declinations and extrapolated fluxes from the 3FHL catalog exceeding 1$\%$ Crab at >200 GeV after correcting for EBL absorption were selected for this survey, resulting in eight targets. Additionally, four FSRQs that were already detected at TeV energies are also included in this survey. In an unbiased fas…
The attenuation of very high energy (VHE) photons by the extragalactic background light (EBL) prevents the observation of high redshift flat spectrum radio quasars (FSRQs). However, the correlation of VHE spectral index with source redshift suggests that EBL intensity may be less than what is predicted. This deviation can draw new constraints on opacity of the universe to VHE gamma-rays. Therefore, more FSRQs may fall above the sensitivity of the forthcoming VHE telescopes than the ones predicted by the existing EBL models. In order to account for the lower EBL intensity predicted by the index…
Flat spectrum radio quasar (FSRQ) is the most luminous blazar at the GeV energies. In this paper, we probe the photon-axion-like particle (ALP) oscillation effect on the latest very-high-energy (VHE) $γ$-ray observations of the FSRQ 4C+21.35 (PKS 1222+216). The $γ$-ray spectra are measured by the collaborations Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC), Very Energetic Radiation Imaging Telescope Array System (VERITAS), and Fermi Large Area Telescope (Fermi-LAT), which cover two activity VHE flares of 4C+21.35 in 2010 and 2014. We show the spectral energy distributions (SEDs)…
With the installation of a new 28-m diameter imaging atmospheric Cherenkov telescope in the middle of the array, the H.E.S.S. instrument has entered since 2012 into its Phase II. The fifth large-size telescope is particularly important to lower the threshold energy of the array, and is thus a unique instrument to observe low-frequency-peaked blazars, such as flat-spectrum radio-quasars (FSRQs), which remain rare in the very-high- energy (VHE; E $>$ 100 GeV) $γ$-ray domain. In this contribution, we report on the discovery with the H.E.S.S. telescopes of VHE $γ$-ray emission from the FSRQ PKS 07…
A model-dependent method is proposed to determine the location of the $γ$-ray emitting region for a given flat spectrum radio quasar (FSRQ). In the model, the extra-relativistic electrons are injected at the base of the jet and non-thermal photons are produced by both synchrotron radiation and inverse-Comtpon (IC) scattering in the energy dissipation region. The target photons dominating inverse-Comtpon scattering originate from both synchrotron photons and external ambient photon fields, and the energy density of external radiation field is a function of the distance between the position of d…
The flat spectrum radio quasar PKS 1510-089 is a monitored target in many wavelength bands due to its high variability. It was detected as a very-high-energy (VHE) $γ$-ray emitter with H.E.S.S. in 2009, and has since been a regular target of VHE observations by the imaging Cherenkov observatories H.E.S.S. and MAGIC. In this paper, we summarize the current state of results focusing on the monitoring effort with H.E.S.S. and the discovery of a particularly strong VHE flare in 2016 with H.E.S.S. and MAGIC. While the source has now been established as a weak, but regular emitter at VHE, no correla…
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…
The location of $γ$-ray emitting region in blazars has been an open issue for several decades and is still being debated. We use the Paliya et al. sample of 619 $γ$-ray-loud flat-spectrum radio quasars with the available spectral energy distributions, and employ a seed photon factor approach, to locate the $γ$-rays production region. This method efficiently set up a relation between the peak frequencies and luminosities for the synchrotron emission and inverse Compton scattering, together with a combination of the energy density and characteristic energy for the external seed photon field, nam…
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…
We report multi-wavelength (IR-O, UV, X-ray, gamma-ray) data of the flat spectrum radio quasar (FSRQ) PKS B1424-418 (z=1.52) gathered during an active phase in 2013 April . As for a few other cases of FSRQs reported in literature, the detection by the Large Area Telescope onboard Fermi of gamma rays with energy above 10 GeV indicates that the emission likely occurs beyond the highly opaque (tau ~10) broad line region. This conclusion is strengthened by a model fit to the spectral energy distribution, which allows us to locate the emission region even beyond the distance generally assumed for t…
In the last couple of years the Magic air Cherenkov telescope has made significant contributions to very high energy $γ$-ray astronomy. These include the detection of the galactic binary system LSI +61 303 and the observation of pulsed emission from the Crab pulsar. Extragalactic objects like the famous FSRQ 3C 279 and the LBL S5 0716+714 have both been detected during optical high states, and the radio galaxy M87 could be observed during an unexpected strong $γ$-ray outburst. Given its low energy trigger threshold (~50 GeV) and fast repositioning time of less than 30s the Magic air Cherenkov …
The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and 2016 August 2 contains 2863 objects located at high Galactic latitudes (|b|>10°). It includes 85% more sources than the previous 3LAC catalog based on 4 years of data. AGNs represent at least 79% of the high-latitude sources in the fourth Fermi-Large Area Telescope Source Catalog (4FGL), which covers the energy range from 50 MeV to 1 TeV. In addition, 344 gamma-ray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (9…
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…
4C +21.35 is a flat-spectrum-radio-quasar-type blazar, in which the rapid variability of very high energy (VHE, $E_γ\gtrsim 100$\,GeV) emission as short as $\sim$ 10 minutes was observed by MAGIC Cherenkov telescopes, and the VHE spectrum extends up to at least 400\,GeV. In this paper, by using a flat broad-line region (BLR) structure, we study the location and properties of $γ$-ray emitting region of 4C +21.35 under the constraints of multiwavelength data. We fit three quasi-simultaneous spectral energy distributions (SEDs) using homogeneous one-zone leptonic model, in which the flat BLR with…
The MAGIC telescopes discovered very high energy (VHE, E>100 GeV) gamma-ray emission coming from the distant Flat Spectrum Radio Quasar (FSRQ) PKS 1222+21 (4C +21.35, z=0.432). It is the second most distant VHE gamma-ray source, with well measured redshift, detected until now. The observation was performed on 2010 June 17 (MJD 55364.9) using the two 17 m diameter imaging Cherenkov telescopes on La Palma (Canary Islands, Spain). The MAGIC detection coincides with high energy MeV/GeV gamma-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The averaged integral…
Very High Energy (VHE) gamma-ray emission from the flat spectrum radio quasar (FSRQ) PKS 1222+21 (4C 21.35, z=0.432) was detected with the MAGIC Cherenkov telescopes during a short observation (~0.5 hr) performed on 2010 June 17. The MAGIC detection coincides with high energy MeV/GeV gamma-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The VHE spectrum measured by MAGIC extends from about 70 GeV up to at least 400 GeV and can be well described by a power law dN/dE \propto E^-Gamma with a photon index Gamma= 3.75+/-0.27stat +/-0.2syst. The averaged integra…
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…
Active Galactic Nuclei can be copious extragalactic emitters of MeV-GeV-TeV gamma rays, a phenomenon linked to the presence of relativistic jets powered by a super-massive black hole in the center of the host galaxy. Most of gamma-ray emitting active galactic nuclei, with more than 1500 known at GeV energies, and more than 60 at TeV energies, are called "blazars". The standard blazar paradigm features a jet of relativistic magnetized plasma ejected from the neighborhood of a spinning and accreting super-massive black hole, close to the observer direction. Two classes of blazars are distinguish…
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…
Optical spectral characterization of OP 313. Constraining the contribution of thermal and non-thermal optical emission
Istituto Nazionale di Fisica Nucleare, Sezione di Padova（意大利国家核物理研究所帕多瓦分部）；Instituto de Astrofísica de Canarias (IAC)（加那利天体物理学研究所）；Universidad de La Laguna (ULL), Departamento de Astrofísica（拉古纳大学天体物理学系）
Characterizing the origins of gamma-ray variability of the jetted active galactic nuclei observed with the Fermi-LAT
Qujing Normal University（曲靖师范学院）；Nanjing University（南京大学）；Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；Anqing Normal University（安庆师范大学）；Kunming University（昆明学院）
We report on broadband observations of CTA 102 ($z=1.037$) during the active states in 2016-2017. In the $γ$-ray band, Fermi LAT observed several prominent flares which followed a harder-when-brighter behavior: the hardest photon index $Γ=1.61\pm 0.10$ being unusual for FSRQs. The peak $γ$-ray flux above 100 MeV $(3.55\pm0.55)\times10^{-5}\:{\rm photon\:cm^{-2}\:s^{-1}}$ observed on MJD 57738.47 within 4.31 minutes, corresponds to an isotropic $γ$-ray luminosity of $L_γ=3.25\times10^{50}\:{\rm erg\:s^{-1}}$, comparable with the highest values observed from blazars so far. The analyses of the S…
Flat-spectrum radio-quasars (FSRQs) are rarely detected at very-high-energies (VHE; E>100 GeV) due to their low-frequency-peaked SEDs. At present, only 6 FSRQs are known to emit VHE photons, representing only 7% of the VHE extragalactic catalog. Following the detection of MeV-GeV gamma-ray flaring activity from the FSRQ PKS 0736+017 (z=0.189) with Fermi, the H.E.S.S. array of Cherenkov telescopes triggered ToO observations on February 18, 2015, with the goal of studying the gamma-ray emission in the VHE band. H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, …
We explore the evolution of the emissions by accelerated electrons in shocked shells driven by jets in active galactic nuclei (AGNs). Focusing on powerful sources which host luminous quasars, we evaluated the broadband emission spectra by properly taking into account adiabatic and radiative cooling effects on the electron distribution. The synchrotron radiation and inverse Compton (IC) scattering of various photons that are mainly produced in the accretion disc and dusty torus are considered as radiation processes. We show that the resultant radiation is dominated by the IC emission for compac…
Detection of the distant quasar OP 313 with the first Large-Sized Telescope of CTAO
Tokai University（东海大学）；University of Tokyo（东京大学）；ETH Zürich（苏黎世联邦理工学院）；Università degli Studi di Siena（锡耶纳大学）；The Barcelona Institute of Science and Technology（巴塞罗那科学技术研究所）；Université Paris-Saclay（巴黎萨克雷大学）；Université Paris Cité（巴黎西岱大学）；Universitat de Barcelona（巴塞罗那大学）；University of Geneva（日内瓦大学）
基于LST-1和MAGIC数据，研究FSRQ OP 313在2013年12月耀发期间的甚高能γ射线发射，并利用多波段观测构建宽带能谱，通过双区轻子模型解释其辐射机制，同时评估河外背景光对VHE光子的衰减。
