The majority of the active galactic nuclei (AGN) detected at very-high-energies above 100 GeV belong to the class of blazars with a small angle between the jet-axis and the line-of-sight. Only about 10 percent of the gamma-ray AGN are objects with a larger viewing angle resulting in a smaller Doppler boosting of the emission. Originally, it was believed that gamma-ray emission can only be observed from blazars and those are variable in its brightness. Instead, the last years have shown that non-blazar active galaxies also show a fascinating variability behaviour which provide important new insights into the physical processes responsible for the gamma-ray production and especially for flaring events. Here, we report on the observation of gamma-ray variability of the active galaxy PKS 0625-354 detected with the H.E.S.S. telescopes in November 2018. The classification of PKS 0625-354 is a still matter of debate. The H.E.S.S. measurements were performed as part of a flux observing program and showed in the first night of the observation a detection of the object with >5sigma. A denser observation campaign followed for the next nine nights resulting in a decrease of the gamma-ray flux. Those observations were accompanied with Swift in the X-ray and UV/optical band allowing for the reconstruction of a multi-band broad-band spectral energy distribution. We will discuss the implications of the gamma-ray variability of the object.
Long-term gamma-ray variability of a non-blazar Active Galactic Nucleus (AGN) PKS 0521-36 is investigated by using Fermi-LAT pass 8 data covering from 2008 August to 2021 March. The results show that the histogram of the gamma-ray fluxes follows a log-normal distribution. Interestingly, in the analysis of about 5.8-year (from MJD 56317 to 58447) LAT data between two outbursts (occurring during 2012 October and 2019 May respectively), a quasi-periodic oscillation (QPO) with a period of about 1.1 years (about 5 sigma of significance) is found in the Lomb-Scargle Periodogram (LSP), the Weighted W…
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 …
We reviewed X-ray flux and spectral variability properties studied to date by various X-ray satellites for Mrk 421 and PKS 2155-304, which are TeV emitting blazars. Mrk 421 and PKS 2155-304 are the most X-ray luminous blazars in the northern and southern hemispheres, respectively. Blazars show flux and spectral variabilities in the complete electromagnetic spectrum on diverse timescales ranging from a few minutes to hours, days, weeks, months and even several years. The flux and spectral variability on different timescales can be used to constrain the size of the emitting region, estimate 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…
The results of the first ever contemporaneous multi-wavelength observation campaign on the BL Lac object PKS2155-304 involving Swift, NuSTAR, Fermi-LAT and H.E.S.S. are reported. The use of these instruments allows us to cover a broad energy range, important for disentangling the different radiative mechanisms. The source, observed from June 2013 to October 2013, was found in a low flux state with respect to previous observations but exhibited highly significant flux variability in the X-rays. The high-energy end of the synchrotron spectrum can be traced up to 40 keV without significant contam…
We present periodicity search analyses on the $γ$-ray lightcurve of the TeV blazar PKS 1510-089 observed by the \textit{Fermi} Large Area Telescope. We report the detection of two transient quasi-periodic oscillations: a 3.6-day QPO during the outburst in 2009 that lasted five cycles (MJD 54906--54923); and a periodicity of 92 days spanning over 650 days from 2018 to 2020 (MJD 58200--58850), which lasted for seven cycles. We employed the Lomb-Scargle periodogram, Weighted Wavelet Z-transform, REDFIT, and the Monte Carlo lightcurve simulation techniques to find any periodicity and the correspon…
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 …
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（教育部现代天文学和天体物理学重点实验室（南京大学））；Centre for Space Research, North-West University（西北大学空间研究中心）
Context: The $γ$-ray blazar PKS 2155-304 has attracted considerable attention because of its extreme TeV variability characteristics during an exceptional flaring period in 2006. Among the observed key findings are (i) a minimum variability timescale as short as $\sim 200$ sec and (ii) highly variable TeV emission, which in the frequency interval [$10^{-4}$ Hz, $10^{-2}$ Hz] can be described by a log-normal distribution and suggests an underlying multiplicative (and not additive) process. Aims: Simultaneously accounting for these findings appears difficult within conventional approaches. Follo…
PKS 1413+135, a compact symmetric object (CSO) with a two-side pc-scale structure in its miniature radio morphology, is spatially associated with 4FGL J1416.1+1320 and recently detected with MAGIC telescopes. We comprehensively analyze its X-ray and GeV gamma-ray observation data for revealing its high energy radiation physics. It is found that the source is in a low-flux stage before MJD 58500 and experiences violent outbursts after MJD 58500 in the GeV band. The flux at 10 GeV varies ~3 orders of magnitude, and the GeV-flux variation is accompanied by the clear spectral variation, which is c…
Context. Blazars are radio-loud active galactic nuclei (AGN) with a jet pointing at small angles towards the observer. The overall emitted spectrum is typically non-thermal, and in some cases the emission and/or absorption lines are so faint as to prevent the determination of the redshift based on optical spectroscopy methods. PKS 0447-439 is a bright blazar recently detected at very high energy. The redshift of the source is still disputed: a recent spectral analysis reports only a lower limit of z > 1.246, which contradicts the previous measure of z = 0.205 reported in the literature. Aims. …
Evidence for the presence of quasi-periodic oscillations (QPOs) in the optical polarization of the blazar PKS 2155-304, during a period of enhanced gamma-ray brightness, is presented. The periodogram of the polarized flux revealed the existence of a prominent peak at $T\sim 13$ min, detected at >99.7% significance, and $T\sim 30$ min, which was nominally significant at >99%. This is the first evidence of QPOs in the polarization of an active galactic nucleus, potentially opening up a new avenue of studying this phenomenon.
The spectral energy distribution (SED) of high-frequency peaked BL Lac objects (HBL) is characterized by two peaks: one in the UV-X-ray and one in the GeV-TeV regime. An interesting object for analyzing these broadband characteristics is PKS 2005-489, which in 2004 showed the softest TeV spectrum ever measured. In 2009, a multi-wavelength campaign has been conducted with, for the first time, simultaneous observations by H.E.S.S. (TeV), Fermi/LAT (GeV), RXTE (keV), Swift (keV, UV, optical) and ATOM (optical) to cover the two peaks of the SED. During this campaign PKS 2005-489 underwent a high s…
We present the redshift lower limit of z>0.6035 for the very-high-energy (VHE; E>100 GeV) emitting blazar PKS 1424+240 (PG 1424+240). This limit is inferred from Lyman beta and gamma absorption observed in the far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph. No VHE-detected blazar has shown solid spectroscopic evidence of being more distant. At this distance, VHE observations by VERITAS are shown to sample historically large gamma-ray opacity values at 500 GeV, extending beyond tau=4 for low-level models of the extragalactic background light (EBL) and beyond…
We present the results of the power spectral density (PSD) analysis for the blazars Mrk\,421 and PKS\,2155$-$304, using good-quality, densely sampled light curves at multiple frequencies, covering 17 decades of the electromagnetic spectrum, and variability timescales from weeks up to a decade. The data were collected from publicly available archives of observatories at radio from OVRO, optical and infrared (B, V, R, I, J, H, and K-bands), X-rays from the {\it Swift} and the {\it Rossi} X-ray Timing Explorer, high and very high energy $γ-$rays from the {\it Fermi} and Very Energetic Radiation I…
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…
Time variability of the photon flux is a known feature of active galactic nuclei (AGN) and in particular of blazars. The high frequency peaked BL Lac (HBL) object PKS 2155-304 is one of the brightest sources in the TeV band and has been monitored regularly with different instruments and in particular with the H.E.S.S. experiment above 200 GeV for more than 11 years. These data together with the observations of other instruments and monitoring programs like SMARTS (optical), Swift-XRT/RXTE/XMM-Newton (X-ray) and Fermi-LAT (100 MeV < E < 300 GeV) are used to characterize the variability of this …
Recently, the IC/CMB X-ray mechanism has been strongly disfavoured for 3C 273 and PKS 0637-752 since the anomalously hard and steady gamma-ray emission predicted by such models violates the observational results from Fermi-LAT. Here we propose the proton synchrotron origin of the X ray-gamma ray flux from the knots of PKS 0637-752 with a reasonable budget in luminosity, by considering synchrotron radiation from an accelerated proton population. Moreover, for the source 3C 273, some of the optical data points could not be explained by electron synchrotron (Meyer et al. 2015), which we have incl…
PKS J2310-437 is an AGN with bright X-ray emission relative to its weak radio emission and optical continuum. It is believed that its jet lies far enough from the line of sight that it is not highly relativistically beamed. It thus provides an extreme test of AGN models. We present new observations aimed at refining the measurement of the source's properties. In optical photometry with the NTT we measure a central excess with relatively steep spectrum lying above the bright elliptical galaxy emission, and we associate the excess wholly or in part with the AGN. A new full-track radio observatio…
We have examined 13 pointed observations of the TeV emitting high synchrotron peak blazar PKS 2155-304, taken by the Suzaku satellite throughout its operational period. We found that the blazar showed large-amplitude intraday variabilities in the soft (0.8 - 1.5 keV) and the hard (1.5 - 8.0 keV) bands in the light curves. Spectral variability on intraday timescales is estimated using the hardness ratio. The blazar usually becomes harder when brighter and vice versa, following the typical behavior of high synchrotron peak blazars. The power spectral density (PSD) analyses of 11 out of 13 light …
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）
Rapid flares from blazars in very high energy (VHE) $γ$-rays challenge the common understanding of jets of active galactic nuclei (AGNs). The same population of ultra-relativistic electrons is often thought to be responsible for both X-ray and VHE emission. We thus systematically searched for X-ray flares at sub-hour timescales of TeV blazars in the entire Rossi X-ray Timing Explorer archival database. We found rapid flares from PKS 2005-489 and S5 0716+714, and a candidate rapid flare from 1ES 1101-232. In particular, the characteristic rise timescale of PKS 2005-489 is less than half a minut…
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 $1…
Following the detection of a $\sim$300 TeV neutrino potentially associated with the flaring blazar TXS 0506+056, an excess of neutrinos around its position in 2014-2015 was revealed by IceCube. However, its contemporaneous quiescence in $γ$-rays is challenging to interpret consistently. Meanwhile, the blazar PKS 0502+049, positioned within the neutrino localization uncertainties, was seen to be flaring in $γ$-rays. We show that dense, line-emitting gas clouds that interact with its jet and induce cosmic ray acceleration and hadronuclear interaction can plausibly explain the 2014-2015 neutrino …
On July 30th, 2019 IceCube detected a high-energy astrophysical muon neutrino candidate, IC-190730A, with a $67\%$ probability of astrophysical origin. The flat spectrum radio quasar (FSRQ) PKS 1502+106 is in the error circle of the neutrino. Motivated by this observation, we study PKS 1502+106 as a possible source of IC-190730A. PKS 1502+106 was in a quiet state in terms of UV/optical/X-ray/gamma-ray flux at the time of the neutrino alert, we therefore model the expected neutrino emission from the source during its average long-term state, and investigate whether the emission of IC-190730A as…
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…
Recently, a new method to constrain the distances of blazars with unknown redshift using combined observations in the GeV and TeV regimes has been developed. The underlying assumption is that the Very High Energy (VHE, E > 100 GeV) spectrum corrected for the absorption of TeV photons by the Extragalactic Background Light (EBL) via photon-photon interaction should still be softer than the gamma-ray spectrum observed by Fermi/LAT. The constraints found are related to the real redshifts by a simple linear relation, that has been used to infer the unknown or uncertain distance of blazars. The samp…
PKS 0521-36 is an Active Galactic Nucleus (AGN) with uncertain classification. We investigate the properties of this source from radio to gamma rays. The broad emission lines in the optical and UV bands and steep radio spectrum indicate a possible classification as an intermediate object between broad-line radio galaxies (BLRG) and steep spectrum radio quasars (SSRQ). On pc-scales PKS 0521-36 shows a knotty structure similar to misaligned AGN. The core dominance and the gamma-ray properties are similar to those estimated for other SSRQ and BLRG detected in gamma rays, suggesting an intermediat…
In the recent years, the new generation of Imaging Atmospheric Cerenkov Telescopes successfully detected very high energy (VHE; E>100 GeV) gamma-ray emission from a growing number of Active Galactic Nuclei (AGNs), mainly belonging to the blazar class. Among these recent results, we will mainly focus in this work on two recent discoveries made with the H.E.S.S. experiment: the tremendously active state of the blazar PKS 2155-304 observed in July 2006 and the discovery of VHE gamma-rays from the radio galaxy Cen A. On the one hand, the observation of very fast variability in PKS 2155-304 challen…
We present a model-dependent method to estimate the redshifts of three TeV BL Lac objects (BL Lacs) through fltting their (quasi-) simultaneous multi-waveband spectral energy distributions (SEDs) by one-zone leptonic synchrotron self-Compton (SSC) model. Considering the impact of electron energy distributions (EEDs) on the results, we use three types of EEDs, such as the power-law EED with exponential cut-ofi (PLC), the log-parabola (PLLP) and the broken power-law (BPL) EEDs, to flt the SEDs. We also use a parameter fi to describe the uncertainties of the extragalactic background light (EBL) m…
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…
The jet of the radio galaxy M87 is misaligned, resulting in a Doppler factor delta~1 for emission of plasma moving parallel to the jet. This makes the observed fast TeV flares on timescales of t_v~5R_g/c harder to understand as emission from the jet. In previous work, we have proposed a jets-in-a-jet model for the ultra-fast TeV flares with t_v<<R_g/c seen in Mrk 501 and PKS 2155-304. Here, we show that about half of the minijets beam their emission outside the jet cone. Minijets emitting off the jet axis result in rapidly evolving TeV (and maybe lower energy) flares that can be observed in ne…
Very high-energy gamma-ray emission from PKS 0447-439 was detected with the H.E.S.S. Cherenkov telescope array in December 2009. This blazar is one of the brightest extragalactic objects in the Fermi Bright Source List and has a hard spectrum in the MeV to GeV range. In the TeV range, a photon index of 3.89 +- 0.37 (stat) +- 0.22 (sys) and a flux normalisation at 1 TeV, Phi(1 TeV) = (3.5 +- 1.1 (stat) +- 0.9 (sys)) x 10^{-13} cm^{-2} s^{-1} TeV^{-1}, were found. The detection with H.E.S.S. triggered observations in the X-ray band with the Swift and RXTE telescopes. Simultaneous UV and optical …
We have examined 40 NuSTAR light curves (LCs) of five TeV emitting high synchrotron peaked blazars: 1ES 0229+200, Mrk 421, Mrk 501, 1ES 1959+650 and PKS 2155-304. Four of the blazars showed intraday variability in the NuSTAR energy range of 3-79 keV. Using an auto correlation function analysis we searched for intraday variability timescales in these LCs and found indications of several between 2.5 and 32.8 ks in eight LCs of Mrk 421, a timescale around 8.0 ks for one LC of Mrk 501, and timescales of 29.6 ks and 57.4 ks in two LCs of PKS 2155-304. The other two blazars' LCs do not show any evid…
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 …
The fast TeV variability of the blazars Mrk 501 and PKS 2155--304 implies a compact emitting region that moves with a bulk Lorentz factor of Gamma_{em}~100 toward the observer. The Lorentz factor is clearly in excess of the jet Lorentz factors Gamma_j\simless 10 measured on sub-pc scales in these sources. We propose that the TeV emission originates from compact emitting regions that move relativistically {\it within} a jet of bulk Gamma_j~10. This can be physically realized in a Poynting flux-dominated jet. We show that if a large fraction of the luminosity of the jet is prone to magnetic diss…
The Fermi Flare Advocate (also known as Gamma-ray Sky Watcher, FA-GSW) service provides for a daily quicklook analysis and review of the high-energy gamma-ray sky seen by the Fermi Gamma-ray Space Telescope. The duty offers alerts for potentially new gamma-ray sources, interesting transients and relevant flares. A public weekly digest containing the main highlights about the GeV gamma-ray sky is published in the web-based Fermi Sky Blog. During the first 3 years of all-sky survey, more than 150 Astronomical Telegrams, several alerts to the TeV Cherenkov telescopes, and targets of opportunity t…
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…
In this paper we present multiband optical polarimetric observations of the VHE blazar PKS 2155-304 made simultaneously with a H.E.S.S./Fermi high-energy campaign in 2008, when the source was found to be in a low state. The intense daily coverage of the dataset allowed us to study in detail the temporal evolution of the emission and we found that the particle acceleration timescales are decoupled from the changes in the polarimetric properties of the source. We present a model in which the optical polarimetric emission originates at the polarised mm-wave core and propose an explanation for the…
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…
PKS 1718$-$649 is one of the closest and most comprehensively studied candidates of a young active galactic nucleus (AGN) that is still embedded in its optical host galaxy. The compact radio structure, with a maximal extent of a few parsecs, makes it a member of the group of compact symmetric objects (CSO). Its environment imposes a turnover of the radio synchrotron spectrum towards lower frequencies, also classifying PKS 1718$-$649 as gigahertz-peaked radio spectrum (GPS) source. Its close proximity has allowed the first detection of extended X-ray emission in a GPS/CSO source with Chandra th…
Most of the ~50 blazars detected to date at TeV energies (E>0.1 TeV) are high-frequency-peaked BL Lacs (HBLs). Only a handful episodic TeV detections of low- and intermediate-frequency-peaked BL Lacs (LBL/IBLs, with synchrotron peak frequencies in the infrared and optical regime) have been reported, typically during high-flux states. The VERITAS array, a ground-based TeV observatory located in southern Arizona has observed five known TeV LBL/IBLs since 2009: 3C 66A, W Comae, PKS 1424+240, S5 0716+714 and BL Lacertae, with at least 5-10 hours/year, which so far resulted in the detection of a br…
We report the first detection of very-high-energy (VHE) gamma-ray emission above 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift. The photon spectrum above 140 GeV measured by VERITAS is well described by a power law with a photon index of 3.8 +- 0.5_stat +- 0.3_syst and a flux normalization at 200 GeV of (5.1 +- 0.9_stat +- 0.5_syst) x 10^{-11} TeV^-1 cm^-2 s^-1, where stat and syst denote the statistical and systematical uncertainty, respectively. The VHE flux is steady over the observation period between MJD 54881 and 55003 (2009 February 19 to June 21). Flux variability…
We present an extensive study on the X-ray intraday variability of ten TeV-emitting high synchrotron peaked blazars (HBLs): 1ES 0229+200, 1ES 0414+009, PKS 0548-322, 1ES 1101-232, 1H 1219+301, H 1426+428, Mrk 501, 1ES 1959+650, PKS 2005-489, and 1ES 2344+514 made with twenty-five XMM-Newton pointed observations during its operational period. Intraday variability has been estimated in three energy bands: soft (0.3--2 keV), hard (2--10 keV) and total (0.3--10 keV). Although seven out of these ten TeV HBLs exhibited some intraday variability at three-sigma levels no major variations exceeding six…
We report on the first simultaneous observations that cover the optical, X-ray, and high energy gamma-ray bands of the BL Lac object PKS 2155-304. The gamma-ray bands were observed for 11 days, between 25 August and 6 September 2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S. atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral energy distribution with the new generation of gamma-ray telescopes. The ATOM telescope and the RXTE and Swift observatories provided optical and X-ray coverage of the low-energy component over the same time period. The o…
The quasar PKS 1510-089 (z=0.361) was observed with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes during high states in the optical and GeV bands, to search for very high energy (VHE, defined as E >= 0.1 TeV) emission. VHE γ-rays were detected with a statistical significance of 9.2 standard deviations in 15.8 hours of H.E.S.S. data taken during March and April 2009. A VHE integral flux of I(0.15 TeV < E < 1.0 TeV) = (1.0 +- 0.2 (stat) +- 0.2 (sys) x 10^{-11} cm^{-2}s^{-1} is measured. The best-fit power law to the VHE data has a photon index of Γ=5.4 +- 0.7 (stat) +- 0.3 (sys)…
Recently observed minute timescale variability of blazar emission at TeV energies has imposed severe constraints on jet models and TeV emission mechanisms. We focus on a robust jet instability to explain this variability. As a consequence of the bulk outflow of the jet plasma, the pressure is likely to be anisotropic, with the parallel pressure $P_{||}$ in the forward jet direction exceeding the perpendicular pressure $P_{\perp}$. Under these circumstances, the jet is susceptible to the firehose instability, which can cause disruptions in the large scale jet structure and result in variability…
We present a detailed study of the X-ray, optical and radio emission from the jet, lobes and core of the quasar PKS 2101-490 as revealed by new Chandra, HST and ATCA images. We extract the radio to X-ray spectral energy distributions from seven regions of the 13 arcsecond jet, and model the jet X-ray emission in terms of Doppler beamed inverse Compton scattering of the cosmic microwave background (IC/CMB) for a jet in a state of equipartition between particle and magnetic field energy densities. This model implies that the jet remains highly relativistic hundreds of kpc from the nucleus, with …
For 3 radio-loud $γ$-ray detected Narrow-Line Seyfert 1 ($γ$-ray NLSy1) galaxies, we report optical variability on intra-night and/or week-like time scales, based on five $\geq$ 3 hours long monitoring sessions for each galaxy. The radio-loudness factors (R$_{1.4 GHz}$) for these galaxies, namely 1H 0323$+$342 (z = 0.0629), PKS J1222$+$0413 (z = 0.966) and PKS J1505$+$0326 (z = 0.408) are $\sim$318, $\sim$1534 and $\sim$3364 at 1.4 GHz, respectively. For the most distant $γ$-ray NLSy1, PKS J1222$+$0413, Intra-Night Optical Variability (INOV) characterisation is presented for the first time. Th…
We describe a Chandra X-ray ToO project designed to isolate the site of TeV flaring in the radio galaxy M87. To date, we have triggered the Chandra observations only once (2010 April) and by the time of the first of our 9 observations, the TeV flare had ended. However, we found that the X-ray intensity of the unresolved nucleus was at an elevated level for our first observation. Of the more than 60 Chandra observations we have made of the M87 jet covering 9 years, the nucleus was measured at a comparably high level only 3 times. Two of these occasions can be associated with TeV flaring, and at…
