The flat spectrum radio quasar 3C~454.3 underwent an extraordinary outburst in December 2009 when it became the brightest gamma-ray source in the sky for over one week. Its daily flux measured with the Fermi Large Area Telescope at photon energies E>100 MeV reached F = 22+/-1 x 10^-6 ph cm^-2 s^-1, representing the highest daily flux of any blazar ever recorded in high-energy gamma-rays. It again became the brightest source in the sky in 2010 April, triggering a pointed-mode observation by Fermi. The correlated gamma-ray temporal and spectral properties during these exceptional events are presented and discussed. The main results show flux variability over time scales less than 3 h and very mild spectral variability with an indication of gradual hardening preceding major flares. No consistent loop pattern emerged in the gamma-ray spectral index vs flux plane. A minimum Doppler factor of ~ 15 is derived, and the maximum energy of a photon from 3C 454.3 is ~ 20 GeV. The spectral break at a few GeV is inconsistent with Klein-Nishina softening from power-law electrons scattering Ly_alpha line radiation, and a break in the underlying electron spectrum in blazar leptonic models is implied.
The magnetar 1E 1547.0-5408 exhibited outbursts in October 2008 and January 2009. In this paper we present in great detail the evolution of the temporal and spectral characteristics of the persistent total and pulsed emission of 1E 1547.0-5408 between ~1 and 300 keV starting in October 3, 2008, and ending in January 2011. We analyzed data collected with the Rossi X-ray Timing Explorer, the International Gamma-Ray Astrophysics Laboratory and the Swift satellite.
The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the Fermi-LAT monitored blazars in the southern sky with closely spaced observations since 2008. We report the discovery of an optical-near-IR (OIR) outburst with no accompanying gamma-rays in the blazar PKS 0208-512, one of the targets of this program. While the source undergoes three outbursts of 1 mag or more at OIR wavelengths lasting for longer than 3 months during 2008-2011, only interval 1 and 3 have corresponding bright phases in GeV energies lasting longer than 1 month. The OIR outburst durin…
The flat-spectrum radio quasar 3C 454.3 underwent an extraordinary 5-day gamma-ray outburst in November 2010 where the daily flux measured with the Fermi Large Area Telescope (LAT) at photon energies E>100 MeV reached (66+/-2) x 10^-6 ph cm^-2 s^-1. This is a factor of 3 higher than its previous maximum flux recorded in December 2009 and ~5 times brighter than the Vela pulsar, which is normally the brightest source in the gamma-ray sky. The 3-hr peak flux was (85+/-5) x 10^-6 ph cm^-2 s^-1, corresponding to an apparent isotropic luminosity of 2.1+/-0.2 10^50 erg s^-1, the highest ever recorded…
The AGILE satellite detected transient high-energy gamma-ray emission from the X-ray binary V404 Cygni, during the June 2015 outburst observed in radio, optical, X-ray and soft gamma-ray frequencies. The activity was observed by AGILE in the 50-400 MeV energy band, between 2015-06-24 UT 06:00:00 and 2015-06-26 UT 06:00:00 (MJD 57197.25 - 57199.25), with a detection significance of ${\sim}4.3σ$. The gamma-ray detection, consistent with a contemporaneous observation by Fermi-LAT, is correlated with a bright flare observed at radio and hard X-ray frequencies, and with a strong enhancement of the …
Analysis of comprehensive monitoring of 34 gamma-ray bright quasars, BL Lac objects, and radio galaxies reveals a close connection between events in the millimeter-wave emission imaged with the VLBA at 43 GHz and flares at gamma-ray and lower frequencies. Roughly 2/3 of the flares are coincident with the appearance of a new superluminal knot and/or a flare in the millimeter-wave "core'" located parsecs from the central engine. This presents a theoretical challenge to explain how the gamma-ray flux can often be variable on intra-day time-scales. Possible answers to this include very narrow open…
We report the discovery of an anomalous flare in a bright blazar, namely, PKS 0208-512, one of the targets of the Yale/SMARTS optical-near-infrared (OIR) monitoring program of Fermi blazars. We identify three intervals during which PKS 0208-512 undergoes outbursts at OIR wavelengths lasting for longer than 3 months. Its brightness increases and then decreases again by at least 1 magnitude in these intervals. In contrast, the source undergoes bright phases in GeV energies lasting for longer than 1 month during intervals 1 and 3 only. The OIR outburst during interval 2 is comparable in brightnes…
Due to its powerful radiation over the entire electromagnetic spectrum and its radio jet activity, the blazar 3C273 offers the opportunity to study the physics of $γ$-ray emission from active galactic nuclei. Since a historically strong outburst in 2009, 3C273 showed relatively weak emission in the gamma-ray band over multiple years. However, recent Fermi-Large Area Telescope observations indicate elevated activity during 2015-2019. We aim at constraining the origin of the gamma-ray outbursts towards 3C273 and investigate their connection to the parsec-scale jet. We generate Fermi-LAT gamma-ra…
We present the light curves and spectral data of two exceptionally luminous gamma-ray outburts observed by the Large Area Telescope (LAT) experiment on board Fermi Gamma-ray Space Telescope from 3C 273 in September 2009. During these flares, having a duration of a few days, the source reached its highest gamma-ray flux ever measured. This allowed us to study in some details their spectral and temporal structures. The rise and decay are asymmetric on timescales of 6 hours, and the spectral index was significantly harder during the flares than during the preceding 11 months. We also found that s…
We test the synchrotron emission scenario for the very bright gamma-ray flare of blazar 3C 279 observed in 2015 June using time-dependent numerical simulations. A bulk Lorentz factor as high as 100 can bring the synchrotron maximum energy above the GeV energy range. We find two possible solutions for the X-ray to gamma-ray spectrum. One is a prompt electron injection model with a hard power-law index as magnetic reconnection models suggest. A too strong magnetic field yields a too bright synchrotron X-ray flux due to secondary electron--positron pairs. Even in the prompt electron injection mod…
Unexpected Gamma-ray Outburst from the CSS Radio Galaxy 3C 216
Istituto Nazionale di Fisica Nucleare, INFN, Roma Tor Vergata（意大利国家核物理研究所，罗马托尔维加塔分部）；Space Science Data Center, SSDC, Agenzia Spaziale Italiana, ASI（意大利航天局空间科学数据中心）；Department of Fundamental Physics, University of Salamanca（萨拉曼卡大学基础物理系）
Nova outbursts take place in binary star systems comprising a white dwarf (WD) and either a low-mass Sun-like star (classical novae) or, a red giant. GeV gamma-ray emission has been detected from a dozen of classical novae and from one novae in symbiotic system (V407 Cyg) by Fermi-LAT. For classical novae, gamma-ray emission is generally thought to be related to internal shocks formed as fast outflow collides with the slow outflow. However, for V407 Cyg, the origin of the gamma-ray emission has been debated, as both internal shock and external shock, resulted from the collision between the nov…
We report on the results of two coordinated multiwavelength campaigns that focused on the blazar Markarian 421 during its 2006 and 2008 outbursts. These campaigns obtained UV and X-ray data using the XMM-Newton satellite, while the gamma-ray data were obtained utilizing three imaging atmospheric Cerenkov telescopes, the Whipple 10m telescope and VERITAS, both based in Arizona, as well as the MAGIC telescope, based on La Palma in the Canary Islands. The coordinated effort between the gamma-ray groups allowed for truly simultaneous data in UV/X-ray/gamma-ray wavelengths during a significant port…
The gamma-ray binary system LS I +61 303 was studied in great detail in VHE gamma-rays in the last years by the MAGIC telescope. The VHE emission of the system exhibited a prominent periodic outburst in the orbital phases 0.6-0.7 between September 2005 to January 2008. In Fall 2008 the Fermi collaboration reported as well periodic emission in the MeV to GeV energy range, but with a shifted outburst in the phases 0.35-0.45. MAGIC observed again LS I+61 303 in 2009 with the twice more sensitive stereo mode to allow for detailed correlation studies between the VHE gamma-ray and Fermi/LAT energy b…
PKS1510-089 is a flat spectrum radio quasar located at a redshift of 0.36. It is one of only a few such sources detected in very-high-energy (VHE, >100 GeV) gamma rays. Though PKS1510-089 is highly variable at GeV energies, until recently no variability has been observed in the VHE band. In 2015 May PKS1510-089 showed a high state in optical and in the GeV range. A VHE gamma-ray flare was detected with MAGIC at that time, showing the first instance of VHE gamma-ray flux variability on the time scale of days in this source. We will present the MAGIC results from this observation, discuss their …
A very bright and fast varying gamma-ray flare has been detected from the blazar 3C 279 on June 16, 2015. We have studied the broadband spectral energy distribution of the source during the flaring episode and in the low activity state using a simple one zone leptonic model. We find that an electron energy distribution described by a broken power law can be used to reproduce the broadband emissions during the high and low activity states. The flux measurements at radio, infrared and optical frequencies are reproduced by the synchrotron emission resulting from the relativistic electrons in a je…
We present timing, spectral, and long-term temporal analysis of the high mass X-ray binary (HMXB) 4U 1036-56 using INTEGRAL and Swift observations. We show that it is a weak hard X-ray source spending a major fraction of the time in quiescence, and only occasionally characterized by X-ray outbursts. The outburst activity we report here lasts several days, with a dynamic range spanned by the luminosity in quiescence and in outburst as high as ~30. We report the detection of pulse period at 854.75+/-4.39 s during an outburst, which is consistent with previous measurements. Finally, we analyze th…
Classical novae produce radioactive nuclei which are emitters of gamma-rays in the MeV range. Some examples are the lines at 478 and 1275 keV (from 7Be and 22Na) and the positron-electron annihilation emission, with the 511 keV line and a continuum. Gamma-ray spectra and light curves are potential unique tools to trace the corresponding isotopes and to give insights on the properties of the expanding envelope. Another possible origin of gamma-rays is the acceleration of particles up to very high energies, so that either neutral pions or inverse Compton processes produce gamma-rays of energies …
W Comae has significant variability in multi-wavelengthes, from the radio to the gamma-ray bands. A bright outburst in the optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae (IBL) source. I find that both the broadband spectral energy distributions (SEDs) quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron + synchrotron-self-Compton (SSC) model. The satisfactory fitting requires a lar…
The variable gamma-ray source HESS J0632+057 is an excellent candidate for a gamma-ray binary. The putative binary system was discovered as a point-like VHE gamma-ray source by HESS. Later measurements by VERITAS yielding no detection, provided evidence for variable emission in the gamma-ray domain. A variable X-ray source as well as a Be star (MWC 148) are found at the location of the gamma-ray source. Recently a periodic X-ray outburst occurring about every 320 days was reported by Swift (ATel 3152). The putative binary system was observed by the MAGIC stereo system in 2010 and 2011. Our mea…
Probing the potential high-energy messengers of the anticipated T Coronae Borealis outburst
INAF - Osservatorio Astronomico di Palermo（意大利国家天体物理研究所巴勒莫天文台）；Institute for Applied Problems in Mechanics and Mathematics, National Academy of Sciences of Ukraine（乌克兰国家科学院应用力学与数学问题研究所）；Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University（密歇根州立大学物理与天文学系数据密集型时域天文中心）；Dipartimento di Fisica e Chimica E. Segrè, Università degli Studi di Palermo（巴勒莫大学E. Segre物理化学系）
We present a detailed Fermi/LAT data analysis for the broad-line radio galaxy 3C 120. This source has recently entered into a state of increased gamma-ray activity which manifested itself in two major flares detected by Fermi/LAT in September 2014 and April 2015 with no significant flux changes reported in other wavelengths. We analyse available data focusing our attention on aforementioned outbursts. We find very fast variability timescale during flares (of the order of hours) together with a significant gamma-ray flux increase. We show that the 6.8 years averaged gamma-ray emission of 3C 120…
We show that the X-ray outburst light curves of four transient anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), namely XTE J1810-197, SGR 0501+4516, SGR 1627-41 and CXOU J164710.2-455216, can be produced by the fallback disk model that was also applied to the outburst light curves of persistent AXPs and SGRs in our earlier work. The model solves the diffusion equation for the relaxation of a disk which has been pushed back by a soft gamma-ray burst. The sets of main disk parameters used for these transient sources are very similar to each other and to those employed in our e…
We present the lightcurves, spectra, and hardness-intensity diagram (HID) of the high mass X-ray binary V0332+53 using Fermi/GBM, MAXI, Swift/BAT, and INTEGRAL through its 2015 Type II outburst. We observe characteristic features in the X-ray emission (2-50 keV) due to periastron passages, the dynamical timescale of the accretion disc, and changes within the accretion column between a radiation-dominated flow and a flow dominated by Coulomb interactions. Based on the HID and the light curves, the critical luminosity is observed to decrease by ~5-7 percent during the outburst, signaling a decre…
We report on Fermi/Large Area Telescope observations of the accreting black hole low-mass X-ray binary V404 Cygni during its outburst in June-July 2015. Detailed analyses reveal a possible excess of $γ$-ray emission on 26 June 2015, with a very soft spectrum above $100$ MeV, at a position consistent with the direction of V404 Cyg (within the $95\%$ confidence region and a chance probability of $4 \times 10^{-4}$). This emission cannot be associated with any previously-known Fermi source. Its temporal coincidence with the brightest radio and hard X-ray flare in the lightcurve of V404 Cyg, at th…
Multi-wavelength light curves of bright gamma-ray blazars (e.g., 3C 454.3) reveal strong correlations across wavebands, yet striking dissimilarities in the details. This conundrum can be explained if the variable flux and polarization result from both (1) modulation in the magnetic field and relativistic electron content imparted at the jet input and (2) turbulence in the flow. In the Turbulent Extreme Multi-Zone (TEMZ) model being developed by the author, much of the optical and high-energy radiation in a blazar is emitted near the 43 GHz core of the jet as seen in VLBA images, parsecs from t…
We present optical photopolarimetric observations of the BL Lac object S4 0954+658 obtained with the 70-cm telescope in Crimea, 40-cm telescope in St.Petersburg, and 1.8-m Perkins telescope at Lowell Observatory (Flagstaff, Az). After a faint state with a brightness level R ~17.6 mag registered in the first half of January 2011, the optical brightness of the source started to rise and reached ~14.8 mag during the middle of March, showing flare-like behavior. The most spectacular case of intranight variability was observed during the night of 2011 March 9, when the blazar brightened by ~0.7 mag…
PKS 1510-089 (z=0.361) is one of only a handful of flat spectrum radio quasars that have been detected at very high energy (VHE, $E>100\,$GeV) gamma rays. It is a very active source across the entire electromagnetic spectrum. VHE observations in May 2016 with H.E.S.S. and MAGIC revealed an exceptionally strong flare, which lasted for less than two nights, and exhibited a peak flux of about 0.8 times the flux of the Crab Nebula above $200\,$GeV. The flare provides the first evidence of intranight variability at VHE in this source. While optical observations with ATOM reveal a counterpart at opt…
XTE J1818-245 is an X-ray nova that experienced an outburst in 2005, first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft gamma-rays with the INTEGRAL satellite, from 2005 February to September. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. We analyzed radio, (N)IR, optical, X-ray and soft gamma-ray observations and constructed simultaneous broad-band X-ray spectra. Analyzing both the light curves in various energy ranges and the hardness-inten…
We analyze the multifrequency behavior of the quasar 3C 454.3 during three prominent γ-ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The data reveal a repeating pattern, including a triple flare structure, in the properties of each γ-ray outburst, which implies similar mechanism(s) and location for all three events. The multi-frequency behavior indicates that the lower frequency events are co-spatial with the γ-ray outbursts, although the γ-ray emission varies on the shortest timescales. We determine that the variability from UV to IR wavelengths during an outburst results from a s…
1A 0535+262 is a Be X-ray binary pulsar and one of the only galactic pulsar systems to show radio jet emission. Characterizing the very high energy emission (VHE, >100 GeV) in these extreme microquasars is critical to understanding their contribution to the origin of galactic cosmic rays. The 2020 giant outburst of this system, where X-ray fluxes exceeded 12 Crab, marked a rare opportunity to investigate the gamma-ray and rapid optical variability of these transient systems while in such an extreme state. This month of activity marked the brightest flare measured in this system. VERITAS's deve…
We report the Fermi-LAT gamma-ray detection of the 2021 outburst of the symbiotic recurrent nova RS Ophiuchi. In this system, unlike classical novae from cataclysmic binaries, the ejecta from the white dwarf form shocks when interacting with the dense circumstellar wind environment of the red giant companion. We find the LAT spectra from 50 MeV to ~20-23 GeV, the highest-energy photons detected in some sub-intervals, are consistent with $π^{\rm 0}$-decay emission from shocks in the ejecta as proposed by Tatischeff & Hernanz (2007) for its previous 2006 outburst. The LAT light-curve displayed a…
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…
A report is made on the luminosity and pulse-period evolution of the Be binary X-ray pulsar, GX 304$-$1, during a series of outbursts from 2009 to 2013 observed by MAXI/GSC, RXTE/PCA, and Fermi/GBM. In total, twelve outbursts repeated by $\sim$ 132.2 days were observed, which is consistent with the X-ray periodicity of this object observed in the 1970s. These 12 outbursts, together with those in the 1970s, were found to all recur with a well defined period of 132.189$\pm$0.02 d, which can be identified with the orbital period. The pulse period at $\sim 275$ s, obtained from the RXTE/PCA and Fe…
We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar 3C454.3. The model is based on the concept of a relativistic jet interacting with compact gas condensations produced when a star (red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelop lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long, variable on timescales of hours, GeV gamma-ray flare from 3C454.3, observed during November 2010 on top of a week…
We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar \c454. The model is based on the concept of a relativistic jet interacting with compact gas condensations produced when a star (red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelop lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long, variable on timescales of hours, GeV gamma-ray flare from \c454, observed during November 2010 on top of a weeks-lo…
There are insufficient catastrophic events (collapse, explosion or merger of stars or compact objects) to explain the cosmologically local rate of apparently non-repeating FRB if each such catastrophic event produces a single FRB. Unless produced by some novel and unsuspected but comparatively frequent event, apparently non-repeating FRB must actually repeat many times in the lifetimes of their sources. Yet no such infrequent repetitions (in contrast to the frequent activity of FRB known to repeat) have been observed, constraining their repetition rates and active lifetimes. The absence of mor…
The microquasar V404 Cygni (also known as GS 2023+338) was previously reported to have weak GeV gamma-ray emission in sub-day time periods during its 2015 outburst. In order to provide more detailed information at the high energy range for this black hole binary system, we conduct analysis to the data obtained with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi). Both LAT database and source catalog used are the latest. In addition to the previously reported detection at the peak of the 2015 outburst, we find possible detection ($\sim 4σ$) of the source durin…
Binary systems are a well-established subclass of gamma-ray sources. The high mass X-ray binary pulsar 1A~0535+262 has been considered to be a possible gamma-ray emitter for a long time, although former gamma-ray searches using \textit{Fermi}-LAT and VERITAS data resulted in upper limits only. We aim at a deep search for gamma-ray emission and pulsations from 1A~0535+262 using more than 13 years of \textit{Fermi}-LAT data. The analysis was performed for both the whole \textit{Fermi}-LAT data set, as well as for the X-ray outbursts that 1A~0535+262 has experienced since the launch of \textit{Fe…
A prominent outburst of the flat spectrum radio quasar 3C~454.3 was observed in 2014 June with the \emph{Fermi} Large Area Telescope. This outburst was characterized by a three-stage light-curve pattern---plateau, flare and post-flare---that occurred from 2014 May to July, in a similar pattern as observed during the exceptional outburst in 2010 November. The highest flux of the outburst reported in this paper occurred during 2014 June 7--29, showing a multiple-peak structure in the light-curves. The average flux in these 22 days was found to be $F[E > 100~\mathrm{MeV}] = (7.2 \pm 0.2) \times 1…
Since 2001, the radio quasar 3C 454.3 has undergone a period of high optical activity, culminating in the brightest optical state ever observed, during the 2004-2005 outburst. The Whole Earth Blazar Telescope (WEBT) consortium has carried out several multifrequency campaigns to follow the source behaviour. The GLAST-AGILE Support Program (GASP) was born from the WEBT to provide long-term continuous optical-to-radio monitoring of a sample of gamma-loud blazars, during the operation of the AGILE and GLAST (now known as Fermi GST) gamma-ray satellites. The main aim is to shed light on the mechani…
V404 Cygni was discovered in 1989 by the $Ginga$ X-ray satellite during its only previously observed X-ray outburst and soon after confirmed as a black hole binary. On June 15, 2015, the Gamma Ray Burst Monitor (GBM) triggered on a new outburst of V404 Cygni. We present 13 days of GBM observations of this outburst including Earth occultation flux measurements, spectral and temporal analysis. The Earth occultation fluxes reached 30 Crab with detected emission to 100 keV and determined, via hardness ratios, that the source was in a hard state. At high luminosity, spectral analysis between 8 and …
Classical novae are runaway thermonuclear burning events on the surfaces of accreting white dwarfs in close binary star systems, sometimes appearing as new naked-eye sources in the night sky. The standard model of novae predicts that their optical luminosity derives from energy released near the hot white dwarf which is reprocessed through the ejected material. Recent studies with the Fermi Large Area Telescope have shown that many classical novae are accompanied by gigaelectronvolt gamma-ray emission. This emission likely originates from strong shocks, providing new insights into the properti…
We report results of an extensive analysis of the X-ray nova XTE J1859+226 observed with BeppoSAX and RXTE during its 1999 outburst. We modelled the source spectrum with a multicolour blackbody-like feature plus the generic Comptonization model BMC which has the advantage of providing spectral description of the emitted-radiation properties without assumptions on the underlying physical process. The multicolour component is attributed to the geometrically thin accretion disk, while the Comptonization spectrum is claimed to originate in the innermost sub-Keplerian region of the system (transiti…
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope discovered a rapid (about 5 days duration), high-energy (E >100 MeV) gamma-ray outburst from a source identified with the blazar PKS 1502+106 (OR 103, S3 1502+10, z=1.839) starting on August 05, 2008 and followed by bright and variable flux over the next few months. Results on the gamma-ray localization and identification, as well as spectral and temporal behavior during the first months of the Fermi all-sky survey are reported here in conjunction with a multi-waveband characterization as a result of one of the first F…
To observationally study spin-period changes of accreting pulsars caused by the accretion torque, the present work analyzes X-ray light curves of 12 Be binary pulsars obtained by the MAXI/GSC all-sky survey and their pulse periods measured by the Fermi/GBM pulsar project, both covering more than 6 years from 2009 August to 2016 March. The 12 objects were selected because they are accompanied by clear optical identification, and accurate measurements of surface magnetic fields. The luminosity $L$ and the spin-frequency derivatives $\dotν$, measured during large outbursts with $L\gtrsim 1\times …
Giant X-ray outbursts, with luminosities of about $ 10^{37}$ erg s$^{-1}$, are observed roughly every 5 years from the nearby Be/pulsar binary 1A 0535+262. In this article, we present observations of the source with VERITAS at very-high energies (VHE; E$>$100 GeV) triggered by the X-ray outburst in December 2009. The observations started shortly after the onset of the outburst, and they provided comprehensive coverage of the episode, as well as the 111-day binary orbit. No VHE emission is evident at any time. We also examined data from the contemporaneous observations of 1A 0535+262 with the F…
After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from gamma-ray to near-infrared frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive GASP-WEBT collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and …
In this paper we report on the two-year-long Fermi-LAT observation of the peculiar blazar 4C +21.35 (PKS 1222+216). This source was in a quiescent state from the start of science operations of the Fermi Gamma-ray Space Telescope in 2008 August until 2009 September, and then became more active, with gradually increasing flux and some moderately-bright flares. In 2010 April and June, 4C +21.35 underwent a very strong GeV outburst composed of several major flares characterized by rise and decay timescales of the order of a day. During the outburst, the GeV spectra of 4C +21.35 displayed a broken …
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…
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 present the earliest X-ray observations of the 2018 outburst of XTE J1810-197, the first outburst since its 2003 discovery as the prototypical transient and radio-emitting anomalous X-ray pulsar (AXP). The Monitor of All-sky X-ray Image (MAXI) detected XTE J1810-197 immediately after a November 20-26 visibility gap, contemporaneous with its reactivation as a radio pulsar, first observed on December 8. On December 13 the Nuclear Spectroscopic Telescope Array (NUSTAR) detected X-ray emission up to at least 30 keV, with a spectrum well-characterized by a blackbody plus power-law model with tem…
Transient X-ray binaries produce major outbursts in which the X-ray flux can increase over the quiescent level by factors as large as $10^7$. The low-mass X-ray binary V 404 Cyg and the high-mass system 4U 0115+634 underwent such major outbursts in June and October 2015, respectively. We present here observations at energies above hundreds of GeV with the VERITAS observatory taken during some of the brightest X-ray activity ever observed from these systems. No gamma-ray emission has been detected by VERITAS in 2.5 hours of observations of the microquasar V 404 Cyg from 2015, June 20-21. The up…
PSR J1119-6127 is a radio pulsar that behaved magnetar like bursts, and we performed a comprehensive investigation of this pulsar using the archival high-energy observations obtained after its outburst in 2016 July. After the 2016 outburst, specific regions on the neutron star surface were heated up to > 0.3 and > 1 keV from about 0.2 keV. A hard non-thermal spectral component with a photon index < 0.5 related to the magnetospheric emission can be resolved from the NuSTAR spectra above 10 keV. We find that the thermal emitting regions did not cool down and gradually shrank by about (20-35)% fo…
