We present a temporal and spectral analysis of the gamma-ray flux from nine of the brightest flat spectrum radio quasars (FSRQs) detected with the Fermi Large Area Telescope (LAT) during its first eight years of operation, with the aim of constraining the location of the emission region. Using the increased photon statistics provided from the two brightest flares of each source, we find evidence of sub-hour variability from B2 1520+31, PKS 1502+106 and PKS 1424-41, with the remaining sources showing variability on timescales of a few hours. These indicate gamma-ray emission from extremely compact regions in the jet, potentially compatible with emission from within the broad line region (BLR). The flare spectra show evidence of a spectral cut-off in 7 of the 18 flares studied, further supporting the argument for BLR emission in these sources. An investigation into the energy dependence of cooling timescales finds evidence for both BLR origin and emission from within the molecular torus (MT). However, Monte Carlo simulations show that the very high energy (VHE) emission from all sources except 3C 279, 3C 454.3 and 4C 21.35 is incompatible with a BLR origin. The combined findings of all the approaches used suggest that the gamma-ray emission in the brightest FSRQs originates in multiple compact emission regions throughout the jet, within both the BLR and the MT.
We present Chandra ACIS-I X-ray observations of 0FGL J1311.9-3419 and 0FGL J1653.4-0200, the two brightest high Galactic latitude (|b|>10 deg) gamma-ray sources from the 3 month Fermi-LAT bright source list that are still unidentified. Both were also detected previously by EGRET, and despite dedicated multi-wavelength follow-up, they are still not associated with established classes of gamma-ray emitters like pulsars or radio-loud active galactic nuclei. X-ray sources found in the ACIS-I fields of view are catalogued, and their basic properties are determined. These are discussed as candidate …
Recent detection of suborbital gamma-ray variability of Flat Spectrum Radio Quasar (FSRQ) 3C 279 by Fermi Large Area Telescope (LAT) is in severe conflict with established models of blazar emission. This paper presents the results of suborbital analysis of the Fermi/LAT data for the brightest gamma-ray flare of another FSRQ blazar 3C 454.3 in November 2010 (MJD 55516-22). Gamma-ray light curves are calculated for characteristic time bin lengths as short as 3 min. The measured variations of the 0.1-10 GeV photon flux are tested against the hypothesis of steady intraorbit flux. In addition, the …
We present a multi-wavelength study of the FSRQ CTA 102 using Fermi-LAT and simultaneous Swift-XRT/UVOT observations. The Fermi-LAT telescope detected one of the brightest flares from this object during Sep, 2016 to Mar, 2017. In the 190 days of observation period the source underwent four major flares. A detailed analysis of the temporal and spectral properties of these flares indicates the flare at MJD 57751.594 has a $γ$-ray flux of (30.12$\pm$4.48)$\times 10^{-6}$ ph cm$^{-2}$ s$^{-1}$ (from 90 minutes binning) in the energy range of 0.1--300 GeV. This has been found to be the highest flux…
Almost 10 yr of $γ$-ray observations with the Fermi Large Area Telescope (LAT) have revealed extreme $γ$-ray outbursts from flat spectrum radio quasars (FSRQs), temporarily making these objects the brightest $γ$-ray emitters in the sky. Yet, the location and mechanisms of the $γ$-ray emission remain elusive. We characterize long-term $γ$-ray variability and the brightest $γ$-ray flares of six FSRQs. Consecutively zooming in on the brightest flares, which we identify in an objective way through Bayesian blocks and a hill-climbing algorithm, we find variability on subhour time scales and as shor…
In an attempt to constrain and understand the emission mechanism of gamma rays, we perform a cross-correlation analysis of 15 blazars using light curves in millimetre, optical and gamma rays. We use discrete correlation function and consider only correlations significant at 99 per cent level. A strong correlation was found between 37 and 95 GHz with a near-zero time delay in most of the sources, and ~1 month or longer in the rest. A similar result was obtained between the optical and gamma-ray bands. Of the 15 sources, less than 50 per cent showed a strong correlation between the millimetre an…
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…
We use the complete MOJAVE 1.5 Jy sample of active galactic nuclei (AGN) to examine the gamma-ray detection statistics of the brightest radio-loud blazars in the northern sky. We find that 23% of these AGN were not detected above 0.1 GeV by the Fermi LAT during the 4-year 3FGL catalog period partly because of an instrumental selection effect, and partly due to their lower Doppler boosting factors. Blazars with synchrotron peaks in their spectral energy distributions located below $10^{13.4}$ Hz also tend to have high-energy peaks that lie below the 0.1 GeV threshold of the LAT, and are thus le…
We address the highly debated issue of constraining the gamma-ray emission region in blazars from cross-correlation analysis using discrete correlation function between radio and gamma-ray light curves. The significance of the correlations is evaluated using two different approaches: simulating light curves and mixed source correlations. The cross-correlation analysis yielded 26 sources with significant correlations. In most of the sources, the gamma-ray peaks lead the radio with time lags in the range +20 and +690 days, whereas in sources 1633+382 and 3C 345 we find the radio emission to lead…
Searching for as yet undetected gamma-ray sources is a major target of the Fermi LAT Collaboration. We present an algorithm capable of identifying such type of sources by non-parametrically clustering the directions of arrival of the high-energy photons detected by the telescope onboard the Fermi spacecraft. n particular, the sources will be identified using a von Mises-Fisher kernel estimate of the photon count density on the unit sphere via an adjustment of the mean-shift algorithm to account for the directional nature of data. This choice entails a number of desirable benefits. It allows us…
We present a high time resolution study of the two brightest $γ$-ray outbursts from a blazar PKS 1222+216 observed by the \textit{Fermi} Large Area Telescope (LAT) in 2010. The $γ$-ray light-curves obtained in four different energy bands: 0.1--3, 0.1--0.3, 0.3--1 and 1--3 GeV, with time bin of 6 hr, show asymmetric profiles with a similar rise time in all the bands but a rapid decline during the April flare and a gradual one during the June. The light-curves during the April flare show $\sim 2$ days long plateau in 0.1--0.3 GeV emission, erratic variations in 0.3--1 GeV emission, and a daily r…
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…
I present a study of the high-energy gamma-ray properties of the flat spectrum radio quasar, PKS 1510-089, based on 3.75 years of observations with the Large Area Telescope (LAT) detector on-board the Fermi gamma-ray Space Telescope. Throughout the observing period, the 0.1 GeV < E < 300 GeV gamma-ray flux was highly variable, undergoing several flaring events where the daily flux exceeded 10^{-5} photons cm^{-2} s^{-1} on 3 separate occasions. The increased photon statistics of these large flares allowed the observations to be re-analysed in 6 and 3 hour intervals, revealing flux doubling tim…
We study GRB 221009A, the brightest gamma-ray burst in the history of observations, using Fermi data. To calibrate them for large inclination angles, we use the Vela X gamma-ray source. Light curves in different spectral ranges demonstrate a 300 s overlap of afterglow and delayed episodes of soft prompt emission. We demonstrate that a relatively weak burst precursor that occurs 3 minutes before the main episode has its own afterglow, i.e., presumably, its own external shock. The main afterglow is the brightest one, includes a photon with an energy of 400 GeV 9 hours after the burst, and is vis…
We present the results of the $γ$-ray flux distribution study on the brightest blazars which are observed by the \emph{Fermi}-LAT. We selected 50 brightest blazars based on the maximum number of detection reported in the LAT third AGN catalog. We performed standard unbinned maximum likelihood analysis on the LAT data during the period between August 2008 and December 2016, in order to obtain the average monthly flux. After quality cuts, blazars for which at least 90\% of the total flux was survived were selected for the further study, and this includes 19 FSRQs and 19 BL Lacs. The Anderson-Dar…
We carried a detailed spectral and temporal study of blazar, \mbox{PKS\,0903-57} using the \emph{Fermi}-LAT and \emph{Swift}-XRT/UVOT observations, during its brightest flaring period MJD\,58931--58970. During this period, the maximum daily averaged $γ$-ray flux ($\rm F_{0.1-500\,GeV}$) of $\rm9.42\times10^{-6}\,ph\,cm^{-2}\,s^{-1}$ is observed on MJD\,58951.5, the highest $γ$-ray flux detected from \mbox{PKS\,0903-57} till now. Several high-energy (HE) photons ($>\,10$\,GeV) consistent with the source location at high probability (>\,99\%) are detected, and the $γ$-ray lightcurve in the activ…
One of the most energetic gamma-ray burst GRB 110731A was observed from optical to GeV energy range. Previous analysis on the prompt phase revealed similarities with the Large Area Telescope (LAT) bursts observed by Fermi: i) a delayed onset of the high-energy emission ($> 100$ MeV), ii) a short-lasting bright peak at later times and iii) a temporally extended component from this phase and lasting hundreds of seconds. Additionally to the prompt phase, multiwavelength observations over different epochs showed that the spectral energy distribution was better fitted by a wind afterglow model. We …
I present a systematic study of gamma-ray flares in blazars. For this purpose, I propose a very simple and practical definition of a flare as a period of time, associated with a given flux peak, during which the flux is above half of the peak flux. I select a sample of 40 brightest gamma-ray flares observed by Fermi/LAT during the first 4 years of its mission. The sample is dominated by 4 blazars: 3C 454.3, PKS 1510-089, PKS 1222+216 and 3C 273. For each flare, I calculate a light curve and variations of the photon index. For the whole sample, I study the distributions of the peak flux, peak l…
In this paper some of the brightest GeV sources observed by the Fermi-LAT were analysed, focusing on their spectral cut-off region. The sources chosen for this investigation were the brightest blazar flares of 3C~454.3 and 3C~279 and the Vela pulsar with a reanalysis with the latest Fermi-LAT software. For the study of the spectral cut-off we first explored the Vela pulsar spectrum, whose statistics in the time interval of the 3FGL catalog allowed strong constraints to be obtained on the parameters. We subsequently performed a new analysis of the flaring blazar SEDs. For these sources we obtai…
One of the brightest regions of diffuse gamma-ray emission in the northern sky is the Cygnus star-forming region, where one can assume the most energetic processes are taking place. The Dragonfly Nebula (MGRO J2019+37) is one of the brightest sources in the Cygnus region. First discovered by MILAGRO, it was later resolved into two sources by VERITAS: the faint point-like source VER J2016+371 and the bright extended source VER J2019+368. The latter accounts for the bulk of the MGRO J2019+37 emission, with the spectrum among the hardest in the TeV range. We report the results of a dedicated MAGI…
The Large Area Telescope (LAT) on-board the Fermi satellite detected emission above 20 MeV only in a small fraction of the long gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Burst Monitor (GBM) at 8 keV-40 MeV. Those bursts that were detected by the LAT were among the brightest GBM bursts. The emission detected by LAT seerms to be delayed respect to the one detected by the GBM. In this review I will show the main implications of these LAT observations on the GRB models and discuss the importance of a synergy between Fermi and other telescopes.
An exceptional $γ$-ray outburst from 3C 279 is detected by {\it Fermi}-Large Area Telescope (LAT) in 2015 June. In the energy range of 0.1$-$300 GeV, the highest flux measured is (39.1$\pm$2.5) $\times$ 10$^{-6}$ \phflux, which is the highest $γ$-ray flux ever detected from 3C 279, exceeding the previous historically brightest flare observed by {\it EGRET} in 1996. The high activity period consists of three major flares with the last one being the brightest. All but one flares show a faster rise and slower decay pattern and at the peak of the activity, the $γ$-ray spectrum is found to show a c…
Locating the missing baryons in the warm-hot intergalactic medium with fast radio bursts and the Sunyaev-Zel'dovich effect
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（教育部现代天文与天体物理重点实验室（南京大学））；Department of Astronomy, School of Physical Sciences, University of Science and Technology of China（中国科学技术大学物理科学学院天文学系）；Center for Cosmology and Computational Astrophysics, Zhejiang University（浙江大学宇宙学与计算天体物理中心）
Galactic center (GC) is expected to be the brightest source of possible dark matter (DM) annihilation signal. Excess gamma-ray emission has been detected by several groups. Both DM and more conventional astrophysical explanations of the excess have been proposed. In this report, we discuss possible effects of modeling the Fermi bubbles at low latitudes on the GC excess. We consider a template of the Fermi bubbles at low latitudes derived by assuming that the spectrum between 1 GeV and 10 GeV at low latitudes is the same as at high latitudes. We argue that the presence of the Fermi bubbles near…
The late-time high-resolution X-ray and radio observations of GRB980425/SN1998bw, the closest known gamma ray burst (GRB) associated with a supernova (SN) explosion, may have actually resolved the hyperluminal source that produced the GRB and its afterglow. Its hyperluminal speed ~350c is consistent with that expected in the cannonball (CB) model of GRBs. The observed superluminal expansion of the late-time radio image of GRB030329/SN2003dh, the GRB with the brightest and longest followed up radio afterglow to date, is also consistent with that expected in the CB model of GRBs and extrapolates…
We present here our results of the temporal and spectral analysis of a sample of 52 bright and hard gamma-ray bursts (GRBs) observed with the Fermi Gamma-ray Burst Monitor (GBM) during its first year of operation (July 2008-July 2009). Our sample was selected from a total of 253 GBM GRBs based on each event peak count rate measured between 0.2 and 40MeV. The final sample comprised 34 long and 18 short GRBs. These numbers show that the GBM sample contains a much larger fraction of short GRBs, than the CGRO/BATSE data set, which we explain as the result of our (different) selection criteria and …
We present the results of a search for high-energy gamma-ray emission from a large sample of galaxy clusters sharing the properties of three existing Fermi-LAT detections (in Perseus, Virgo and Abell 3392), namely a powerful radio source within their brightest cluster galaxy (BCG). From a parent, X-ray flux-limited sample of clusters, we select 114 systems with a core-dominated BCG radio flux above 50 or 75 mJy, stacking data from the first 45 months of the Fermi mission, to determine statistical limits on the gamma-ray fluxes of the ensemble of candidate sources. For a >300 MeV selection, the…
Shocks in gamma-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the brightest GeV gamma-ray emission to date. The nova is detected in hard X-rays while it is still gamma-ray bright, but contrary to simple theoretical expectations, the detected 3.5-78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the gamma-rays are likely had…
Observations of high energy emission from gamma-ray bursts (GRBs) constrain the extreme physical conditions associated with these energetic cosmic explosions. The Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope, a pair conversion telescope, observes energetic quanta from 30 MeV to > 300 GeV. Synergy of the LAT with the Gamma-ray Burst Monitor (GBM) enlarges the energy coverage to ~ 7.5 decades, very useful for studying the GRB emission itself. Prompt optical observations and their possible correlations with photon emission at LAT energies help our understanding of the ph…
The Large Area Telescope (LAT), Fermi's main instrument, is providing a new view of the local energetic pulsar population. In addition to identifying a pulsar origin of a large fraction of the bright unidentified Galactic EGRET sources, the LAT results provide a great opportunity to study a sizable population of high-energy pulsars. Correlations of their physical properties, such as the trend of the luminosity versus the rotational energy loss rate, help identify global features of the gamma-ray pulsar population. Several lines of evidence, including the light curve and spectral features, sugg…
Although not designed primarily as a polarimeter, the \textit{Fermi}-Large Area Telescope (LAT) has the potential to detect high degrees of linear polarization from some of the brightest gamma-ray sources. To achieve the needed accuracy in the reconstruction of the event geometry, low-energy ($\leq200$ MeV) events converting in the silicon detector layers of the LAT tracker have to be used. We present preliminary results of the ongoing effort within the LAT collaboration to measure gamma-ray polarization. We discuss the statistical and systematic uncertainties affecting such a measurement. We …
Supernova (SN) 1987A is the only young SN in which H_2 has been detected in the ejecta. The properties of the H_2 are important for understanding the explosion and the ejecta chemistry. Here, we present new VLT/SINFONI observations of H_2 in SN 1987A, focussing on the 2.12 μm (1,0)S(1) line. We find that the 3D emissivity is dominated by a single clump in the southern ejecta, with weaker emission being present in the north along the plane of the circumstellar ring. The lowest observed velocities are in the range 400-800 km/s, in agreement with previous limits on inward mixing of H. The brighte…
This paper presents light curves and the first systematic characterization of variability of the 106 objects in the Fermi Large Area Telescope (LAT) Bright AGN Sample (LBAS). Weekly light curves obtained during the first 11 months of survey (August 04, 2008 - July 04, 2009), are tested for variability, and their properties are quantified through autocorrelation and structure function analysis. For the brightest sources power density spectra (PDS) and fit of the temporal structure of major flares is performed. More than 50% of the sources are variable, where high states do not exceed 1/4 of the…
The brightest blazars detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (Fermi/LAT) show significant breaks in their spectra at a few GeV. The sharpness and the position of the breaks can be well reproduced by absorption of $γ$-rays via photon--photon pair production on He ii and H i Lyman recombination continua (LyC) produced in the broad-line region (BLR). Using 138 weeks of LAT observations of the brightest GeV blazar 3C 454.3 we find a power-law dependence of the peak energy on flux and discover anti-correlation between flux and the column density of the He ii LyC which …
The past decade has seen a dramatic improvement in the quality of data available at both high (HE: 100 MeV to 100 GeV) and very high (VHE: 100 GeV to 100 TeV) gamma-ray energies. With three years of data from the Fermi Large Area Telescope (LAT) and deep pointed observations with arrays of Cherenkov telescope, continuous spectral coverage from 100 MeV to $\sim10$ TeV exists for the first time for the brightest gamma-ray sources. The Fermi-LAT is likely to continue for several years, resulting in significant improvements in high energy sensitivity. On the same timescale, the Cherenkov Telescope…
