In examining a select sample of over 200 blazars of known jet kinetic power (L_kin) and well-characterized SEDs, we found (Meyer et al., 2011) that Intermediate synchrotron-peaking (ISP) blazars may have lower gamma-ray output than high synchrotron-peaking (HSP) blazars of similar L_kin, consistent with our hypothesis that ISP blazars are less-beamed versions of HSP blazars, rather than a distinct population. Further, by using the radio core dominance as a measure of relative beaming, we find that gamma-ray luminosity depends on beaming in a consistent way for blazars ranging over all jet kinetic powers (10^42 - 10^46 ergs/s). We re-examine the gamma-ray properties of this core sample of blazars using the 1-year LAT catalog (Abdo et al., 2010). We find that for weak jets, the ratio of inverse Compton to synchrotron emission remains constant with increased beaming, consistent with an SSC model for the jet emission, while the most powerful jets show a strong increase in Compton dominance with orientation, consistent with an external Compton (EC) emission model.
Current models suggest gamma-ray bursts could be used as a way of probing Population III stars - the first stars in the early Universe. In this paper we use numerical simulations to demonstrate that late time radio observations of gamma-ray burst afterglows could provide a means of identifying bursts that originate from Population III stars, if these were highly massive, independently from their redshift. We then present the results from a pilot study using the Australia Telescope Compact Array at 17 GHz, designed to test the hypothesis that there may be Population III gamma-ray bursts amongst…
The quasi-simultaneous multi-wavelength emission of TeV blazar 3C 66A is studied by using a one-zone multi-component leptonic jet model. It is found that the quasi-simultaneous spectral energy distribution (SED) of 3C 66A can be well reproduced, especially its {\it Fermi}-LAT first 3 months average spectrum can be well reproduced by the synchrotron-self Compton (SSC) component plus external Compton (EC) component of the broad line region (BLR). Clues on its redshift and gamma-ray emission location are obtained. The results indicate the following. (i) On the redshift; The theoretical intrinsic …
If gamma-ray bursts are sources of ultra-high energy cosmic rays, then radiative signatures of hadronic acceleration are expected in GRB data. Observations with the Fermi Gamma-ray Space Telescope offer the best means to search for evidence of UHECRs in GRBs through electromagnetic channels. Various issues related to UHECR acceleration in GRBs are reviewed, with a focus on the question of energetics.
Misaligned Active Galactic Nuclei (MAGNs), i.e., radio galaxies and quasars with the jet not directly pointing at the observer, are a new class of GeV emitters. In low power radio galaxies (i.e., FRIs), gamma-rays are mainly produced in compact jet regions, although in at least one case, Centaurus A, high energy photons from the radio lobes have been also observed. The first localization of the gamma-ray dissipation zone in a high power radio galaxy (i.e., FRII) excludes major contributions from extended regions. The study of the FRII source 3C111 indicates that gamma-ray photons are produced …
The physical origin of the >0.1 GeV emission detected from Gamma-Ray Bursts (GRBs) by the Fermi satellite has not yet been completely understood. In this work we consider the GeV light curves of ten GRBs with measured redshift detected by the Fermi-LAT. These light curves are characterised by a long-lived ($\gtrsim10^2$ seconds) emission, whose luminosity decays in time as a power-law. While the decay rate is similar for all GRBs (i.e. $L_{LAT}\propto t^{-1.2}$), the normalisation spans about two orders of magnitude in luminosity. However, after re-normalising the luminosities to the prompt en…
GRB 121027A is un-usual with its extremely long-lasting, energetic X-ray flares. The total energy release in X-ray flares is about one order of magnitude higher than prompt gamma-rays, making it special from most long GRBs. We show that while the prompt gamma-ray emission satisfies the empirical E_{iso}-E_{p} relation of typical long GRBs, the X-ray flares, whose spectra can be fit with a cutoff-power-law model with well-constrained E_p, significantly deviate from such a relation. Nonetheless, a time-resolved spectral analysis of X-ray flares suggest that the X-ray emission is consistent with …
The prompt emission of short gamma ray bursts (sGRBs) with known redshifts are analyzed using the model of multi-color blackbody which is interpreted as the emission from a non-dissipative photosphere taking into account a power law jet structure and the viewing geometry of the jet. We find nearly $69\%$ and $26\%$ of the sample are consistent with multi-color blackbody and a pure blackbody model, respectively. Using this interpretation, we infer that nearly $57\% \, (18\%)$ of the sGRBs in our sample are observed within (or along the edge of) the jet core. The sGRB jets are deduced to possess…
Neutron Star Merger Rates from Multi-messenger Observations: Clues to the Physical Origin of the Short and Long-short Gamma-ray Bursts
Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）；School of Astronomy and Space Science, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
We present the analysis of a large sample of gamma-ray burst (GRB) X-ray light curves in the rest frame to characterise their intrinsic properties in the context of different theoretical scenarios. We determine the morphology, time scales, and energetics of 64 long GRBs observed by \emph{Swift}/XRT \emph{without} flaring activity. We furthermore provide a one-to-one comparison to the properties of GRBs \emph{with} X-ray flares. We find that the steep decay morphology and its connection with X-ray flares favour a scenario in which a central engine origin. We show that this scenario can also acc…
The origin of GeV emission from the early epoch of gamma-ray bursts (GRBs) is a widely discussed issue. The long gamma-ray burst GRB 170405A, observed by the Fermi Gamma-ray Space Telescope showed high-energy emission delayed by $\sim$20 s with respect to the X-ray emission, followed by temporally fading gamma-ray emission lasting for $\sim$1,000 s, as commonly observed in high-energy GRBs. In addition, a high-energy spectral cutoff at $\sim$50 MeV was detected in the prompt emission phase. If this feature is caused by pair-production opacity, the bulk Lorentz factor of the GRB ejecta can be e…
The recent detection of gravitational waves and electromagnetic counterparts from the double neutron star merger event GW+EM170817, supports the standard paradigm of short gamma-ray bursts (SGRBs) and kilonovae/macronovae. It is important to reveal the nature of the compact remnant left after the merger, either a black hole or neutron star, and their physical link to the origin of the long-lasting emission observed in SGRBs. The diversity of the merger remnants may also lead to different kinds of transients that can be detected in future. Here we study the high-energy emission from the long-la…
Analyzing light curves of a complete sample of bright Swift long gamma-ray bursts (LGRBs) of which the peak photon fluxes constructed with the bin width of 1 second in the Swift 15-350 keV energy band exceed 2.6 photons cm$^{-2}$s$^{-1}$, we confirm that there do exist the third class in GRBs in addition to short and long GRBs. Being different from previous works based on the duration, fluence, etc. our classification method is based on two properties both quantified with light curve shapes of the prompt emission: the Absolute Deviation from the Constant Luminosity of their cumulative light cu…
We present all-sky simulated Fermi maps of gamma-rays from dark matter decay and annihilation in the Local Universe. The dark matter distribution is obtained from a constrained cosmological simulation of the neighboring large-scale structure provided by the CLUES project. The dark matter fields of density and density squared are then taken as an input for the Fermi observation simulation tool to predict the gamma-ray photon counts that Fermi would detect in 5 years of all-sky survey for given dark matter models. Signal-to-noise sky maps have also been obtained by adopting the current Galactic …
The Globular Cluster 47 Tucanae was firstly detected in gamma-rays by the Large Area Telescope (LAT) onboard the \emph{Fermi} Gamma-ray Space Telescope, and the gamma-ray emission has been widely attributed to the millisecond pulsars. In this work, we analyze the Fermi-LAT pass 8 data ranging from 2008 August to 2017 May and report the detection of a modulation with a period of $18.416\pm0.008$ hours at a significance level of $\sim 4.8σ$. This is the first time to detect a significant modulation with a period much longer than that of millisecond pulsars in gamma-rays from Globular Clusters. T…
The pulsed radiation from PSR B0833-45 (Vela) has a phased-averaged spectral energy distribution (SED) of an apparently simple structure across a wide energy range, from optical light to hard gamma-rays. However, the Vela pulses in narrow energy bands reveal astonishing complexity of the directional pattern of the radiation. These pulses are, therefore, a unique clue to the underlying radiative processes. We present the results of a 3D modeling of the Vela radiation properties within an outer-gap scenario. We show how the inverse Compton scattering of photons by primary and secondary particles…
Prompt γ-ray emissions from gamma-ray bursts (GRBs) exhibit a vast range of extremely complex temporal structures with a typical variability time-scale significantly short - as fast as milliseconds. This work aims to investigate the apparent randomness of the GRB time profiles making extensive use of nonlinear techniques combining the advanced spectral method of the Singular Spectrum Analysis (SSA) with the classical tools provided by the Chaos Theory. Despite their morphological complexity, we detect evidence of a non stochastic short-term variability during the overall burst duration - seemi…
A thermal component is suggested to be the physical composition of the ejecta of several bright gamma-ray bursts (GRBs). Such a thermal component is discovered in the time-integrated spectra of several short GRBs as well as long GRBs. In this work, we present a comprehensive analysis of ten very short GRBs detected by Fermi Gamma-Ray Burst Monitor to search for the thermal component. We found that both the resultant low-energy spectral index and the peak energy in each GRB imply a common hard spectral feature, which is in favor of the main classification of the short/hard versus long/soft dich…
The Fermi Large Area Telescope (Fermi LAT) provides long term systematic monitoring observations of the gamma-ray emission from blazars. The variability properties and the correlation with other wavelength bands are important clues for the evaluation of blazar models. We present results from timing and multiwavelength correlation analysis and discuss differences between blazar classes.
Very high energy gamma-ray observations offer indirect methods for studying the highest energy cosmic rays in our Universe. The origin of cosmic rays at energies greater than $10^{18}$ eV remains a mystery, and many questions in particle astrophysics exist. The VERITAS observatory in southern Arizona, USA, carries out an extensive observation program of the gamma-ray sky at energies above 85 GeV. Observations of Galactic and extragalactic sources in the TeV band provide clues to the highly energetic processes occurring in these objects, and could provide indirect evidence for the origin of cos…
We investigate the vertical structure and elements distribution of neutrino-dominated accretion flows around black holes in spherical coordinates with the reasonable nuclear statistical equilibrium. According our calculations, heavy nuclei tend to be produced in a thin region near the disk surface, whose mass fractions are primarily determined by the accretion rate and the vertical distribution of temperature and density. In this thin region, we find that $^{56}\rm Ni$ is dominant for the flow with low accretion rate (e.g., $0.05 $$M_{\odot}$ $\rm s^{-1}$) but $^{56}\rm Fe$ is dominant for the…
The gravitational wave (GW) memory from a radiating and decelerating point mass is studied in detail. It is found that for isotropic photon emission the memory generated from the photons is essentially the same with the memory from the point mass that radiated the photons so that it is anti-beamed. On the other hand, for anisotropic emission the memory from the photons may have a non-vanishing amplitude even if it is seen with small viewing angles. In the decelerating phases of gamma-ray burst (GRB) jets the kinetic energy of the jet is converted into the energy of gamma-ray photons. Then it w…
VERITAS is an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes located in southern Arizona. Its aim is to study the very high energy (VHE: E > 100 GeV) gamma-ray emission from astrophysical objects. The study of Active Galactic Nuclei (AGN) is intensively pursued through the VERITAS blazar key science project, but also through the large MWL observational campaigns on radio galaxies. The successful VERITAS AGN research program has provided insights to the jet inner structures and started a more detailed classification of blazars. Moreover, the synergy between Fermi and VERIT…
In our search for gamma-ray emission from Be X-ray binaries from analysis of the data obtained with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope, we find likely detection of GRO J1008-57. The binary has an orbital period of 249.48 days, and it is only significantly detected in its orbital phase 0.8--0.9 (>4 sigma). Further light curve analysis indicates that the detection is probably largely due to an emitting event in one orbital cycle around year 2012--2013, following a giant X-ray outburst of the source. This property of having occasional gamma-ray emitting ev…
The optical light that is generated simultaneously with the x-rays and gamma-rays during a gamma-ray burst (GRB) provides clues about the nature of the explosions that occur as massive stars collapse to form black holes. We report on the bright optical flash and fading afterglow from the powerful burst GRB 130427A and present a comparison with the properties of the gamma-ray emission that show correlation of the optical and >100 MeV photon flux light curves during the first 7,000 seconds. We attribute this correlation to co-generation in an external shock. The simultaneous, multi-color, optica…
It is shown that the contribution of faint gamma-ray bursts (GRB) to the total allsky GRB energy flux is small. The allsky flux of GRB appears to be $5.3 \times 10^{-3}$ erg/$\rm cm^2 y$, with little additional component hidden within weak or otherwise undetectable GRB. This significantly constrains physical models of GRB and dirty fireballs, suggesting a rather sharp dichotomy between them should the latter exist.
Multi-messenger observations of GW170817 have not conclusively established whether the merger remnant is a black hole (BH) or a neutron star (NS). We show that a long-lived magnetized NS with a poloidal field $B\approx 10^{12}$G is fully consistent with the electromagnetic dataset, when spin down losses are dominated by gravitational wave (GW) emission. The required ellipticity $ε\gtrsim 10^{-5}$ can result from a toroidal magnetic field component much stronger than the poloidal component, a configuration expected from a NS newly formed from a merger. Abrupt magnetic dissipation of the toroida…
Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) detected the gamma-ray afterglow of GRB 190114C, which can constrain microscopic parameters of the shock-heated plasma emitting non-thermal emission. Focusing on the early afterglow of this event, we numerically simulate the spectrum and multi-wavelength light curves with constant and wind-like circumstellar medium using a time-dependent code. Our results show that the electron acceleration timescale at the highest energies is likely shorter than 20 times the gyroperiod to reproduce the GeV gamma-ray flux and its spectral index repor…
The identification of low-energy counterparts for gamma-ray sources is one of the biggest challenge in modern gamma-ray astronomy. Recently, we developed and successfully applied a new association method to recognize gamma-ray blazar candidates that could be possible counterparts for the unidentified gamma-ray sources above 100 MeV in the second Fermi Large Area Telescope (LAT) catalog (2FGL). This method is based on the Infrared (IR) colors of the recent Wide-Field Infrared Survey Explorer (WISE) all-sky survey. In this letter we applied our new association method to the case of unidentified …
The gravitational wave event GW170817 and the slowly-rising afterglows of short gamma-ray burst GRB 170817A clearly suggest that the GRB jet has an angular structure. However the actual jet structure remains unclear as different authors give different structures. We formulate a novel method to inversely reconstruct the jet structure from off-axis GRB afterglows, without assuming any functional form of the structure in contrast to the previous studies. The jet structure is uniquely determined from the rising part of a light curve for a given parameter set by integrating an ordinary differential…
Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic-ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. In this paper, we analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. In particular, we investigate the characteristics of the bright pulsar PSR J1023-5746 that dominates the gamma-ray emission below a few GeV at the position of Westerlund 2, and th…
Short gamma-ray bursts (SGRBs) are now known to be the product of the merger of two compact objects. However, two possible formation channels exist: neutron star -- neutron star (NS -- NS) or NS -- black hole (BH). The landmark SGRB 170817A provided evidence for the NS -- NS channel, thanks to analysis of its gravitational wave signal. We investigate the complete population of SGRBs with an associated redshift (39 events), and search for any divisions that may indicate that a NS -- BH formation channel also contributes. Though no conclusive dichotomy is found, we find several lines of evidence…
A clue to finding the long-sought sources of cosmic rays is the recent observation of an astrophysical flux of high-energy neutrinos by the IceCube detector, since these possibly originate in hadronic interactions at cosmic-ray accelerators. While the neutrino sky map shows no significant indication of point sources so far, it is possible to utilize the sensitivity of Imaging Air Cherenkov Telescope (IACT) arrays, such as VERITAS, to search for hadronic gamma-ray emission from the neutrino directions. Over the last 2 years, the positions of neutrino events detected by IceCube have been observe…
The Origin of Cross-Energy-Similar FRED Profiles in Gamma-Ray Bursts Pulses
State Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所粒子天体物理国家重点实验室）；University of Chinese Academy of Sciences（中国科学院大学）；Department of Physics and Earth Sciences, University of Ferrara（费拉拉大学地球科学系）；INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna（意大利国家天体物理研究所博洛尼亚天文与空间科学观测站）；School of Physics and Electronic Science, Guizhou Normal University（贵州师范大学物理与电子科学学院）
The High Energy Stereoscopic System (H.E.S.S.) has revealed many TeV (10 12 eV) gamma-ray sources along the Galactic Plane and around 30 percent of these sources remain unidentified. The morphology and dynamics of dense gas coincident and surrounding the gamma-ray emission can provide clues about the nature of the TeV emission. The H2O Southern Galactic Plane Survey (HOPS) undertaken with the Mopra radio telescope, includes several dense gas tracers, such as NH3 (n,n) transitions and HC3N (3-2), star formation tracers including H2O masers, and radio recombination lines which trace ionised gas.…
The Fermi bubbles are gigantic gamma-ray structures in our Galaxy. The physical origin of the bubbles is still under debate. The leading scenarios can be divided into two categories. One is the nuclear star forming activity similar to extragalactic starburst galaxies and the other is the past active galactic nucleus (AGN) like activity of the Galactic center supermassive black hole. In this paper, we propose that metal abundance measurements will provide an important clue to probe their origin. Based on a simple spherically symmetric bubble model, we find that the generated metallicity and abu…
The new and extreme population of GRBs detected by Fermi-LAT shows several new features in high energy gamma-rays that are providing interesting and unexpected clues into GRB prompt and afterglow emission mechanisms. Over the last 6 years, it has been Swift that has provided the robust dataset of UV/optical and X-ray afterglow observations that opened many windows into components of GRB emission structure. The relationship between the LAT detected GRBs and the well studied, fainter, less energetic GRBs detected by Swift-BAT is only beginning to be explored by multi-wavelength studies. We explo…
The BL Lac object 1ES 0229+200 (z=0.14) has been detected by HESS during observations taking place in 2005-2006. The TeV spectrum, when corrected for the absorption of gamma-ray photons through the interaction with the extragalactic background light, is extremely hard, even if the most conservative level for the background is considered. The case of 1ES 0229+200 is very similar to that of 1ES 1101-232, for which a possible explanation, in the framework of the standard one-zone synchrotron-self Compton model, is that the high-energy emission is synchrotron-self Compton radiation of electrons di…
X-ray flares detected in nearly half of gamma-ray burst (GRB) afterglows are one of the most intriguing phenomena in high-energy astrophysics. All the observations indicate that the central engines of bursts, after the gamma-ray emission has ended, still have long periods of activity, during which energetic explosions eject relativistic materials, leading to late-time X-ray emission. It is thus expected that X-ray flares provide important clues to the nature of the central engines of GRBs, and more importantly, unveil the physical mechanism of the flares themselves, which has so far remained m…
The most energetic gamma-ray bursts (GRBs) are remarkable sources releasing huge amounts of energy on short timescales. Their prompt emission, which usually lasts few seconds, is so bright that it is visible across the whole observable universe. Studying these extreme events may provide clues on the nature of GRB progenitors and on the physical processes at work in relativistic jets. In this paper, we study the bright end of the isotropic energy distribution of long GRBs. We use two samples of long GRBs with redshift detected by Fermi/GBM or Konus-Wind, two instruments which measure the spectr…
Future missions for long gammma-ray burst (GRB) observations at high redshift such as HiZ-GUNDAM and THESEUS will provide clue to the star formation history in our universe. In this paper focusing on high redshift (z>8) GRBs, we calculate the detection rate of long GRBs by future observations, considering both Population (Pop) I&II stars and Pop III stars as GRB progenitors. For the Pop I&II star formation rate (SFR), we adopt an up-to-date model of high-redshift SFR based on the halo mass function and dark matter accretion rate obtained from cosmological simulations. We show that the Pop I&II…
For decades, gamma-ray bursts (GRBs) have been broadly divided into `long'- and `short'-duration bursts, lasting more or less than 2s, respectively. However, this dichotomy does not map perfectly to the two progenitor channels that are known to produce GRBs -- the merger of compact objects (merger-GRBs) or the collapse of massive stars (collapsar-GRBs). In particular, the merger-GRBs population may also include bursts with a short, hard $\lesssim$2s spike and subsequent longer, softer extended emission (EE). The recent discovery of a kilonova -- the radioactive glow of heavy elements made in n…
Optical re-brightenings in the afterglows of some gamma-ray bursts (GRBs) are unexpected within the framework of the simple external shock model. While it has been suggested that the central engines of some GRBs are newly born magnetars, we aim to relate the behaviors of magnetars to the optical re-brightenings. A newly born magnetar will lose its rotational energy in the form of Poynting-flux, which may be converted into a wind of electron-positron pairs through some magnetic dissipation processes. As proposed by Dai (2004), this wind will catch up with the GRB outflow and a long-lasting reve…
Despite their different nature and physics, blazars and gamma-ray bursts have in common very powerful relativistic jets, which make them the most luminous sources in the Universe. The energy extraction from the central compact object, the jet collimation, the role and geometry of the magnetic fields, the structure of the jet itself represent still big enough questions, that a complete paradigm cannot yet be drawn. This article is concerned with the main observational facts about blazars and gamma-ray burst jets, based on multi-wavelength campaigns, and on the clues one can glean from these on …
The prompt Gamma-Ray Bursts' (GRBs) efficiency is an important clue on the emission mechanism producing the $γ$-rays. Previous estimates of the kinetic energy of the blast waves, based on the X-ray afterglow luminosity $L_X$, suggested that this efficiency is large, with values above 90\% in some cases. This poses a problem to emission mechanisms and in particular to the internal shocks model. These estimates are based, however, on the assumption that the X-ray emitting electrons are fast cooling and that their Inverse Compton (IC) losses are negligible. The observed correlations between $L_X$…
The Fermi bubbles (FBs) are large gamma-ray emitting lobes extending up to $55°$ in latitude above and below the Galactic center (GC). Although the FBs were discovered 8 years ago, their origin and the nature of the gamma-ray emission are still unresolved. Understanding the properties of the FBs near the Galactic plane may provide a clue to their origin. Previous analyses of the gamma-ray emission at the base of the FBs, what remains after subtraction of Galactic foregrounds, have shown an increased intensity compared to the FBs at high latitudes, a hard power-law spectrum without evidence of …
A large sample of known and likely gamma-ray blazars has been monitored twice per week since late 2007 at 15 GHz with the Owens Valley Radio Observatory (OVRO) 40-meter Telescope. The sample contains about 1700 sources, including the initial sample of 1158 sources above declination -20 degrees from the Candidate Gamma-Ray Blazar Survey (CGRaBS) plus all the blazars associated with Fermi-LAT detections as released in the Fermi AGN catalogs. Using statistical likelihood analyses, we compare the variability amplitude for various sub-populations within our sample. These include comparisons of gamm…
Gravitational waves from a merger of two neutron stars (NSs) were discovered for the first time in GW170817, together with diverse electromagnetic counterparts, providing a direct clue to the origin of short gamma-ray bursts (sGRBs). The associated sGRB 170817A was much fainter than typical, suggesting off-axis emission from a relativistic jet. However the observed prompt spectrum is inconsistent with the spectral (Amati) relation and causes the compactness problem in the simplest off-axis model. We focus on this spectral puzzle and suggest a possible key that the off-axis emission generally c…
We present a detailed analysis of the GeV gamma-ray emission toward the supernova remnant (SNR) G8.7-0.1 with the Large Area Telescope (LAT) onboard the \emph{Fermi} Gamma-ray Space Telescope. An investigation of the relationship among G8.7-0.1 and the TeV unidentified source HESS J1804-216 provides us with an important clue on diffusion process of cosmic rays if particle acceleration operates in the SNR. The GeV gamma-ray emission is extended with most of the emission in positional coincidence with the SNR G8.7-0.1 and a lesser part located outside the western boundary of G8.7-0.1. The region…
Unusually high velocities (< ~0.1c) and correspondingly high kinetic energies have been observed in a subset of Type Ic supernovae (so-called "broad-lined Ic" supernovae; SNe Ic-BL), prompting a search for a central engine model capable of generating such energetic explosions. A clue to the explosion mechanism may lie in the fact that all supernovae that accompany long-duration gamma-ray bursts belong to the SN Ic-BL class. Using a combination of two-dimensional relativistic hydrodynamics and radiation transport calculations, we demonstrate that the central engine responsible for long gamma-ra…
We present the results of hydrodynamical simulations of gamma-ray burst jets propagating through their stellar progenitor material and subsequently through the surrounding circumstellar medium. We consider both jets that are injected with constant properties in the center of the star and jets injected with a variable luminosity. We show that the variability properties of the jet outside the star are a combination of the variability injected by the engine and the variability caused by the jet propagation through the star. Comparing power spectra for the two cases shows that the variability inje…
X-ray Flash (XRF) 100316D, a nearby super-long under-luminous burst with a peak energy E_p \sim 20 keV, was detected by Swift and was found to be associated with an energetic supernova SN 2010bh. Both the spectral and the temporal behavior of this burst are rather similar to that of XRF 060218, except that the latter was associated with a "less energetic" SN 2006aj and had a prominent soft thermal emission component in the spectrum. We analyze the spectral and temporal properties of this burst, and interpret the prompt gamma-ray emission and the early X-ray plateau emission as synchrotron emis…
We systematically analyze three GRB samples named as radio-loud, radio-quiet and radio-none afterglows, respectively. It is shown that dichotomy of the radio-loud afterglows is not necessary. Interestingly, we find that the intrinsic durations ($T_{int}$), isotropic energies of prompt gamma-rays ($E_{γ, iso}$) and redshifts ($z$) of their host galaxies are log-normally distributed for both the radio-loud and radio-quiet samples except those GRBs without any radio detections. Based on the distinct distributions of $T_{int}$, $E_{γ, iso}$, the circum-burst medium density ($n$) and the isotropic …
The RoboPol instrument and the relevant program was developed in order to conduct a systematic study of the optical polarisation variability of blazars. Driven by the discovery that long smooth rotations of the optical polarisation plane can be associated with the activity in other bands and especially in gamma rays, the program was meant to investigate the physical mechanisms causing them and quantify the optical polarisation behaviour in blazars. Over the first three nominal observing seasons (2013, 2014 and 2015) RoboPol detected 40 rotations in 24 blazars by observing a gamma-ray-loud and …
X-ray flares are generally believed to be produced by the reactivation of the central engine, and may have the same energy dissipation mechanism as the prompt emission of gamma-ray bursts (GRBs). X-ray flares can therefore provide important clues to understanding the nature of the central engines of GRBs. In this work, we study for the first time the physical connection between differential size and return distributions of X-ray flares of GRBs with known redshifts. We find that the differential distributions of duration, energy, and waiting time can be well fitted by a power-law function. In p…
We report the detection of a faint optical flash by the 2-m Faulkes Telescope North simultaneously with the second of two prompt gamma-ray pulses in INTEGRAL gamma-ray burst (GRB) 080603A, beginning at t_rest = 37 s after the onset of the GRB. This optical flash appears to be distinct from the subsequent emerging afterglow emission, for which we present comprehensive broadband radio to X-ray light curves to 13 days post-burst and rigorously test the standard fireball model. The intrinsic extinction toward GRB 080603A is high (A_V,z = 0.8 mag), and the well-sampled X-ray-to-near-infrared spectr…
The solar disk is a bright source of multi-GeV gamma rays, due to the interactions of hadronic cosmic rays with the solar atmosphere. However, the underlying production mechanism is not understood, except that its efficiency must be greatly enhanced by magnetic fields that redirect some cosmic rays from ingoing to outgoing before they interact. To elucidate the nature of this emission, we perform a new analysis of solar atmospheric gamma rays with 9 years of Fermi-LAT data, which spans nearly the full 11-year solar cycle. We detect significant gamma-ray emission from the solar disk from 1 GeV …
The solar disk is a bright gamma-ray source. Surprisingly, its flux is about one order of magnitude higher than predicted. As a first step toward understanding the physical origin of this discrepancy, we perform a new analysis in 1-100 GeV using 6 years of public Fermi-LAT data. Compared to the previous analysis by the Fermi Collaboration, who analyzed 1.5 years of data and detected the solar disk in 0.1-10 GeV, we find two new and significant results: 1. In the 1-10 GeV flux (detected at $>5σ$), we discover a significant time variation that anticorrelates with solar activity. 2. We detect gam…
