The Synchrotron-self-Compton Radiation Accompanying Shallow Decaying X-Ray Afterglow： the Case of GRB 940217

High energy emission (> tens MeV) of Gamma-Ray Bursts (GRBs) provides an important clue on the physical processes occurring in GRBs that may be correlated with the GRB early afterglow. A shallow decline phase has been well identified in about half of Swift Gamma-ray Burst X-ray afterglows. The widely considered interpretation involves a significant energy injection and possibly time-evolving shock parameter(s). We calculate the synchrotron-self-Compton (SSC) radiation of such an external forward shock and show that it could explain the well-known long term high energy (i.e., tens MeV to GeV) afterglow of GRB 940217. We propose that cooperation of Swift and GLAST will help to reveal the nature of GRBs.     

Comptel Constraints on Unidentified EGRET Sources

A major legacy of the EGRET experiment aboard the Compton Gamma-Ray Observatory (CGRO) is the detection of a large number of unidentified Gamma-ray sources. Out of the 271 EGRET sources are 170 not identified yet [Hartman et al. ApJS (123) (1999) 79]. To provide additional information on these unidentified EGRET sources, we derived simultaneous MeV fluxes or flux limits for each source by analyzing the contemporaneous COMPTEL data. Then we compare these MeV fluxes to the extrapolations of the published EGRET spectra. Our results can be grouped into 4 categories [Zhang et al. A&A (421) (2004) 983]: (1) non-constraining upper limits (∼120 sources), (2) hints (> 2 sigma) or detections with consistent MeV fluxes (∼16 sources), (3) enhanced MeV emission (2 sources), and (4) constraining MeV flux limits, requiring a spectral break (∼22 sources). In summary, for about 40 of the unidentified EGRET sources we can provide supplementary spectral information in the neighboring gamma-ray band, which – together with other source parameters – might provide clues to the underlying source nature.     

Variability in GRB afterglows and GRB 021004

We present general analytic expressions for GRB afterglow light curves arising from a variable external density profile and/or a variable energy in the blast wave. The former could arise from a clumpy ISM or a variable stellar wind; The latter could arise from refreshed shocks or from an angular dependent jet structure (patchy shell). Both scenarios would lead to a variable light curve. Our formalism enables us to invert the observed light curve and obtain possible density or energy profiles. The optical afterglow of GRB 021004 was detected 537 s AB (after the burst) [GCN (2002) 1564]. Extensive follow up observations revealed a significant temporal variability. We apply our formalism to the R-band light curve of GRB 021004 and we find that several models provide a good fit to the data. We consider the patchy shell model with p=2.2 as the most likely explanation. According to this model our line of sight was towards a ‘cold spot’ that has lead to a relativity low γ-ray flux and an initially weak afterglow (while the X-ray afterglow flux after a day was above average). Observations above the cooling frequency, ν_c, could provide the best way to distinguish between our different models.     

GRB 170817A: a short GRB seen off-axis

The angular distribution of gamma-ray burst(GRB)jets is not yet clear.The observed luminosity of GRB 170817A is the lowest among all known short GRBs,which is best explained by the fact that our line of sight is outside of the jet opening angle,θ_(obs)θ_j,whereθ_(obs) is the angle between our line of sight and the jet axis.As inferred by gravitational wave observations,as well as radio and X-ray afterglow modeling of GRB 170817A,it is likely that θ_(obs)~20°–28°.In this work,we quantitatively consider two scenarios of angular energy distribution of GRB ejecta:a top-hat jet and a structured jet with a power law index s.For the top-hat jet model,we get a large θ_j(e.g.,θ_j10°),a rather high local (i.e., z 0.01) short GRB rate ~8–15×10~3 Gpc~(-3)yr~(-1((estimated to be 90~1850 Gpc~(-3)yr~(-1) in Fong et al.)and an extremely high(on-axis,V(~500 ke V for a typical short GRB).For the structured jet model,we use θ_(obs) to give limits on s and θ_j for typical on-axis luminosity of a short GRB(e.g.,10~(49)erg s(-1) 1051erg s(-1)),and a low on-axis luminosity case(e.g.,1049erg s(-1))gives more reasonable values of s.The structured jet model is more feasible for GRB170817A than the top-hat jet model due to the rather high local short GRB rate,and the extremely high on-axis E_(peak,0) almost rules out the top-hat jet model.GRB 170817A is likely a low on-axis luminosity GRB(1049erg s(-1))with a structured jet.     

Metallicities of high redshift GRB hosts: The case of GRB 100219A

GRB 100219A at z = 4.667 has been the highest redshift gamma‐ray burst observed with the X‐shooter spectrograph up to now. The spectrum covering the range from 5000 to 24000 Å and a large number of absorption lines allows to make a detailed study of the interstellar medium in a high redshift galaxy. The ISM in the low ionisation state and the kinematics of the absorption line components reveal a complex velocity field. The metallicity measured from different absorption lines is around 0.1 solar. Other GRB hosts at redshift beyond ∼3 have similar metallicities albeit with a large scatter in the metallicity distribution. X‐shooter will allow us to determine metallicities of a larger number of GRB hosts beyond redshift 5, to probe the early chemical enrichment of the Universe and to study its evolution from redshift 2 to beyond 10 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrodynamic Evolution of GRB Afterglow

1 INTRODUCTIONMom the detection at X-rap optical and radio wavelengths of ganuna-ray bursts (GRBs)since 1997 (Costa et al. 1997; van Paradijs et al. 1997; Sahu et al. 1997; Djorgovsld et al. 1997;Metzger et al. 1997; Frail et al. 1997; Taylor et al. 1997; Kulforni et al. 1998; Halpern et al.1998; Castro-Tirado et al. 1999; Kulkalni et al. 1999; Galama et al. 1999), we have come toknow that the GRBs can release 1051 ~ 1054 ergs in a few seconds and that the fireball modelcan describ…     

GRB sky distribution puzzles

We analyze the randomness of the sky distribution of cosmic gamma-ray bursts. These events are associated with massive galaxies, spiral or elliptical, and therefore their positions should trace the large-scale structure, which, in turn, could show up in the sky distribution of fluctuations of the cosmicmicrowave background (CMB). We test this hypothesis by mosaic correlation mapping of the distributions of CMB peaks and burst positions, find the distribution of these two signals to be correlated, and interpret this correlation as a possible systematic effect.     

Swift observations of GRB 050712

We present the results of X-ray and optical observations of GRB 050712 performed by Swift . The X-ray light curve of this burst exhibits episodes of flares in the first 1000 s, the same epoch at which the UVOT detected an optical counterpart. A shallow X-ray decay, with a decay slope of  α=−0.73  , followed and lasted ∼70 ks. This behaviour can be explained in terms of activity of the gamma-ray burst 'inner engine', with the possibility that the last flare is caused by the interaction of the ejecta with the surrounding medium.
We also find interesting spectral parameters for the X-ray emission. In particular, data suggest the presence of an intrinsic absorption in the first 1000 s, which can be explained if circumburst medium clouds lie along the line of sight.     

A model for GRB jets

By using relativistic, axisymmetric, ideal MHD, we examine the motion of the baryon/e^±/ photon fluid that emanates from a stellar-mass compact object/debris-disk system (a common outcome of many progenitor models). We prove that the motion can be described as a frozen pulse, which permits the study of each shell of the pancake-shaped outflow using steady-state equations. The ejected energy flux is dominated by the electromagnetic (Poynting) contribution, but it can also have a non negligible e^±/radiation (thermal fireball)component. We demonstrate, through exact self-similar solutions, that the flow is first thermally and subsequently magnetically accelerated up to equipartition between kinetic and Poynting fluxes, i.e., ～ 50% of the total energy is converted into baryonic kinetic energy. The electromagnetic forces also collimate the flow, reaching a cylindrical structure asymptotically.     

GRB jet beaming angle statistics

Existing theory and models suggest that a Type I (merger) GRB should have a larger jet beaming angle than a Type II (collapsar) GRB, but so far no statistical evidence is available to support this suggestion. In this paper, we obtain a sample of 37 beaming angles and calculate the probability that this is true. A correction is also devised to account for the scarcity of Type I GRBs in our sample. The probability is calculated to be 83% without the correction and 71% with it.     

VLA observations of GRB error boxes

Very Large Array (VLA) radio observations of precisely-located GRB error boxes have been performed to search for fading and quiescent emission associated with -ray bursts. These observations were made as quickly as 23 hours and as late as 13 years after the time of the burst. Our measurements presented here have found GRB error boxes to be empty of sources to the 80 µJy level ( = 3.6 cm) at 9 months, to 1 mJy (20 cm) at 9 days, and probably to 5 mJy (20 cm) at 23 hours after the bursts.     

Study of WATCH GRB error boxes

We have studied the first WATCH GRB Catalogue of-ray Bursts in order to find correlations between WATCH GRB error boxes and a great variety of celestial objects present in 33 different catalogues. No particular class of objects has been found to be significantly correlated with the WATCH GRBs.     

A GRB Follow-up System at the Xinglong Observatory and Detection of the High-Redshift GRB 060927

A gamma-ray burst (GRB) optical photometric follow-up system at the Xinglong Observatory of National Astronomical Observatories of China (NAOC) has been constructed. It uses the 0.8-m Tsinghua-NAOC Telescope (TNT) and the 1-m EST telescope, and can au-tomatically respond to GRB Coordinates Network (GCN) alerts. Both telescopes slew rela-tively fast, being able to point to a new target field within ～ 1 min upon a request. Whenever available, the 2.16-m NAOC telescope is also used. In 2006 the system responded to 15 GRBs and detected seven early afterglows. In 2007 six GRBs have been detected among 18 follow-up observations. TNT observations of the second most distant GRB 060927 (z = 5.5) are shown, which started as early as 91 s after the GRB trigger. The afterglow was detected in the combined image of the first 19 × 20 s unfiltered exposures. This GRB follow-up system has joined the East-Asia GRB Follow-up Observation Network (EAFON).     

Deriving the GRB distance scale

We describe a method to address the burster origin problem by determining their distance scale. We show that this can be easily carried out at soft X-ray wavelengths by measuring the effective column densities of a representative sample of burst spectra. We demonstrate the effectiveness of the technique by simulating the performance of a small CATSAT-type mission (Forrestet al., 1995).     

Onboard GRB trigger algorithms of SVOM-GRM

The Gamma-Ray Monitor （GRM） is a high energy detector onboard the future Chinese-French satellite named the Space-based multi-band astronomical Variable Object Monitor which is dedicated to studies of gamma-ray bursts （GRBs）. This paper presents an investigation of the algorithms that look for GRBs by searching for a significant increase in the photon count rate for the computer onboard GRM. The trigger threshold and trigger efficiency, which are based on a given sample of GRBs, are calculated with the algorithms. The trigger characteristics of onboard instruments GRM and ECLAIRs are also analyzed. In addition, the impact of solar flares on GRM is estimated, and a method to distinguish solar flares from GRBs is investigated.     

GRB satellite triggers for neutrino telescopes

The real‐time distribution of alert messages from satellites that detect gamma‐ray bursts are of key importance to neutrino telescopes.We describe how the distribution network of these alert messages is used by the ANTARES neutrino telescope, and the resulting increase in detection efficiency for neutrinos from gamma‐ray bursts. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Light Curves of GRB Afterglows

We present gamma-ray burst afterglow light curves in X-ray, optical and radio bands for various distributions of accelerated electrons behind the shock. The effects of lateral expansion of the jet and of winds in typical Wolf-Rayet star on the evolution are discussed. The light curves in the radiative case decline more rapidly than those in the adiabatic case. Under the combined effect of jet expansion and wind environment, the light curves have the greatest deviation from those of the standard model. All these results refer to the relativistic phase.     

Search for GRB counterparts: non-optical

The present status of the search for counterparts of gamma-ray bursts is given. Particular emphasis will be put on the search for flaring counterparts, i.e. emission during or shortly after the gamma-ray emission. In view of a separate review on optical counterparts I will concentrate on GeV/TeV, X-ray, radio and neutrino searches.     

Spectra and light curves of GRB afterglows

We performed accurate numerical calculations of angle-, time-, and frequency-dependent radiative transfer for the relativistic motion of matter in gamma-ray burst (GRB) models. Our technique for solving the transfer equation, which is based on the method of characteristics, can be applied to the motion of matter with a Lorentz factor up to 1000. The effect of synchrotron self-absorption is taken into account. We computed the spectra and light curves from electrons with a power-law energy distribution in an expanding relativistic shock and compare them with available analytic estimates. The behavior of the optical afterglows from GRB 990510 and GRB 000301c is discussed qualitatively.