S. McGlynn

IDENTIFICATION AND PROPERTIES OF THE PHOTOSPHERIC EMISSION IN GRB090902B

The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover th ...

The Fermi GBM Gamma-Ray Burst Spectral Catalog: Four Years of Data

In this catalog we present the updated set of spectral analyses of gamma-ray bursts (GRBs) detected by the Fermi Gamma-Ray Burst Monitor during its first four years of operation. It contains two types of spectra, time-integrated spectral fits and spectral fits at the brightest time bin, from 943 triggered GRBs. Four different spectral models were fitted to the data, resulting in a compendium of more than 7500 spectra. The analysis was performed similarly but not identically to Goldstein et al. All 487 GRBs from the first two years have been re-fitted using the same methodology as that of the 456 GRBs in years three and four. We describe, in detail, our procedure and criteria for the analysis and present the results in the form of parameter distributions both for the observer-frame and rest-frame quantities. The data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center.

The Fermi GBM Gamma-Ray Burst Spectral Catalog: The First Two Years

We present systematic spectral analyses of gamma-ray bursts (GRBs) detected by the Fermi Gamma-Ray Burst Monitor (GBM) during its first two years of operation. This catalog contains two types of spectra extracted from 487 GRBs, and by fitting four different spectral models, this results in a compendium of over 3800 spectra. The models were selected based on their empirical importance to the spectral shape of many GRBs, and the analysis performed was devised to be as thorough and objective as possible. We describe in detail our procedure and criteria for the analyses, and present the bulk results in the form of parameter distributions. This catalog should be considered an official product from the Fermi GBM Science Team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center.

The Second Fermi GBM Gamma-Ray Burst Catalog: The First Four Years

This is the second of a series of catalogs of gamma-ray bursts (GRBs) observed with the Fermi Gamma-ray Burst Monitor (GBM). It extends the first two-year catalog by two more years, resulting in an overall list of 953 GBM triggered GRBs. The intention of the GBM GRB catalog is to provide information to the community on the most important observables of the GBM detected GRBs. For each GRB the location and main characteristics of the prompt emission, the duration, peak flux and fluence are derived. The latter two quantities are calculated for the 50-300 keV energy band, where the maximum energy release of GRBs in the instrument reference system is observed and also for a broader energy band from 10-1000 keV, exploiting the full energy range of GBMs low-energy detectors. Furthermore, information is given on the settings and modifications of the triggering criteria and exceptional operational conditions during years three and four in the mission. This second catalog is an official product of the Fermi GBM science team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center.

Observational evidence of dissipative photospheres in gamma-ray bursts

The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape ...

Prospects for GRB Science with the Fermi Large Area Telescope

The Large Area Telescope (LAT) instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst (GRB) phenomena in the >100 MeV band. The synergy with Fermis Gamma-ray Burst Monitor detectors will link these observations to those in the well explored 10-1000 keV range; the addition of the >100 MeV band observations will resolve theoretical uncertainties about burst emission in both the prompt and afterglow phases. Trigger algorithms will be applied to the LAT data both onboard the spacecraft and on the ground. The sensitivity of these triggers will differ because of the available computing resources onboard and on the ground. Here we present the LATs burst detection methodologies and the instruments GRB capabilities.

TIME-RESOLVED ANALYSIS OF FERMI GAMMA-RAY BURSTS WITH FAST- AND SLOW-COOLED SYNCHROTRON PHOTON MODELS

Time-resolved spectroscopy is performed on eight bright, long gamma-ray bursts (GRBs) dominated by single emission pulses that were observed with the Fermi Gamma-Ray Space Telescope. Fitting the prompt radiation of GRBs by empirical spectral forms such as the Band function leads to ambiguous conclusions about the physical model for the prompt radiation. Moreover, the Band function is often inadequate to fit the data. The GRB spectrum is therefore modeled with two emission components consisting of optically thin non-thermal synchrotron radiation from relativistic electrons and, when significant, thermal emission from a jet photosphere, which is represented by a blackbody spectrum. To produce an acceptable fit, the addition of a blackbody component is required in five out of the eight cases. We also find that the low-energy spectral index α is consistent with a synchrotron component with α = –0.81 ± 0.1. This value lies between the limiting values of α = –2/3 and α = –3/2 for electrons in the slow- and fast-cooling regimes, respectively, suggesting ongoing acceleration at the emission site. The blackbody component can be more significant when using a physical synchrotron model instead of the Band function, illustrating that the Band function does not serve as a good proxy for a non-thermal synchrotron emission component. The temperature and characteristic emission-region size of the blackbody component are found to, respectively, decrease and increase as power laws with time during the prompt phase. In addition, we find that the blackbody and non-thermal components have separate temporal behaviors as far as their respective flux and spectral evolutions.

Global characteristics of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs ,

Context. INTEGRAL has two sensitive gamma-ray instruments that have detected and localised 47 gamma-ray bursts (GRBs) from its launch in October 2002 up to July 2007. Aims. We present the spectral, spatial, and temporal properties of the bursts in the INTEGRAL GRB catalogue using data from the imager, IBIS, and spectrometer, SPI. Methods. Spectral properties of the GRBs are determined using power-law and, where appropriate, Band model and quasithermal model fits to the prompt emission. Spectral lags, i.e. the time delay in the arrival of low-energy γ-rays with respect to high-energy γ-rays, are measured for 31 of the GRBs. Results. The photon index distribution of power-law fits to the prompt emission spectra is presented and is consistent with that obtained by Swift. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ∼0.15 ph cm −2 s −1 is ∼1400 yr −1 in the fully coded field of view of IBIS. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags 0.75 s. Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies, and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q = −0.225 ± 0.090 and hence reflect the local large-scale structure of the Universe. Conclusions. The spectral, spatial, and temporal properties of the 47 GRBs in the INTEGRAL catalogue are presented and compared with the results from other missions. The rate of long-lag GRBs with inferred low luminosity is ∼25% of type Ib/c supernovae. Some of these bursts could be produced by the collapse of a massive star without a supernova. Alternatively, they could result from a different progenitor, such as the merger of two white dwarfs or a white dwarf with a neutron star or black hole, possibly in the cluster environment without a host galaxy.

Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere

Fermi Gamma-ray Space Telescope observations of GRB110721A have revealed two emission components from the relativistic jet: emission from the photosphere, peaking at 100 keV and a non-thermal component, which peaks at 1000 keV. We use the photospheric component to calculate the properties of the relativistic outow. We nd a strong evolution in the ow properties: the Lorentz factor decreases with time during the bursts from 1000 to 150 (assuming a redshift z = 2; the values are only weakly dependent on unknown eciency parameters). Such a decrease is contrary to the expectations from the internal shocks and the isolated magnetar birth models. Moreover, the position of the ow nozzle measured from the central engine, r0, increases by more than two orders of magnitude. Assuming a moderately magnetised outow we estimate that r0 varies from 10 6 cm to 10 9 cm during the burst. We suggest that the maximal value reects the size of the progenitor core. Finally, we show that these jet properties naturally explain the observed broken power-law decay of the temperature which has been reported as a characteristic for GRB pulses.

The Spectral Lag of GRB 060505: A Likely Member of the Long-Duration Class

Two long -ray bursts, GRB060505 and GRB060614, occurred in nearby galaxies at redshifts of 0.089 and 0.125 respectively. Due to their proximity and durations, deep follow-up campaigns to search for supernovae (SNe) were initiated. However none were found in either case, to limits more than two orders of magnitude fainter than the prototypical GRB-associated SN, 1998bw. It was suggested that the bursts, in spite of their durations (� 4 and 102s), belonged to the population of short GRBs which has been shown to be unrelated to SNe. In the case of GRB060614 this argument was based on a number of indicators, including the negligible spectral lag, which is consistent with that of short bursts. GRB060505 has a shorter duration, but no spectral lag was measured. We present the spectral lag measurements of GRB060505 using Suzaku’s Wide Area Monitor and the Swift Burst Alert Telescope. We find that the lag is 0.36 ± 0.05s, inconsistent with the lags of short bursts and consistent with the properties of long bursts and SN-GRBs. These results support the association of GRB060505 with other low-luminosity GRBs also found in star-forming galaxies and indicates that at least some massive stars may die without bright SNe. Subject headings: gamma rays: bursts