G. Fishman

BATSE observations of gamma-ray burst spectra. I: Spectral diversity

We studied the time-averaged gamma-ray burst spectra accumulated by the spectroscopy detectors of the Burst and Transient Source Experiment. The spectra are described well at low energy by a power-law continuum with an exponential cutoff and by a steeper power law at high energy. However, the spectral parameters vary from burst to burst with no universal values. The break in the spectrum ranges from below 100 keV to more than 1 MeV, but peaks below 200 keV with only a small fraction of the spectra breaking above 400 keV; it is therefore unlikely that a majority of the burst spectra are shaped directly by pair processes, unless bursts originate from a broad redshift range. The correlations among burst parameters do not fulfill the predictions of the cosmological models of burst origin. No correlations with burst morphology or the spatial distribution were found. We demonstrate the importance of using a complete spectral description even if a partial description (e.g., a model without a high-energy tail) is statistically satisfactory.

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.

A Possible X-Ray Counterpart to SGR 1900+14

The location of the soft gamma repeater SGR 1900+14 was recently reduced to two ~5 arcmin2 alternate error boxes by the network synthesis method. We have used the ROSAT High Resolution Imager to observe the error box that is closest to the supernova remnant G42.8+0.6. A quiescent, steady, point X-ray source was found at α(2000) = 19h07m1415, δ(2000) = 9°191906, whose unabsorbed flux is 3 × 10-12 ergs cm-2 s-1. Its position is also consistent with a peculiar double infrared source described in a companion paper. We have also examined this region using the VLA, and have obtained upper limits to the extreme ultraviolet flux of this object using the Extreme-Ultraviolet Explorer.

TheUlysses Supplement to the BATSE 4Br Catalog of Cosmic Gamma-Ray Bursts

We present Interplanetary Network localization information for 147 gamma-ray bursts observed by the Burst and Transient Source Experiment between the end of the 3rd BATSE catalog and the end of the 4th BATSE catalog, obtained by analyzing the arrival times of these bursts at the Ulysses and Compton Gamma-Ray Observatory (CGRO) spacecraft. For any given burst observed by these two spacecraft, arrival time analysis (or triangulation) results in an annulus of possible arrival directions whose half-width varies between 7 arcseconds and 2.3 degrees, depending on the intensity and time history of the burst, and the distance of the Ulysses spacecraft from Earth. This annulus generally intersects the BATSE error circle, resulting in an average reduction of the error box area of a factor of 25.

Are Abell Clusters Correlated with Gamma-Ray Bursts?

A recent study has presented marginal statistical evidence that gamma-ray burst (GRB) sources are correlated with Abell clusters, based on analyses of bursts in the BATSE 3B catalog. Using precise localization information from the Third Interplanetary Network, we have reanalyzed this possible correlation. We find that most of the Abell clusters that are in the relatively large 3B error circles are not in the much smaller IPN/BATSE error regions. We believe that this argues strongly against an Abell cluster-GRB correlation.

All-Sky Search for Transient Sources Near 0.5 MeV with the Burst and Transient Source Experiment (BATSE)

We present a search for bright, transient emission lines near 0.5 MeV in nearly two years of data from the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory. Such features have been reported from black hole candidates Nova Muscae and 1E 1740.7-2942 and from the Crab, lasting for ≈ 1 day. Our survey covers the whole sky and is sensitive to events with durations from 0.5-3.0 day. No transients are observed, and the systematic errors are low enough that the upper limits are significantly below the fluxes of the two most significant events previously reported.

EXIST'S Gamma-Ray Burst Sensitivity

We use semianalytic techniques to evaluate the burst sensitivity of designs for the EXIST hard X-ray survey mission. Applying these techniques to the mission design proposed for the Beyond Einstein program, we find that with its very large field of view and faint gamma-ray burst detection threshold, EXIST will detect and localize approximately two bursts per day, a large fraction of which may be at high redshift. We estimate that EXISTs maximum sensitivity will be ~4 times greater than that of Swifts Burst Alert Telescope. Bursts will be localized to better than 40 at threshold, with a burst position as good as a few arcseconds for strong bursts. EXISTs combination of three different detector systems will provide spectra from 3 keV to more than 10 MeV. Thus, EXIST will enable a major leap in the understanding of bursts, their evolution, environment, and utility as cosmological probes.

The Interplanetary Network Supplement to the BATSE Catalogs of Untriggered Cosmic Gamma-Ray Bursts

We present Interplanetary Network (IPN) detection and localization information for 211 gamma-ray bursts (GRBs) observed as untriggered events by the Burst and Transient Source Experiment (BATSE) and published in catalogs by Kommers et al. and Stern et al. IPN confirmations have been obtained by analyzing the data from 11 experiments. For any given burst observed by BATSE and one other distant spacecraft, arrival time analysis (or triangulation) results in an annulus of possible arrival directions whose half-width varies between 14 and 56, depending on the intensity, time history, and arrival direction of the burst, as well as the distance between the spacecraft. This annulus generally intersects the BATSE error circle, resulting in a reduction of the area of up to a factor of ~650. When three widely separated spacecraft observed a burst, the result is an error box whose area is as much as 30,000 times smaller than that of the BATSE error circle. Because the IPN instruments are considerably less sensitive than BATSE, they generally did not detect the weakest untriggered bursts but did detect the more intense ones, which failed to trigger BATSE when the trigger was disabled. In a few cases, we have been able to identify the probable origin of bursts as soft gamma repeaters. The vast majority of the IPN-detected events, however, are GRBs, and the confirmation of them validates many of the procedures utilized to detect BATSE untriggered bursts.

Possible association of a quiescent x-ray source with a gamma-ray burster

We report on the first repeatedly detected statistically significant coincidence (chance probability {approx_equal}10{sup {minus}2}{endash}10{sup {minus}3}) between an X-ray source and a gamma-ray burst error box. We present three {ital ROSAT} observations of the field of the gamma-ray burst of 1992 May 1. The first, a 2000 s target of opportunity observation, was carried out 18 days after the burst. A weak X-ray source was identified, but with too few photons to determine its spectral characteristics. The second, a 30 ks PSPC observation, resulted in the detection of 118 net photons over the 0.07{endash}2.4 keV energy range. We find that the spectrum is consistent with thermal bremsstrahlung from a 7{times}10{sup 6} K plasma with about 10{sup 22} cm{sup {minus}2} HI column density. The unabsorbed flux is {approximately}9.4{times}10{sup {minus}13} ergscm{sup {minus}2}s{sup {minus}1} (corresponding absorbed flux 4.8{times}10{sup {minus}14} ergscm{sup {minus}2}s{sup {minus}1}). Analysis of the photon arrival times indicates that the source may be variable. Using the HI column density from the spectral fit, we set a lower limit to the source distance of at least several kpc; an extragalactic source cannot be ruled out. If the gamma-ray burst is indeed related to the X-ray source, its total energy output would have been atmorexa0» least 2{times}10{sup 37} ergs. The third observation, 6200 s with the HRI, defines a source error circle of 6{double_prime} radius. We discuss optical observations of this region, and consider various possibilities for the nature of the X-ray source. {copyright} {ital 1996 The American Astronomical Society.}«xa0less

THE INTERPLANETARY NETWORK SUPPLEMENT TO THE BURST AND TRANSIENT SOURCE EXPERIMENT 5B CATALOG OF COSMIC GAMMA-RAY BURSTS

We present Interplanetary Network localization information for 343 gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE) between the end of the 4th BATSE catalog and the end of the Compton Gamma-Ray Observatory (CGRO) mission, obtained by analyzing the arrival times of these bursts at the Ulysses, Near Earth Asteroid Rendezvous (NEAR), and CGRO spacecraft. For any given burst observed by CGRO and one other spacecraft, arrival time analysis (or triangulation) results in an annulus of possible arrival directions whose half-width varies between 11 arcsec and 21{sup 0}, depending on the intensity, time history, and arrival direction of the burst, as well as the distance between the spacecraft. This annulus generally intersects the BATSE error circle, resulting in an average reduction of the area of a factor of 20. When all three spacecraft observe a burst, the result is an error box whose area varies between 1 and 48,000 arcmin{sup 2}, resulting in an average reduction of the BATSE error circle area of a factor of 87.