arXiv subjects
Publications and source records attributed to David L. Band.
The transients following GRB970228 and GRB970508 showed that these (and probably all) GRBs are cosmological. However, the host galaxies expected to be associated with these and other bursts are largely absent, indicating that either bursts are further than expected or the host galaxies are underluminous. This apparent discrepancy does not invalidate the cosmological hypothesis, but instead host galaxy observations can test more sophisticated models.
If gamma-ray bursts originate in galaxies at cosmological distances, the host galaxy should be detected if a burst error box is searched deep enough; are the host galaxies present? We present and implement a statistical methodology which evaluates whether the observed galaxy detections in a burst's error box are consistent with the presence of the host galaxy, or whether all the detections can be attributed to unrelated background galaxies. This methodology requires the model-dependent distribution of host galaxy fluxes. While our methodology was derived for galaxies in burst error boxes, it can be applied to other candidate host objects (e.g., active galaxies) and to other types of error boxes. As examples, we apply this methodology to two published studies of burst error boxes. We find that the nine error boxes observed by Larson and McLean (1997) are too large to discriminate between the presence or absence of host galaxies, while the absence of bright galaxies in the four significantly smaller error boxes observed by HST (Schaefer et al. 1997) does confirm that there is a "no-host galaxy" problem within the "minimal" host galaxy model.
Gamma-ray burst spectra usually show hard-to-soft evolution within intensity spikes and from spike to spike. The techniques used to study spectral evolution often lack sufficient temporal resolution to determine the nature of this evolution. By comparing the auto- and crosscorrelations between the time histories of the BATSE Large Area Detector discriminator rates I have characterized the spectral evolution of a sample of 209 bursts. I find that hard-to-soft evolution is ubiquitous, and only ~10% of the bursts show clear soft-to-hard evolution.
A computer-based technique has been developed to search bright BATSE gamma-ray bursts for spectral lines in a comprehensive manner. The first results of the search are discussed and an example line candidate shown.
To investigate the repeater content of gamma ray burst samples I develop two models where sources burst at a constant average rate. I find that the sky coverage affects the number of repeaters in a sample predominantly through the detector livetime, and that the number of bursts in the sample is the primary parameter. Thus the repeater content of burst samples should be compared within the context of a repetition model; a direct comparison between two samples is possible only if the samples have similar sizes. The observed repeater fraction may not be the actual fraction if the sources burst on average less than once during the detector livetime. Sources which burst repeatedly during active phases separated by more than the observation period must be treated separately.