Locations and spectra of cosmic gamma-ray bursts

The COMPTEL instrument aboard the Compton Gamma Ray Observatory is used to study the phenomena of cosmic gamma-ray bursts. Three years of observations from April 1991 through April 1994 reveal 18 significant gamma-ray burst detections. The locations (mean accuracy $\sim$1$\sp\circ$) and spectra (0.75-30 MeV) of these bursts are measured and are used to investigate the spatial distribution of burst sources and the characteristics of their emission at MeV energies. Rapidly determined COMPTEL burst localizations obtained through direct imaging are used to search for fading burst counterpart emission in a coordinated effort with ground-based optical and radio telescopes. The COMPTEL burst localizations are consistent with an isotropic angular distribution of sources, yet the spatial coincidence of two bursts indicates the possibility of repetition from at least one source. The combination of COMPTEL burst images with Interplanetary Network triangulation data significantly reduces the uncertainty in burst directions. The lack of an observable parallax between COMPTEL and Interplanetary Network localizations indicates that two of the strongest bursts must have originated more than $\sim$100 AU from the earth. Nearly all of the time-averaged COMPTEL burst spectra are consistent with a single power law model with spectral index in the range 1.5-3.5. Exponential, thermal bremsstrahlung and thermal synchrotron models are statistically inconsistent with the full COMPTEL burst sample, although they can adequately describe some of the individual burst spectra. Comparisons of simultaneous and near-simultaneous burst spectra measured by COMPTEL, BATSE and EGRET show wide-band emission that is characterized by a variable turnover around a few hundred keV, followed by a single power law out to $\sim$100 MeV. The relation between burst emission measured by COMPTEL at MeV energies and that measured by EGRET at GeV energies is still unclear, but there is no evidence to indicate a spectral change or temporal delay between the two. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby isotropic emitters within the Galactic disk or the gamma-ray emission is relativistically beamed to avoid the opacity of two-photon pair production. No obvious fading burst counterpart emission has been identified in the deepest optical and radio searches ever performed with time delays of hours. The upper limits on such emission suggest that fading optical counterparts after delays of $\sim$hours are fainter than 16$\sp{\rm th}$ visual magnitude and radio emission is weaker than $\sim$0.2 Jy. These results indicate that low-energy burst emission (if it exists) is very weak and/or very short-lived. Future low-energy burst counterpart search efforts will have to concentrate on obtaining deep measurements with time-delays significantly shorter than a few hours