Spectroscopy of gamma-ray bursts: An overview

Abstract

More than two decades of accumulated data on the spectra of gamma-ray bursts (GRB's) are now in hand. In general, burst spectra display a wide variety of both spectral and temporal behavior. In essentially all bursts where spectral measurements are possible, the spectrum evolves during the burst. This spectral complexity and variability has presented researchers with a challenging task. Early spectral measurements contained evidence for cyclotron-line-like absorption and emission features. These results were the main anchor for the galactic neutron star paradigm for the origin of gamma-ray bursts. Ginga provided striking confirmation with evidence in three bursts for cyclotron absorption with both first and second harmonics present in two of them. The absorption line energies were consistent with the teragauss magnetic fields expected for neutron stars. However, the BATSE experiment on CGRO has dealt a severe blow to this paradigm. BATSE has found no evidence for the expected galactic anisotropy and also no evidence for cyclotron lines. This paper will: 1) provide an historical overview of GRB spectroscopic measurements 2) a discussion of the BATSE/Ginga controversy and 3) a sampling of the most recent results and analyses of GRB spectral data.