Batse Observations Of Grb Spectra At Low Energies

The BATSE spectroscopy detectors were designed to detect photons with energies as low as 5 keV. For gamma-ray burst studies, the discriminators provide a broad-band energy bin available for spectral fitting, which lies below the burst high resolution spectroscopy data. The energy range of this bin can be as low as 5-10 keV depending on the gain setting of the detector. We have surveyed roughly 100 strong bursts which satisfy the following criteria: 1) sufficient signal-to-noise, 2) a detector observing the source with a viewing angle less than 60 degrees and 3) a detector gain sufficient to bring the low-energy data coverage down to the 5 - 20 keV range of interest. Spectra integrated over the bright portions were fit with one of several spectral forms and the agreement between the fitted model and the low-energy data is determined. We find evidence that x-ray excesses exist in approximate to 10% of the bursts. In most cases, the excess can be accounted for by a spectral model with upward curvature, such as has been proposed by Brainerd, or by a separate spectral component.