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 (BATSE). The spectra are described well at low energy by a power-law continuum with an exponential cutoff N_E_(E) is proportional to E^alpha^ exp (- E/E_0_), and by a steeper power law, N_E_(E) is proportional to E^beta^ with α > β at high energy. However, the spectral parameters α, β, and E_0_ vary from burst to burst with no universal values. The break in the spectrum, E_0_, 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. Consequently, it is unlikely that a majority of the burst spectra are shaped directly by pair processes, unless bursts originate from a broad redshift range. We find that the correlations among burst parameters do not fulfill the predictions of the cosmological models of burst origin, but our burst sample may not be appropriate for such a test. No correlations with burst morphology or the spatial distribution were found. We also studied the process of fitting the BATSE spectral data. For example, 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.