BATSE OBSERVATIONS OF GAMMA-RAY BURST SPECTRA .3. LOW-ENERGY BEHAVIOROF TIME-AVERAGED SPECTRA

We analyze time-averaged spectra from 86 bright gamma-ray bursts from the first 5 years of the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory to determine whether the l owest energy data are consistent with a standard spectra form fit to t he data at all energies. The BATSE Spectroscopy Detectors have the cap ability to observe photons as low at 5 keV. Using the gamma-ray burst locations obtained with the BATSE Large Area Detectors, the Spectrosco py Detectors' low-energy response can be modeled accurately. This, tog ether with a postlaunch calibration of the lowest energy Spectroscopy Detector discriminator channel, which can lie in the range 5-20 keV, a llows spectral deconvolution over a broad energy range, similar to 5 k eV to 2 MeV. The additional coverage allows us to search for evidence of excess emission, or for a deficit, below 20 keV. While no burst has a significant (greater than or equal to 3 sigma) deficit relative to a standard spectra model, we find that 12 bursts have excess low-energ y emission, ranging between 1.2 and 5.8 times the model flux, that exc eeds 5 sigma in significance. This is evidence for an additional low-e nergy spectral component in at least some bursts, or for deviations fr om the power-law spectral form typically used to model gamma-ray burst s at energies below 100 keV.