X-ray Spectral Characteristics of GINGA Gamma-Ray Bursts

We have investigated the spectral characteristics of a sample of bright γ-ray bursts detected with the γ-ray burst sensors aboard the satellite Ginga. This instrument employed a proportional and scintillation counter to provide sensitivity to photons in the 2 400 keV region and as such provided a unique opportunity to characterize the largely unexplored X-ray properties of γ-ray bursts. The photon spectra of the Ginga bursts are well described by a low energy slope, a bend energy, and a high energy slope. In the energy range where they can be compared, this result is consistent with burst spectral analyses obtained from the BATSE experiment aboard the Compton Observatory. However, below 20 keV we find evidence for a positive spectral number index in approximately 40% of our burst sample, with some evidence for a strong rolloff at lower energies in a few events. There is a correlation (Pearson’s r = -0.62) between the low energy slope and the bend energy. We find that the distribution of spectral bend energies extends below 10 keV. There has been some concern in cosmological models of GRBs that the bend energy covers only a small dynamic range. Our result extends the observed dynamic range and, since we observe bend energies down to the limit of our instrument, perhaps observations have not yet limited the range. The Ginga trigger range was virtually the same as BATSE’s, yet we find a different range of fit parameters. One possible explanation might be that GRBs have two break energies, one often in the 50 to 500 keV range and the other near 5 keV. Both BATSE and Ginga fit with only a single break energy so BATSE tends to find breaks near the center of its 1 Current address: GSFC, [email protected] 2 [email protected]