X-ray Spectral Characteristics of Ginga Gamma-Ray Bursts

Abstract

The proportional counter and scintillator on Ginga provided unprecented spectral coverage of GRBs, from 2 - 400 keV. 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 energy range and we tend to find breaks in our energy range. The observed ratio of energy emitted in the x-rays relative to the gamma-rays can be much larger than a few percent and, in fact, is sometimes larger than unity. The average for our 22 bursts is 24%. There is one example of a strong x-ray excess at low energy.