γ-ray sources as comptonized X-ray sources

γ-ray burst spectra have often been fit by optically thin thermal bremsstrahlung. However, at the high temperatures implied by such fits (kT∼300 keV), the free–free cross-section is so much smaller than the Compton cross-section that Compton scattering might dominate the spectral formation processes. We have investigated the possibility that emission mechanisms based on Compton scattering can also fit the data. In particular, Monte Carlo calculations have been used to compare the γ-ray burst spectral data with black-body spectra which have undergone inverse comptonization by a much hotter, overlying plasma. The best-fit parameters suggest that the underlying black body is an X-ray source (kTBB∼2.4 keV) while the overlying, comptonizing plasma has kTe∼150 keV and a column density of 4 × 1024electrons cm−2. Such a model would also naturally explain some of the unusual γ-ray burst spectra. In particular, a low energy (∼400 keV) cutoff in the spectrum of GB781119 can be explained as a Wien peak produced by a higher column density in the comptonizing region, and two-component spectra (for example, GB790329) can be explained by a lower column density.