The X‐Ray Characteristics of a Classical Gamma‐Ray Burst and Its Afterglow

The serendipitous observation of GRB 780506 by co-aligned gamma-ray (HEAO 1A-4 0.02 - 6 MeV) and X-ray (HEAO 1 A-2 2-60 keV) instruments during a six hourpointing at a blank section of the sky gave us unprecedented highsignal-to-noise X-ray spectra and light curves of a gamma-ray burst and itsafterglow. We observed two breaks in the initial spectrum, one consistent witha peak in nu-F-nu of ~45 keV, and one below 4 keV, consistent with strongabsorption, followed by dramatic spectral variability. The initial strongturnover below a few keV evolved into a slight excess. The spectral shapevaried widely outside low energy limits prescribed by current relativisticshock models. Two minutes after the burst ended, HEAO 1 A-2 detected a faintresurgence of 2-10 keV flux, rising to a peak ~seven minutes after burst onset,followed by irregular emission with best-fit decay time of half an hour. Weestimated that this entire afterglow radiated between 3 and 30% of the >1 keVenergy radiated during the burst.Comment: 45 pages, 18 of which are figures. References corrected, figures clarified, minor wording changes, to correspond to the form in which it will appear in the Astrophysical Journal, July 1, 501. Contains a detailed Appendix on a Bayesian Method for finding the position of a variable sourc