The serendipitous observation of GRB 780506 by coaligned gamma-ray (HEAO 1 A-4 0.02-6 MeV) and X-ray (HEAO 1 A-2 2-60 keV) instruments during a 6 hr pointing at a blank section of the sky gave us unprecedented high signal-to-noise ratio X-ray spectra and light curves of a gamma-ray burst and its afterglow. First, the spectra: although the time resolution was only 10.24 s, it was possible to derive unambiguous position constraints and a reliable instrument response. We therefore could see two breaks in the initial spectrum, one consistent with a peak in nu F(nu) of similar to 45 keV and one below 4 keV, consistent with strong absorption. The event exhibited dramatic spectral variability, hardening as it rose to each peak, and softening as it fell, similar to behavior reported above 100 keV from other bursts. The initial strong turnover below a few keV evolved into a slight excess as the burst progressed. The spectral shape varied widely outside low-energy limits prescribed by current relativistic shock models. At no time was there evidence for any lines, either in emission or absorption. Overall, similar to 20% of the total energy was radiated in nonthermal 2-30 keV X-rays. Second, the light curve: the X-ray proportional counters were able to monitor the flux for an hour before and some hours after burst onset. Two minutes after it ended, HEAO 1 A-2 detected a faint resurgence of 2-10 keV flux, rising to a peak similar to 7 minutes after burst onset, followed by irregular emission with a best-fit decay time of 1/2 hr. The position of the source of the extended flux was consistent with that of the burst and was probably not from a serendipitous transient. We estimated that the entire afterglow radiated between 3% and 30% of the >1 keV energy radiated during the burst. In this paper, we present the 2-300 keV HEAO 1 A-2 and A-4 light curves and spectra from GRB 780506 and its apparent similar to 1 keV afterglow.
