The giant flare of 1998 August 27 from SGR 1900+14.: I.: An interpretive study of BeppoSAX and Ulysses observations

The giant flare of 1998 August 27 from SGR 1900+14 was extraordinary in many ways: it was the most intense flux of gamma rays ever detected from a source outside our solar system; it was longer than any previously detected burst from a soft gamma repeater (SGR) in our Galaxy by more than an order of magnitude; and it showed a remarkable four-peaked, periodic pattern in hard X-rays with the same rotation period that was found modulating soft X-rays from the star in quiescence. The event was detected by several gamma-ray experiments in space, including the Ulysses gamma-ray burst detector and the BeppoSAX Gamma-Ray Burst Monitor. These instruments operate in different energy ranges, and comparisons of their measurements reveal complex patterns of spectral evolution as the intensity varies. In this paper, we present a joint analysis of the BeppoSAX and Ulysses data and discuss some implications of these results for the SGRs. We also present newly analyzed Venera/SIGNE and ISEE-3 data on the 1979 March 5 giant flare from an SGR in the Large Magellanic Cloud (SGR 0526-66) and compare them with the August 27 event. Our results are consistent with the hypothesis that giant flares are due to catastrophic magnetic instabilities in highly magnetized neutron stars, or "magnetars." In particular, observations indicate that the initial hard spike involved a relativistic outflow of pairs and hard gamma rays, plausibly triggered by a large propagating fracture in the crust of a neutron star with a field exceeding 10(14) G. Later stages in the light curve are accurately fitted by a model for emission from the envelope of a magnetically confined pair-photon fireball, anchored to the surface of the rotating star, which contracts as it emits X-rays and then evaporates completely in a finite time. The complex four-peaked shape of the light curve likely provides the most direct evidence known for a multipolar geometry in the magnetic field of a neutron star.