Progress in gamma-ray burst astronomy with GINGA

Abstract

We report two important new observational results made possible by the wide energy range (1.5 to 375 keV) of the gamma-ray burst detectors on board the GINGA satellite. 1. 1) Lower energy X-ray emission has a much longer decay time than higher energy emission. The spectrum of the tail of the gamma-ray burst is possibly from a blackbody at a temperature of about 1 to 2 keV. By assuming a blackbody spectrum from a surface with a definite emissivity, we are able to estimate that the size of the emission area is about 1 km in radius for a source distance of 1 kpc. 2. 2) Spectral features were clearly seen at about 20 and 40 keV with harmonic. Interpreting these features as the fundamental and first harmonics of the cyclotron absorption lines seem to require a strong magnetic field (about 10 12 gauss) at the emission region of the gamma-ray burst. These two results strongly support the hypothesis that the gamma-ray bursts originate on the surface of strongly magnetized neutron stars. This work was done by an international collaboration between LANL and ISAS.