Astronomy and Astrophysics – Astronomy
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...182.3305s&link_type=abstract
American Astronomical Society, 182nd AAS Meeting, #33.05; Bulletin of the American Astronomical Society, Vol. 25, p.846
Astronomy and Astrophysics
Astronomy
Scientific paper
We report the detection of an unusual high-energy transient by the gamma -ray burst detector onboard the satellite Ginga. The spectral properties of this event, which we refer to as a Medium Energy Burst (MEB900129), place it between those of Type I X-ray bursts and classical gamma -ray bursts. The event was much softer than classical gamma -ray bursts, with a spectrum that peaks between 16-18 keV, and decreases both above and below the peak. A thermal bremstrahlung fit to the 20-400 keV emission gives a characteristic temperature of ~ 24 keV; much softer (cooler) than the >= 100 keV temperatures characteristic of the classical gamma -ray bursts, but similar to that of the soft gamma -ray repeaters (SGRs). Although the shape of the spectrum is similar to that of type I X-ray bursts, the peak photon energy is higher by about a factor of 5. We have established with high confidence that the spectrum rolls over below 16 keV. Several spectral models which include photoelectric absorption from a neutral medium (with cosmic abundance of elements) have been fit to the data. Column densities of ~ 10(24) \ cm(-2) give acceptable fits to the low energy roll-over, however, a power law fit in this energy range with index alpha ~ -2.3 cannot be excluded by the data. In addition, such power law models with slopes greater than -1.5 are strongly rejected by the data (A Rayleigh-Jeans spectrum has slope -1.0). The time history of the event is simple, with a rise-time of ~ 0.7 seconds followed by an exponential decay with a 3 second time-scale, similar to those seen from type I X-ray bursts as well as some classical gamma -ray bursts, but not characteristic of the SGRs. A source radiating at 10(38) erg/sec would have to be about 1 kpc distant in order to produce the measured peak flux of ~ 1.5 times 10(-6) erg/cm(2) /sec.
Fenimore Edward E.
Murakami Tadayoshi
Strohmayer Tod E.
Yoshida Akio
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