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Gamma-ray bursts

At the time of the launch of ASCA in 1993 the origin of gamma-ray bursts (GRBs) was unknown so deep searches were made for X-ray counterparts to those GRBs with small error boxes. Several faint X-ray sources were detected. One is clearly a distant galaxy, but is likely to be a chance coincidence. The 1996 discovery of X-ray afterglows to GRBs and their localization to arc minute accuracy using Beppo-SAX opened a new era in the observations of GRBs. While ASCA operations are not optimized for a rapid response on a timescale of hours (the satellite is available for commanding only a few times a day), it has been possible through the quick efforts of the operations team to follow some X-ray afterglows. In particular, for GRB 970828 ASCA observed a flare in the X-ray afterglow that challenges the current relativistic fireball model.

In 1993, ASCA identified a soft gamma-ray repeater (SGR 1806-20) with a young SNR. This was the first firm identification of a SGR with a SNR. A stable X-ray source in the SNR leads to the suspicion that the SGR activities come from a central X-ray source in the SNR and this should be a pulsar. The ASCA identification established that GRBs and SGRs were in different categories. Recently, RXTE has observed the same SGR and found a 7.47 second pulsar. This led to a reanalysis of the ASCA data and the discovery of a large change in periodicity between the ASCA and RXTE observations. This large change requires this neutron star to have a super strong magnetic field of roughly 10^15 gauss. This is extraordinary as a magnetic field for a neutron star, but might be the reason for the SGR activity. Further ASCA observations of other SGRs may lead to similar conclusions (Kouveliotou et al. 1998 Nature 393, 236).

Last modified: Tuesday, 26-Jun-2001 14:22:36 EDT

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This file was last modified on Tuesday, 26-Jun-2001 14:22:36 EDT

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