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Session 119 - Gamma Ray Bursts.
Oral session, Saturday, January 10
International Ballroom Center,
Two recent GRBs, GRB 970228 and GRB 970508, were observed with the Hubble Space Telescope. HST observations of GRB 970228 confirmed the association of GRB970228 with a possible host galaxy and showed that the GRB is situated off-center in the galaxy. This suggests that GRBs are probably not related to nuclear activity. The observations taken more than 200 days after the outburst showed no movement of the GRB, no fading of the extended component, and confirmed that the optical decay behaviour continued until such late times. This places strong contraints on the energetics of the gamma-ray burst, and further suggests a cosmological origin of the gamma-ray burst. HST observations of GRB970508, on the other hand, did not reveal any host galaxy. The observations show that any underlying galaxy must be fainter than R 25.5 which, coupled with its observed redshift of >0.8, indicates that the host galaxy must be at least 10 times fainter than L*. The implications will be discussed.
