Title: Gamma Ray Bursts
Speaker: Jan Van Paradijs
Abstract: On average once a day a burst of gamma rays is detected with the BATSE experiment on the Compton Gamma Ray Observatory. Most of them last between a tenth of a second and several minutes, and emit the bulk of their energy between 0.1 and 1 MeV. For a long time since their discovery in the 1960's it was generally believed that these gamma-ray bursts (GRBs) originate from galactic neutron stars. However, the completely isotropic sky distribution, and the relative dearth of very weak bursts, as established with BATSE in the early 1990's exclude that GRB sources are located in the galactic disk. Two possible models remained to explain the sky and brightness distributions of GRBs: they come from either a very large (several 100 kpc) halo around the galaxy, or from Gpc ('cosmological') distances. Agreement on which distance scale is the correct one could not be reached; it was generally felt that this would require the detection of counterparts in other wavelength bands. In the first half of 1997 this was accomplished when the BeppoSAX Wide Field Cameras provided the first rapid (several hours) accurate (several arcminutes) GRB locations. Rapid X-ray, optical and radio follow-up observations have led to the detections of GRB afterglows whose properties establish that GRBs come from cosmological distances: GRBs are by far the most powerful photon emitters known in the Universe.
Reference for students: The same Gamma Ray Burst chapter from "Unsolved Problems in Astrophysics," by Bahcall & Ostriker, as before, plus Jean has copies of two recent Nature letters (389, 261-265).