HOW FAR AWAY ARE GAMMA-RAY BURSTERS?



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HOW FAR AWAY ARE GAMMA-RAY BURSTERS?

Bohdan Paczynski

Princeton University Observatory, Princeton, NJ 08544-1001

Abstract:

The positions of over 1000 gamma-ray bursts detected with the BATSE experiment on board of the Compton Gamma Ray Observatory are uniformly and randomly distributed in the sky, with no significant concentration to the galactic plane or to the galactic center. This is consistent with the previous, less precise experiments. The strong gamma-ray bursts have the intensity distribution consistent with the number density independent of distance in the Euclidean space. The weak gamma-ray bursts are relatively rare, indicating that either their number density is reduced at large distances or that the space in which they are distributed is non-Euclidean. In other words, we appear to be at the center of a spherical and bounded distribution of bursters. This is consistent with the distribution of all objects that are known to be at cosmological distances (like galaxies and quasars), but inconsistent with the distribution of any objects which are known to be in our galaxy (like stars and globular clusters). If the bursters are indeed at cosmological distances then a simple prediction follows: the weakest bursts should be redshifted, i.e. on average their durations should be longer and their spectra should be softer than the corresponding quantities for the strong bursts. There is evidence for both effects in the BATSE data.

At this time the cosmological distance scale is favored above the galactic one, but is not proven. A definite proof (or dis-proof) of the galactic distance scale could be provided with the results of a search for very weak bursts in the Andromeda galaxy (M31) with an instrument times more sensitive than BATSE.

If the bursters are indeed at cosmological distances then they are the most luminous sources of electromagnetic radiation known in the universe. At this time we have no clue as to their nature, even though well over a hundred suggestions were published in the scientific journals. An experiment providing arc second positions would greatly improve the likelihood that counterparts of gamma-ray bursters are finally detected. A new interplanetary network would offer the best opportunity.





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Jerry Bonnell
Tue Jun 6 14:33:23 EDT 1995