Gamma Ray Bursts

Gamma ray bursts are short (~10 ms to ~100s) flashes of gamma rays from beyond the Solar System. Most of their energy is emitted around 1 MeV, with only a small fraction in the X-ray band. Absorption features have been reported in the 15-100 keV range which have been interpreted as cyclotron absorption in 10^12 gauss magnetic fields. In addition, emission lines around ~430 keV have also been claimed; these lines were attributed to redshifted annihilation emssion.

Over the years after bursts were discovered the consensus developed that bursts originated on nearby (~100 pc) neutron stars. Various theories were developed which favored nearby sources on energetic grounds. The absorption lines were a major foundation of these theories since neutron stars are the only known anchor of teragauss magnetic fields. Consequently BATSE was launched on CGRO to confirm the local neutron star hypothesis and to further study burst phenomenology.

However, the BATSE observations deepened the mystery. The local neutron star hypothesis predicted that faint bursts would be localized to the plane of the Galaxy. However, BATSE found that there are fewer faint bursts than expected for a uniform source density in three dimensional Euclidean space, yet the bursts are still isotropically distributed on the sky. Thus the key prediction of the local neutron star hypothesis failed, invalidating the hypothesis. We appear to be at the center of a spherical, bounded source distribution. The primary post-BATSE theories were that bursts originate in a large Galactic halo (at a large enough radius such that our offset from the center of the Galaxy is not evident), or at cosmological distances (where the spatial curvature reduces the apparent density of faint sources). Coalescing neutron star-neutron star or neutron star-black hole binaries are popular scenarios for cosmological models.

Compounding the mystery has been the absence of a definitive detection of an absorption line in the BATSE spectra. Although the spectra from ~250 bursts have been searched, only ~50 have been intense enough so that lines in their spectra may be detectable. Given the relatively small number of bursts searched, the apparent discrepancy between BATSE and previous missions is not yet compelling, and the validity of the previous claims of line detections should not be questioned on the basis of the BATSE results. In addition, we are analyzing a new group of line candidates identified by a more systematic, computerized screening procedure than previously applied; some of these candidates are very promising.

Controversy has characterized the study of gamma-ray bursts since BATSE's launch. Whether bursts repeat (which would be difficult for cosmological models to explain) and whether cosmological signatures are evident in the burst ensemble have been debated.

Recently the Italian-Dutch X-ray satellite Beppo-SAX localized a number of bursts to 3 arcminutes using its Wide Field Cameras (which are orthogonal to the main telescopes). The positions were disseminated within hours, and the main telescopes were pointed at the bursts' locations. In a number of cases X-ray transients were observed and localized to 50 arcseconds. Subsequently optical and radio transients were discovered. A somewhat extended source (a host galaxy?) was detected under the optical transient associated with GRB 970228, and more significantly, absorption lines consistent with gas at z=0.835 were detected in a spectrum of the GRB 970508 optical transient. Thus almost definitely some, and probably all, bursts originate at cosmological distances.

The brightnesses of the Beppo-SAX transients are not proportional to the burst intensities, and transients have not been detected for all these bursts. Host galaxies of the expected brightnesses have not been observed near the Beppo-SAX burst positions nor in the smaller burst error boxes accumulated over years of burst observations. Thus the counterparts associated with the Beppo-SAX bursts have advanced the study of gamma-ray bursts, but have not eliminated the mystery of this phenomenon.

Last updated August 3, 1997.

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