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Models of Gamma-ray bursts

Models of gamma-ray bursts
On the theoretical side, it was once strongly believed that neutron stars within the Milky Way were the sources of the gamma-ray bursts. But data from recent missions like CGRO have caused many of the supporting arguments for this idea to be proven wrong. The matter is one of the most interesting, and unresolved, debates in high-energy astrophysics today.

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A theoretical understanding of gamma-ray bursts will comprise several components: a site, an energy source, and an emission mechanism. The sites must be consistent with the observed isotropy and inhomogeneity, the energy source must be sufficient to produce the observed intensities for the distances assumed, and the emission mechanism must be able to reproduce the time scales and the spectra observed in bursts. Satisfying even these minimal observational prerequisites has proved difficult. There have been over a hundred theoretical papers proposing a wide range of scenarios for gamma-ray bursts. None have provided complete details specifying the site, the energy source, and an analysis of the energy emission processes.

Are the sources of Gamma-Ray Bursts in our Galaxy?

The particular Galactic model that some scientists are now studying is motivated by the recent discovery that many neutron stars are born with a higher velocity than previously thought (>500 km/s rather than <200 km/s). This would allow them to escape from the disk and move into a halo or corona around the Galaxy at a distance of about 100 kpc. This distant corona contains an ample population of sources which appear isotropic when viewed from Earth, and can therefore easily account for the angular and brightness distributions of the BATSE bursts. The Galactic corona model has the attractive feature that it can easily explain rapid time variability, cyclotron lines, repetition, and the lack of bright optical counterparts.

Are the sources of GRBs outside our Galaxy?

Recent observations of apparent counterparts in other parts of the electromagnetic spectrum seem to imply that at least some, if not all, GRB occur in galaxies other than the Milky Way. The observed isotropy is also a necessary requirement of cosmological models. The apparent inhomogeneity would result from redshift effects, and possibly source evolution. If gamma-ray bursts are cosmological, however, their energy release must be gigantic. For the brightest bursts, if the intrinsic emission is isotropic the total energy in gamma-rays must be 1053 to 1054 ergs, which is at least as great as that produced in supernovae. The difference, of course, is that in supernovae only 1051 ergs comes out in kinetic energy and visible light, and almost all of the photons are well below X-ray energies. Cosmological models are being developed which can get all that energy into high-energy photons; they are currently favored by the majority of astrophysicists.

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