Why ‘Galactic’ gamma-ray bursts might depend on environment: Blast waves around neutron stars

Although galactic models for gamma-ray bursts are hard to reconcile withthe isotropy data, the issue is still sufficiently open that both options should beexplored. The most likely 'triggers' for bursts in our Galaxy would be violentdisturbances in the magnetospheres of neutron stars. Any event of this kind islikely to expel magnetic flux and plasma at relativistic speed. Such ejecta wouldbe braked by the interstellar medium (ISM), and a gamma-ray flash may resultfrom this interaction. The radiative efficiency, of this mechanism would dependon the density of the circumstellar ISM. Therefore, even if neutron stars wereuniformly distributed in space (at least within 1-2 kpc of the Sun), the observedlocations of bursts would correlate with regions of above-average ISM density.1. INTRODUCTIONBefore there was any firm evidence on the isotropy of classical gamma-raybursts (GRBs), the most plausible interpretations involved magnetospheres ofneutron stars within our Galaxy. Indeed, on the basis of general theoretical plau-sibility, many people would have bet strongly against a cosmological interpreta-tion. The remarkable isotropy discovered by the BATSE experiment (togetherwith the 'flatter than Newtonian' counts) clearly shifts the odds substantially.If one lays aside theoretical preconceptions, the cosmological interpretation maynow seem strongly favoured. However, Bayesians who allow their assessment tobe influenced by some prior view of the relative plausibility of the alternativehypotheses may now find the arguments quite evenly balanced. For example,we may think the isotropy is 100 times easier to account for in a cosmologicalthan in a galactic model; however, if we previously would have bet 100 to 1 infavour of a galactic origin, we end up betting 'evens'. This is our rationale forcontinuing to consider both options.If they are not cosmological, GRBs would most likely populate a relativelynearby region of the Galactic disk, at distances ^ (1 - 2) kpc. [As noted byother speakers, 'halo' models entail (at least in a mild form) many of the sameInstitute of Astronomy, Madingley Road, Cambridge CBS OHA, England.Pennsylvania State University, 525 Davey Lab, University Park, PA 16803.