THE DISTANCE SCALE TO GAMMA-RAY BURSTS

Abstract

ABSTRACT We do not know the distance scale to gamma-ray bursts. Here I discuss several observational results and theoretical calculations which provide evidence about the distance scale. First, I describe the recent discovery that many neutron stars have high enough velocities to escape from the Milky Way. These high veloicy neutron stars form a distant, previously unknown Galactic "corona." This distant corona is isotropic when viewed from Earth, and consequently, the population of neutron stars in it can easily explain the angular and brightness distributions of the BATSE bursts. If this were all of the evidence that we considered, we could not distinguish between the cosmological and Galactic hypotheses. I contend that we can go further, by considering other important evidence. I draw attention to the many similarities between soft gamma-ray repeaters, which are known to be high velocity neutron stars, and gamma-ray bursts. I point out that the source of the famous 1979 March 5 event, which is a high velocity neutron star 50 kpc away from us, demonstrates that high velocity neutron stars are capable of producing bursts which have the energy, the duration, and the spectrum of gamma-ray bursts. Finally, I comment that high velocity neutron stars in a distant Galactic corona can account for cylotron lines and repeating, naturally explain the absence of bright optical counterparts in gamma-ray burst error boxes, whereas all of these present major difficulties for cosmological models. I conclude that when we consider all of the evidence, it adds up to a strong case for the Galactic hypothesis.