Are H [CSC]i[/CSC] Supershells the Remnants of Gamma-Ray Bursts?

Abstract

Gamma-ray bursts (GRBs) are thought to originate at cosmological distances from the most powerful explosions in the universe. If GRBs are not beamed, then the distribution of their number as a function of γ-ray flux implies that they occur once per ~(0.3-40) million years per bright galaxy and that they deposit ≳1053 ergs into their surrounding interstellar medium. The blast wave generated by a GRB explosion would be washed out by interstellar turbulence only after tens of millions of years when it finally slows down to a velocity of ~10 km s-1. This rather long lifetime implies that there could be up to several tens of active GRB remnants in each galaxy at any given time. For many years, radio observations have revealed the enigmatic presence of expanding neutral-hydrogen (H I) supershells of near-kiloparsec radius in the Milky Way and in other nearby galaxies. The properties of some supershells cannot be easily explained in terms of conventional sources such as stellar winds or supernova explosions. However, the inferred energy and frequency of the explosions required to produce most of the observed supershells agree with the above GRB parameters. More careful observations and analysis might reveal which fraction of these supershells are GRB remnants. We show that if this link is established, the data on H I supershells can be used to constrain the energy output, the rate per galaxy, the beaming factor, and the environment of GRB sources in the universe.