Fireballs in the Galactic Halo and $\gamma$-Ray Bursts

Abstract

If gamma ray burst sources are in the galactic halo they inevitably involve a creation of an opaque pair plasma fireball, just like in cosmological sources. We find that the typical physical conditions in a galactic halo fireball are: optical depth $\ap 10^8$, thermal energy $\ap 100 KeV$, maximal relativstic expansion $\G \ap 300 $ and a maximal baryonic load of $\ap 10^{-15} M_\odot$. This does not rule out galctic halo models but it poses an addtional severe constraint on all such sources. A comparison of these conditions with the physical conditions at cosmological fireballs reveal that galactic halo fireballs are less favorable than cosmological ones as sources of gamma ray bursts.