Fireballs in the Galactic Halo and $\gamma$-Ray Bursts
  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. 
Tsvi Piran and Amotz Shemi