Fireballs in the galactic halo and gamma-ray bursts

If gamma-ray burst sources are in the Galactic halo, they inevitably involve the 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 ≈ 10⁸, thermal energy ≈ 100 keV, maximal relativistic expansion Γ ≈ 300, and a maximal baryonic load of ≈ 10^-15 M_⊙. This does not rule out Galactic halo models, but it poses an additional 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.