ar X iv : a st ro - p h / 99 09 02 0 v 1 1 S ep 1 99 9 Dust sublimation by GRBs and its implications

The prompt optical flash recently detected accompanying GRB990123 suggests that, for at least some GRBs, γ-ray emission is accompanied by prompt optical-UV emission with luminosity L(1 − 7.5 eV) ≈ 1 × 10 49 (∆Ω/4π)erg/s, where ∆Ω is the solid angle into which γ-ray and optical-UV emission is beamed. Such an optical-UV flash can destroy dust in the beam by sublimation out to an appreciable distance, ≈ 10 pc, and may clear the dust out of as much as ∼ 10 7 (∆Ω/4π)M ⊙ of molecular cloud material on an apparent time scale of ∼ ten seconds. Detection of time dependent extinction on this time scale would therefore provide strong constraints on the GRB source environment. Dust destruction implies that existing, or future, observations of not-heavily-reddened fireballs are not inconsistent with GRBs being associated with star forming regions. In this case, however, if γ-ray emission is highly beamed, the expanding fireball would become reddened on a ∼1 week time scale. If the optical depth due to dust beyond ≈ 8 pc from the GRB is 0.2 ∼ < τ V ∼ < 2, most of the UV flash energy is converted to infra-red, λ ≈ 1µm, radiation with luminosity L IR ≈ 10 41 erg/s extending over an apparent duration of ≈ 20(1 + z)(∆Ω/0.01) day. Dust infra-red emission may already have been observed in GRB970228 and GRB980326, and may possibly explain their unusual late time behavior.