Dust sublimation by gamma-ray bursts and its implications

The prompt optical flash recently detected accompanying GRB 990123 suggests that, for at least some gamma-ray bursts (GRBs) gamma-ray emission is accompanied by prompt optical-UV emission with luminosity L(1-7.5 eV) approximate to 1 x 10(49)(Delta Omega/4 pi) ergs s(-1) where Delta Omega is the solid angle into which gamma-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, approximate to 10 pc, and may clear the dust out of as much as similar to 10(7)(Delta Omega/4 pi) M. of molecular cloud material on an apparent timescale of similar to 10 s. Detection of time-dependent extinction on this timescale would therefore provide strong constraints on the GRB source environment. Dust destruction implies that existing or future observations of fireballs that are not heavily reddened are not inconsistent with GRBs being associated with star-forming regions. In this case, however, if the initial flash is highly beamed, the expanding fireball would become reddened on a similar to 1 week timescale. If the optical depth due to dust beyond approximate to 8 pc from the GRB is 0.2 less than or similar to tau(V) less than or similar to 2, most of the UV flash energy is converted to infrared (lambda approximate to 1 mu m) radiation with luminosity L-IR approximate to 10(41) ergs s(-1) extending over an apparent duration of approximate to 20 (1 + z)(Delta Omega/0.01) days. Dust infrared emission may already have been observed in GRB 970228 and GRB 980326 and may possibly explain their unusual late-time behavior.