Dust sublimation by gamma-ray bursts and its implications

The prompt optical flash recently detected accompanying GRB 990123 suggests that, for at least some γ-ray bursts (GRBs), γ-ray emission is accompanied by prompt optical-UV emission with luminosity L(1-7.5 eV) ≈ 1 × 10⁴⁹(ΔΩ/4π) ergs s^-1, where ΔΩ 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 ∼ 10⁷(ΔΩ/4π) M_⊙ of molecular cloud material on an apparent timescale of ∼ 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 ∼ 1 week timescale. If the optical depth due to dust beyond ≈ 8 pc from the GRB is 0.2 ≲ κ_V ≲ 2, most of the UV flash energy is converted to infrared (λ ≈ 1 μm) radiation with luminosity L_IR ≈ 10⁴¹ ergs s^-1 extending over an apparent duration of ≈ 20(1 + z)(ΔΩ/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.