Gamma-ray burst in a molecular cloud: Destruction of dust and H ₂ and the emergent spectrum

A gamma-ray burst (GRB) with strong optical-UV emission occurring in a molecular cloud will photodissociate H₂, photoionize H₂, H, and He, and destroy dust grains. We model these processes, including time-dependent radiative transfer, in both continuum radiation and the resonance lines of H₂- The UV will pump H₂ into vibrationally excited levels. We calculate the absorption spectrum imprinted on radiation from the GRB at various times. In addition to the strong absorption lines of v = 0 H₂ at λ < 1110 Å due to cold ambient gas, we find that radiation reaching us from the GRB and its afterglow will show strong absorption lines due to vibrationally excited H₂ at 1110 Å < λ < 1705 Å. These absorption lines, if observed, would provide unequivocal evidence for the association of the GRB with molecular gas. Low-resolution spectra will exhibit conspicuous features due to clustering of individual lines; a list of the strongest such absorption features is given for the spectral resolution R ≈ 350 characteristic of the grism on the Swift UV-optical telescope.