Gamma-ray burst in a molecular cloud:: Destruction of dust and H2 and the emergent spectrum

A gamma-ray burst (GRB) with strong optical-UV emission occurring in a molecular cloud will photodissociate H-2, photoionize H-2, 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 H-2. The UV will pump H-2 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 H-2 at lambda < 1110 angstrom 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 H-2 at 1110 angstrom < lambda < 1705 angstrom. 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 approximate to 350 characteristic of the grism on the Swift UV-optical telescope.