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 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 A 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 A < λ < 1705 A. 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.

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