Gamma‐Ray Bursts in Molecular Clouds: H2Absorption and Fluorescence

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Abstract
If a gamma-ray burst (GRB) with strong UV emission occurs in a molecular cloud, excitation of the surrounding H2 will have observable consequences. The UV will pump H2 into vibrationally excited levels that produce strong absorption at wavelengths λ 1650 Å. As a result, both the prompt flash and later afterglow will exhibit strong absorption shortward of 1650 Å, with specific spectroscopic features. Such a cutoff in the emission from GRB 980329 may have already been observed by Fruchter and coworkers; if so, GRB 980329 was at redshift 3.0 z 4.4. BVRI photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510 was at redshift 1.6 z 2.3. Fluorescence accompanying the UV pumping of the H2 will result in UV emission from the GRB which can extend over days or months, with 7.5-13.6 eV fluorescent luminosity ~1042 ergs s-1. Spectroscopy can distinguish this fluorescence from other possible sources of transient optical emission, such as a supernova.
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