Gamma‐Ray Bursts in Molecular Clouds: H2Absorption and Fluorescence

If a gamma ray burst with strong UV emission occurs in a molecular cloud,there will be observable consequences resulting from excitation of thesurrounding H2. The UV pulse from the GRB will pump H2 intovibrationally-excited levels which produce strong absorption at wavelengths <1650 A. As a result, both the prompt flash and later afterglow will exhibitstrong absorption shortward of 1650 A, with specific spectroscopic features.Such a cutoff in the emission from GRB 980329 may already have been observed byFruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRIphotometry of GRB 990510 could also be explained by H2 absorption if GRB 990510is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping ofthe H2 will result in UV emission from the GRB which can extend over days ormonths, depending on parameters of the ambient medium and beaming of the GRBflash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s forstandard estimates of the parameters of the GRB and the ambient medium.Spectroscopy can distinguish this fluorescent emission from other possiblesources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters