Optical reprocessing of gamma-ray bursts

One model for the optical flashes associated with three cosmic gamma-ray burst sources invokes the reprocessing of some of the gamma-radiation emitted by a hypothesized collapsed object in the surface layers of a nearby companion star. This model was investigated by carrying out detail, fully hydrodynamical calculations of such reprocessing in the surface layers of very low mass stars. It is found that, at most, 7 percent of the gamma-ray fluence incident on the companion star is reprocessed into the blue band; the time scale for this reprocessing is typically 100 s, which is long compared to the duration of the gamma-ray burst itself. Using this result, it is shown that there is marginal agreement between the observed and calculated ratios of gamma-ray fluence to optical fluence at earth.