The Gamma-Ray Burst GRB 090417b and its Host Galaxy: A Step Towards an Understanding of Optically-Dark Gamma-Ray Bursts

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GRB 090417B was an unusually long gamma-ray burst with a T90 duration of at least 2131 s and a multi-peaked light curve at energies of 15-150 keV. It was an optically dark burst and has been convincingly associated with a bright star-forming galaxy at a redshift of z = 0.345 that is broadly similar to the Milky Way. This is one of the few cases where a host galaxy has been clearly identified for a dark gamma-ray burst and thus it is an ideal candidate for studying the origin of dark bursts. We find that the dark nature of GRB 090417B can not be explained by high redshift, incomplete observations, or unusual physics in the production of the afterglow. The Swift/XRT X-ray data are consistent with the afterglow being obscured by a dense, localized sheet of dust approximately 30-80 pc from the burst along the line of sight. Assuming the standard relativistic fireball model for the afterglow we find that the optical flux is at least 2.5 mag fainter than predicted by the X-ray flux. We are able to explain the lack of an optical afterglow, and the evolution of the X-ray spectrum, by assuming localized dust along the line of sight. Our results suggest that this dust imparts an extinction of A_V > 12 mag, which is sufficient to explain the missing optical flux. GRB 090417B is an example of a gamma-ray burst that is dark due to the localized dust structure in its host galaxy.

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