GRB 090417B and its Host Galaxy: A Step Towards an Understanding of
  Optically-Dark Gamma-Ray Bursts
  GRB 090417B was an unusually long burst with a T_90 duration of at least 2130
s and a multi-peaked light curve at energies of 15-150 keV. It was optically
dark and has been associated with a bright star-forming galaxy at a redshift of
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 an ideal candidate for studying the origin of dark bursts. We find that
the dark nature of GRB 090417B cannot be explained by high redshift, incomplete
observations, or unusual physics in the production of the afterglow. 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. 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. Our results suggest that this dust sheet 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.
