GRB051022: physical parameters and extinction of a prototype dark burst

Abstract

GRB051022 was undetected to deep limits in early optical observations, but precise astrometry from radio and X-ray showed that it most likely originated in a galaxy at z~0.8. We report radio, optical, near infra-red and X-ray observations of GRB051022. Using the available X-ray and radio data, we model the afterglow and calculate the energetics of the afterglow, finding it to be an order of magnitude lower than that of the prompt emission. The broad-band modeling also allows us to precisely define various other physical parameters and the minimum required amount of extinction, to explain the absence of an optical afterglow. Our observations suggest a high extinction, at least 2.3 magnitudes in the infrared (J) and at least 5.4 magnitudes in the optical (U) in the host-galaxy restframe. Such high extinctions are unusual for GRBs, and likely indicate a geometry where our line of sight to the burst passes through a dusty region in the host that is not directly co-located with the burst itself.