Dust extinctions for an unbiased sample of gamma-ray burst afterglows

Abstract

In this paper we compute rest-frame extinctions for the afterglows of a\nsample of gamma-ray bursts complete in redshift. The selection criteria of the\nsample are based on observational high-energy parameters of the prompt emission\nand therefore our sample should not be biased against dusty sight-lines. It is\ntherefore expected that our inferences hold for the general population of\ngamma-ray bursts. Our main result is that the optical/near-infrared extinction\nof gamma-ray burst afterglows in our sample does not follow a single\ndistribution. 87% of the events are absorbed by less than 2 mag, and 50% suffer\nfrom less than 0.3-0.4 mag extinction. The remaining 13% of the afterglows are\nhighly absorbed. The true percentage of gamma-ray burst afterglows showing high\nabsorption could be even higher since a fair fraction of the events without\nreliable redshift measurement are probably part of this class. These events may\nbe due to highly dusty molecular clouds/star forming regions associated with\nthe gamma-ray burst progenitor or along the afterglow line of sight, and/or to\nmassive dusty host galaxies. No clear evolution in the dust extinction\nproperties is evident within the redshift range of our sample, although the\nlargest extinctions are at z~1.5-2, close to the expected peak of the star\nformation rate. Those events classified as dark are characterized, on average,\nby a higher extinction than typical events in the sample. A correlation between\noptical/near-infrared extinction and hydrogen-equivalent column density based\non X-ray studies is shown although the observed NH appears to be well in excess\ncompared to those observed in the Local Group. Dust extinction does not seem to\ncorrelate with GRB energetics or luminosity.\n