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 sample of Swift γ-ray bursts complete in redshift. The selection criteria of the sample are based on observational high-energy parameters of the prompt emission and therefore our sample should not be biased against dusty sight-lines. It is therefore expected that our inferences hold for the general population of γ-ray bursts. Our main result is that the optical/near-infrared extinction of γ-ray burst afterglows in our sample does not follow a single distribution. 87% of the events are absorbed by less than 2 mag, and 50% suffer from less than 0.3-0.4 mag extinction. The remaining 13% of the afterglows are highly absorbed. The true percentage of γ-ray burst afterglows showing high absorption could be even higher since a fair fraction of the events without reliable redshift measurement are probably part of this class. These events may be due to highly dusty molecular clouds/star forming regions associated with the γ-ray burst progenitor or along the afterglow line of sight, and/or to massive dusty host galaxies.No clear evolution in the dust extinction properties is evident within the redshift range of our sample, although the largest extinctions are at z ∼ 1.5 − 2, close to the expected peak of the star formation rate. Those events classified as dark are characterized, on average, by a higher extinction than typical events in the sample. A correlation between optical/near-infrared extinction and hydrogen-equivalent column density based on X-ray studies is shown although the observed N H appears to be well in excess compared to those observed in the Local Group. Dust extinction does not seem to correlate with GRB energetics or luminosity.

GRB hosts through cosmic time

“…We observe a mild trend with redshift such that GRB hosts at z = 1.5 tend to have larger dust content (A med V ∼ 1.2 mag) and line broadening (σ med ∼ 70 kms) on average than low-redshift hosts (A med V ∼ 0.6 mag, and σ med ∼ 40 kms). The significance of each of these results is only around 2σ, but the evolution in A V is remarkably consistent with a similar behavior observed for GRB afterglows (Covino et al 2013). 2.…”

“…Interestingly, however, a very similar trend -highest visual extinctions at z ∼ 1.5-2 -exists in the complete sample of A V values derived from afterglow data (Covino et al 2013) and in the evolution of the far UV attenuation in field galaxies (Cucciati et al 2012). It is also consistent with the somewhat higher fraction of dusty GRBs at higher redshift (Sect.…”

“…As representative comparison sample of the dust content towards GRBs (A GRB V ), we avail of the union of the afterglows from Greiner et al (2011) and Covino et al (2013). After excluding duplicates, and limiting the samples to the same redshifts as the host spectroscopy (z < 3.6), 75 GRBs remain to constrain the z < 1 1 < z < 2 2 < z < 3 3 < z < 3.…”

“…3. Redshift dependent fractions with errors of dusty afterglows from our X-Shooter emission-line spectroscopy (XES) in blue against representative GRB samples in black Covino et al 2013). …”

Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of theSwift/BAT6 complete sample of bright LGRBs

“…We used light curves published by Wiersema et al (2012) and Filgas et al (2011) for GRBs 091018 and 091127, respectively. For GRB 071112C, the joint data sets of Huang et al (2012) and Covino et al (2013) were used.…”