Dust depletion and extinction in a gamma-ray burst afterglow

We put stringent constraints for the first time on the dust properties in the circumburst medium of a gamma-ray burst (GRB) afterglow. This is based on the optical spectrum of GRB 020813 (z = 1.255) obtained with the Keck I LRIS 4.65 hr after the burst. From the absorption lines in the spectrum, we derive very high column densities for six heavy elements with different refractory properties. The relative abundances resemble the dust depletion patterns in the Milky Way, from which we infer a visual extinction of A(V) similar or equal to 0.4 and A(V) > 0.3 at 95% confidence level. However, the high columns of metals and dust contrast with an observed UV continuum spectrum that is remarkably close to a power law of the form F-nu proportional to nu(-0.9), with no sign of curvature or a 2200 Angstrom extinction feature, suggesting low reddening. The Milky Way or Magellanic Cloud reddenings are possible only for very low visual extinctions (A(V) < 0.08 or A(V) < 0.2, respectively, at 95% confidence), inconsistent with the A(V) values inferred from the depletion analysis. If we assume a GRB intrinsic spectrum and an extinction law of the forms F-lambda(i) = F-V(5500/lambda)(alpha) and A(lambda) = A(V) (5500/lambda)(gamma), we obtain (for A(V) = 0.4) the constraints from continuum spectrum gamma < 0.85 and α < 1.72.