ApJ in press Preprint typeset using LATEX style emulateapj v. 3/25/03 DUST DEPLETION AND EXTINCTION IN A GAMMA-RAY BURST AFTERGLOW

ApJ in press We put stringent constraints for the ¯rst 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 Keck I LRIS 4.65 h after the burst. From the absorption lines in the spectrum, we derive very high column densities for six heavy elements with di®erent refractory properties. The relative abundances resemble the dust depletion patterns in the Milky Way, from which we infer a visual extinction of AV ' 0:4 and AV> 0:3 at 95 % con¯dence 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 Fº / º ¡0:9, with no sign of curvature, or a 2200 ºA extinction feature, suggesting low reddening. The Milky Way or Magellanic Cloud reddenings are possible only for very low visual extinctions (AV < 0:08 or AV < 0:2, respectively at 95 % con¯dence), inconsistent with the AV values inferred from the depletion analysis. If we assume a GRB intrinsic spectrum and an extinction law of the forms F i ¸ = FV (5500=¸) ® and A ¸ = AV (5500=¸) ° , we obtain (for AV = 0:4) the constraints from continuum spectrum: ° < 0:85 and ® < 1:72