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 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 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 } \simeq 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 } \propto \nu ^ { -0.9 } , with no sign of curvature , or a 2200 Å 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 \alpha < 1.72 .