Extinction properties of the X-ray bright/optically faint afterglow of GRB 020405

Received ; accepted Abstract. We present an optical-to-X-ray spectral analysis of the aft erglow of GRB 020405. The optical spectral energy dis- tribution not corrected for the extragalactic extinction i s significantly below the X-ray extrapolation of the single p owerlaw spectral model suggested by multiwavelength studies. We investigate whether considerable extinction could explain t he ob- served spectral "mismatch" by testing several types of extinction curves. For the first time we test extinction curves co mputed with time-dependent numerical simulations of dust grains destruction by the burst radiation. We find that an extinction law weakly dependent on wavelength can reconcile the unabsorbed optical and X-ray data with the expected synchrotron spectrum. A gray extinction law can be provided by a dust grain size distribution biased toward large grains.

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