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

¹ LATT, Laboratoire d'Astrophysique Toulouse-Tarbes, 14 Av. Edouard Belin, 31400 Toulouse, France e-mail: gstratta@ast.obs-mip.fr
² JILA and Department of Astrophysical and Planetary Sciences, University of Colorado at Boulder, 440 UCB, Boulder, CO 80309, USA
³ Department of Astrophysical Sciences, 4 Ivy Lane, Princeton, NJ 08544, USA
⁴ INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone, 00040, Rome, Italy

Received: 25 April 2005
Accepted: 15 June 2005

Abstract

We present an optical-to-X-ray spectral analysis of the afterglow of GRB 020405. The optical spectral energy distribution not corrected for the extragalactic extinction is significantly below the X-ray extrapolation of the single powerlaw spectral model suggested by multiwavelength studies. We investigate whether considerable extinction could explain the observed spectral “mismatch” by testing several types of extinction curves. For the first time we test extinction curves computed 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.