A&A 396, L5-L9 (2002)
DOI: 10.1051/0004-6361:20021618

Letter

The afterglow of GRB 021004: Surfing on density waves

D. Lazzati¹, E. Rossi¹, S. Covino², G. Ghisellini² and D. Malesani<sup>2

1</sup>  Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
²  INAF - Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy

(Received 15 October 2002 / Accepted 6 November 2002)

Abstract
We present a model for the early optical afterglow of GRB 021004. This burst had one of the earliest detected optical afterglows, allowing for a dense optical sampling. The lightcurve was peculiar, since bright bumps were superimposed to the regular power-law decay observed in many other events. We show that, given their time scale and shape, the bumps are likely due to the interaction of the fireball with moderate density enhancements in the ambient medium. The enhancements have a density contrast of order 10, modifying only slightly the dynamics of the fireball, which therefore surfs on them rather than colliding into them. A relativistic reverse shock does not develop. Since the interaction takes place when the fireball is still hyper-relativistic it is not possible to understand if the overdensities are localized in clumps or are spherically symmetric around the GRB progenitor. The monotonic decrease of the contrast of successive rebrightenings suggests however the presence of clumps embedded in a uniform environment. Such an interpretation, complemented by the detection of several high velocity absorption systems in the optical spectrum, strongly suggests that GRB 021004 exploded within the remnant of a previous explosion.

Key words: gamma rays: bursts -- radiation mechanisms: non-thermal -- ISM: structure

Offprint request: D. Lazzati, lazzati@ast.cam.ac.uk

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