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. Malesani21 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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