EDP Sciences
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Volume 399, Number 3, March I 2003
Page(s) 913 - 918
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021870

A&A 399, 913-918 (2003)
DOI: 10.1051/0004-6361:20021870

Thermal components in the early X-ray afterglow of GRBs

D. Lazzati

Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
(Received 15 October 2002 / Accepted 11 December 2002)

The presence of thermal components in the early X-ray afterglows of $\gamma$-ray bursts is investigated. We discuss both the presence of a thermal continuum and, in particular, of collisional X-ray emission lines. We compute the predicted luminosity by a thin plasma for a range of metallicities for the continuum and the K $_\alpha$ lines of the elements Mg, Si, S, Ar, Ca and Fe. We show that light travel effects are dominant in the determination of the thermal continuum and line luminosities, and derive the relevant equations. We conclude that thermal lines and continua are unlikely to dominate the early afterglow of GRBs, unless the explosion site is surrounded by a very massive and extremely clumped shell of material. Such conditions are difficult to envisage in the close environment of the GRB progenitor, unless they are excited by some strong precursor activity, like in the Supranova scenario.

Key words: gamma-rays: burst -- line: formation -- radiation mechanisms: thermal

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