Journal Article

Optical afterglows of gamma-ray bursts: a bimodal distribution?

Marco Nardini, Gabriele Ghisellini and Giancarlo Ghirlanda

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 383, issue 3, pages 1049-1057
Published in print January 2008 | ISSN: 0035-8711
Published online January 2008 | e-ISSN: 1365-2966 | DOI: https://dx.doi.org/10.1111/j.1365-2966.2007.12588.x

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The luminosities of the optical afterglows of gamma-ray bursts, 12 h (rest-frame time) after the trigger, show a surprising clustering, with a minority of events being at a significantly smaller luminosity. If real, this dichotomy would be a crucial clue to understand the nature of optically dark afterglows, i.e. bursts that are detected in the X-ray band, but not in the optical. We investigate this issue by studying bursts of the pre-Swift era, both detected and undetected in the optical. The limiting magnitudes of the undetected ones are used to construct the probability that a generic burst is observed down to a given magnitude limit. Then, by simulating a large number of bursts with pre-assigned characteristics, we can compare the properties of the observed optical luminosity distribution with the simulated one. Our results suggest that the hints of bimodality present in the observed distribution reflect a real bimodality: either the optical luminosity distribution of bursts is intrinsically bimodal, or there exists a population of bursts with a quite significant grey absorption, i.e. wavelength-independent extinction. This population of intrinsically weak or grey-absorbed events can be associated with dark bursts.

Keywords: radiation mechanisms: non-thermal; dust, extinction; gamma-rays: bursts

Journal Article.  6940 words.  Illustrated.

Subjects: Astronomy and Astrophysics