Volume 526, February 2011
|Number of page(s)||10|
|Published online||16 December 2010|
The nature of “dark” gamma-ray bursts
Max-Planck-Institut für extraterrestrische Physik,
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2 Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
3 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
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4 Clemson Univ., Dept. of Physics and Astronomy, Clemson, SC 29634, USA
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5 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
Accepted: 11 October 2010
Context. Thirteen years after the discovery of the first afterglows, the nature of dark gamma-ray bursts (GRB) still eludes explanation: while each long-duration GRB typically has an X-ray afterglow, optical/NIR emission is only seen for 40–60% of them.
Aims. Here we use the afterglow detection statistics of the systematic follow-up observations performed with GROND since mid-2007 in order to derive the fraction of “dark bursts” according to different methods, and to distinguish between various scenarios for “dark bursts”.
Methods. Observations were performed with the 7-channel “Gamma-Ray Optical and Near-infrared Detector” (GROND) at the 2.2 m MPI/ESO telescope. We used the afterglow detection rate in dependence on the delay time between GRB and the first GROND exposure.
Results. For long-duration Swift bursts with a detected X-ray afterglow, we achieve a 90% (35/39) detection rate of optical/NIR afterglows whenever our observations started within less than 240 min after the burst. Complementing our GROND data with Swift/XRT spectra we construct broad-band spectral energy distributions and derive rest-frame extinctions.
Conclusions. We detect 25–40% “dark bursts”, depending on the definition used. The faint optical afterglow emission of “dark bursts” is mainly due to a combination of two contributing factors: (i) moderate intrinsic extinction at moderate redshifts, and (ii) about 22% of “dark” bursts at redshift >5.
Key words: gamma-rays burst: general / techniques: photometric
© ESO, 2010
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