The nature of “dark” gamma-ray bursts

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: jcg@mpe.mpg.de; kruehler@mpe.mpg.de; pafonso@mpe.mpg.de; cclemens@mpe.mpg.de; filgas@mpe.mpg.de; nardini@mpe.mpg.de; felipe@mpe.mpg.de; arau@mpe.mpg.de; pschady@mpe.mpg.de
² Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
e-mail: klose@tls-tautenburg.de; rossi@tls-tautenburg.de
⁴ Clemson Univ., Dept. of Physics and Astronomy, Clemson, SC 29634, USA
e-mail: dieter@clemson.edu; aupdike@clemson.edu
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
e-mail: aky@ast.cam.ac.uk

Received: 22 July 2010
Accepted: 11 October 2010

Abstract

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.