Volume 560, December 2013
|Number of page(s)||14|
|Published online||06 December 2013|
1 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
2 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
3 Universitá degli studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
4 Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, 128 00 Prague 2, Czech Republic
5 International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
6 Shanghai Astronomical Observatory, Chinese Academy of Sciences (SHAO), 80 Nandan Road, 200030 Shanghai, PR China
7 ESO, Alonso de Córdoba 3107, Vitacura, Casilla 19001, Santiago, Chile
8 ALMA JAO, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
9 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
10 American River College, Physics Dept., 4700 College Oak Drive, Sacramento, CA 95841, USA
11 Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
12 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
13 European Southern Observatory, Schwarzschild-Str. 2, 85748 Garching, Germany
14 Roger Williams Univ., One Old Ferry Road, Bristol, RI 02809, USA
15 CSIRO Astronomy & Space Science, Locked Bag 194, Narrabri, NSW 2390, Australia
Received: 13 February 2013
Accepted: 17 April 2013
Aims. With the afterglow of GRB 100621A being the brightest detected so far in X-rays, and superb GROND coverage in the optical/near-infrared during the first few hours, an observational verification of basic fireball predictions seemed possible.
Methods. In order to constrain the broad-band spectral energy distribution of the afterglow of GRB 100621A, dedicated observations were performed in the optical/near-infrared with the 7-channel Gamma-Ray Burst Optical and Near-infrared Detector (GROND) at the 2.2 m MPG/ESO telescope, in the sub-millimeter band with the large bolometer array LABOCA at APEX, and at radio frequencies with ATCA. Utilizing also Swift X-ray observations, we attempt an interpretation of the observational data within the fireball scenario.
Results. The afterglow of GRB 100621A shows a very complex temporal and spectral evolution. We identify three different emission components, the most spectacular one causing a sudden intensity jump about one hour after the prompt emission. The spectrum of this component is much steeper than the canonical afterglow. We interpret this component using a two-shell collision prescription after the first shell has been decelerated by the circumburst medium. We use the fireball scenario to derive constraints on the microphysical parameters of the first shell. Long-term energy injection into a narrow jet seems to provide an adequate description. Another noteworthy result is the large (AV = 3.6 mag) line-of-sight host extinction of the afterglow in an otherwise extremely blue host galaxy.
Conclusions. Some GRB afterglows have shown complex features, and that of GRB 100621A is another good example. Yet, detailed observational campaigns of the brightest afterglows promise to deepen our understanding of the formation of afterglows and the subsequent interaction with the circumburst medium.
Key words: gamma-ray burst: general / gamma-ray burst: individual: GRB 100621A / techniques: photometric
Based on data acquired with the Atacama Pathfinder Experiment (APEX) under ESO programme 285.D-5035(A).
Tables of the photometry are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/560/A70
© ESO, 2013
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