Volume 568, August 2014
|Number of page(s)||9|
|Published online||20 August 2014|
1 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
2 Excellence Cluster Universe, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
3 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001 Santiago 19, Chile
4 Ioffe Physical-Technical Institute, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia
5 Columbia University, Physics Dept., 538 W 120th Street, New York, NY 10027, USA
6 Center for Space Plasma and Aeronomic Research, Univ. Alabama, Huntsville, AL 35805, USA
7 Technische Universität München, Physik Dept., James-Franck-Str., 85748 Garching, Germany
8 Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
9 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
Received: 22 May 2014
Accepted: 11 July 2014
Aims. Prompt or early optical emission in gamma-ray bursts (GRBs) is notoriously difficult to measure, and observations of the dozen cases show a large variety of properties. Yet, such early emission promises to help us achieve a better understanding of the GRB emission process(es).
Methods. We performed dedicated observations of the ultra-long duration (T90 about 7000 s) Swift GRB 130925A 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.
Results. We detect an optical/near-infrared flare with an amplitude of nearly 2 mag which is delayed with respect to the keV−MeV prompt emission by about 300−400 s. The decay time of this flare is shorter than the duration of the flare (500 s) or its delay.
Conclusions. While we cannot offer a straightforward explanation, we discuss the implications of the flare properties and suggest ways toward understanding it.
Key words: gamma rays: stars / radiation mechanisms: non-thermal
Partly based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ID 092.A-0231(B).
The GROND photometry table is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/568/A75
© ESO, 2014
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