GROND coverage of the main peak of gamma-ray burst 130925A
van Eerten, H.; Greiner, J.; Hurley, K.; Klose, S.; von Kienlin, A.; Schady, P.; Rau, A.; Krühler, T.; Nicuesa Guelbenzu, A.; Elliott, J.; Kann, D. A.; Sudilovsky, V.; Zhang, X. -L.; Frederiks, D. D.; Svinkin, D. S.; Schmidl, S.; Aptekar, R. L.; Golenetskii, S. V.; Ulanov, M. V.; Graham, J. F.; Tanga, M.; Varela, K.; Bhat, P. N.; Yu, H. -F.; Beloborodov, A.; Bolmer, J.
Germany, Chile, Russia, United States
Abstract
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.