GROND coverage of the main peak of gamma-ray burst 130925A

J. Greiner, H.-F. Yu, T. Krühler, D. D. Frederiks, A. Beloborodov, P. N. Bhat, J. Bolmer, H. van Eerten, R. L. Aptekar, J. Elliott, S. V. Golenetskii, J. F. Graham (+14 others)
2014 Astronomy and Astrophysics  
Prompt or early optical emission in gamma-ray bursts 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). We performed dedicated observations of the ultra-long duration (T90 about 7000 s) GRB 130925A in the optical/near-infrared with the 7-channel "Gamma-Ray Burst Optical and Near-infrared Detector" (GROND) at the 2.2m MPG/ESO
more » ... 2.2m MPG/ESO telescope. We detect an optical/NIR 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. While we cannot offer a straightforward explanation, we discuss the implications of the flare properties and suggest ways toward understanding it.
doi:10.1051/0004-6361/201424250 fatcat:xwoutziy5zafxm26kvf3q32tlm