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

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. © ESO, 2014.