GROND coverage of the main peak of Gamma-Ray Burst 130925A
  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 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. 
J. Greiner, H.-F. Yu, T. Kr\"uhler, D. D. Frederiks, A. Beloborodov,   P. N. Bhat, J. Bolmer, H. van Eerten, R. L. Aptekar, ...
astro-ph.HE