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.
- gamma rays
- radiation mechanisms