GROND Coverage of the Main Peak of Gamma-Ray Burst 130925A

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 Nearinfrared 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 a possible interpretation.