GROND coverage of the main peak of Gamma-Ray Burst 130925A

TL;DR

This paper reports on optical/near-infrared observations of the ultra-long GRB 130925A, detecting a delayed flare that provides insights into the early emission processes of gamma-ray bursts.

Contribution

First dedicated optical/NIR observation of an ultra-long GRB's main peak, revealing a delayed flare and discussing its implications for GRB emission models.

Findings

Detected a nearly 2 mag optical/NIR flare delayed by 300-400 s

Flare decay time is shorter than its duration and delay

Discussed potential implications for understanding GRB emission processes

Abstract

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.