Modelling the prompt optical emission of GRB 180325A: the evolution of a spike from the optical to gamma-rays

TL;DR

This paper analyzes multi-wavelength observations of GRB 180325A, revealing that early optical flashes likely originate from reverse shocks, while gamma-ray and X-ray emissions are consistent with internal dissipation processes.

Contribution

It provides detailed modeling of the prompt optical emission evolution and clarifies the different origins of optical and high-energy emissions in GRB 180325A.

Findings

Optical flashes at 50s and 120s are from reverse shocks.

Gamma-ray and X-ray emissions are from internal dissipation.

Optical emission shows evolution consistent with shock models.

Abstract

The transition from prompt to the afterglow emission is one of the most exciting and least understood phases in gamma-ray bursts (GRBs). Correlations among optical, X-ray and gamma-ray emission in GRBs have been explored, to attempt to answer whether the earliest optical emission comes from internal and/or external shocks. We present optical photometric observations of GRB 180325A collected with the TAROT and RATIR ground-based telescopes. These observations show two strong optical flashes with separate peaks at $\sim50\;$s and $\sim120\;$s, followed by a temporally extended optical emission. We also present X-rays and gamma-ray observations of GRB 180325A, detected by the Burst Alert Telescope (BAT) and X-ray Telescope (XRT), on the Neil Gehrels Swift observatory, which both observed a narrow flash at $\sim80\;$s. We show that the prompt gamma- and X-ray early emission shares similar temporal and spectral features consistent with internal dissipation within the relativistic outflow (e.g. by internal shocks or magnetic reconnection), while the early optical flashes are likely generated by the reverse shock that decelerates the ejecta as it sweeps up the external medium.