GRB 190919B: Rapid optical rise explained as a flaring activity

TL;DR

This paper analyzes the unusual rapid optical rise in GRB 190919B, attributing it to two superimposed flares from inner engine activity and external shock, providing insights into complex GRB emission mechanisms.

Contribution

It introduces a two-flare model to explain the steep optical rise in GRB 190919B, linking inner engine activity with external shock afterglow.

Findings

Optical light curve shows a steep rise ~100 s after trigger.

The first flare is linked to inner engine activity, supported by marginal hard X-ray detection.

The second flare follows closure relations for a slow cooling plasma in the ISM.

Abstract

Following the detection of a long GRB 190919B by INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory), we obtained an optical photometric sequence of its optical counterpart. The light curve of the optical emission exhibits an unusually steep rise ~100 s after the initial trigger. This behaviour is not expected from a 'canonical' GRB optical afterglow. As an explanation, we propose a scenario consisting of two superimposed flares: an optical flare originating from the inner engine activity followed by the hydrodynamic peak of an external shock. The inner-engine nature of the first pulse is supported by a marginal detection of flux in hard X-rays. The second pulse eventually concludes in a slow constant decay, which, as we show, follows the closure relations for a slow cooling plasma expanding into the constant interstellar medium and can be seen as an optical afterglow sensu stricto.