Modeling GRB 170202A fireball from continuous observations with the Zadko and the Virgin Island Robotic Telescopes

TL;DR

This study uses coordinated optical observations from Zadko and Virgin Island Robotic Telescopes to model GRB 170202A's fireball, revealing insights into its expansion, shock interactions, and energy parameters within the standard fireball framework.

Contribution

It demonstrates that comprehensive, coordinated observations enable detailed modeling of GRB fireballs, including parameters like electron and magnetic energies, which are typically underestimated.

Findings

GRB 170202A's fireball expands in a constant density medium.

Early optical rise and late flares observed in the afterglow.

Electron and magnetic energy parameters are much smaller than usual.

Abstract

We present coordinated observations of GRB 170202A carried out by the Zadko and the Virgin Island Robotic Telescopes. The observations started 59s after the event trigger, and provided nearly continuous coverage for two days due to the unique location of these telescopes. We clearly detected an early rise in optical emission, followed by late optical flares. By complementing these data with archival observations, we show GRB 170202A is well described by the standard fireball model if multiple reverse shocks are taken into account. Its fireball is evidenced to expand within a constant density interstellar medium, with most burst parameters consistent with the usual ranges found in literature. The electron and magnetic energy parameters (\epsilon_e, \epsilon_B) are orders of magnitude smaller than commonly assumed values. We argue that the global fit of the fireball model achieved by our study should be possible for any burst, pending the availability of a sufficiently comprehensive data set. This conclusion emphasizes the crucial importance of coordinated observation campaigns of GRBs, such as the one central to this work, to answer outstanding questions about the underlying physics driving these phenomena.