GRB$\,$220831A: a hostless, intermediate Gamma-ray burst with an unusual optical afterglow

TL;DR

GRB 220831A is a unique, hostless gamma-ray burst with an unusual optical afterglow, possibly originating from a collapsar or a compact merger, exhibiting atypical afterglow evolution.

Contribution

This study presents comprehensive multi-wavelength observations of GRB 220831A, revealing its hostless nature and unusual afterglow behavior, and explores its possible origins and physical mechanisms.

Findings

No underlying host galaxy detected to deep limits

Optical afterglow shows a steep post-break decay inconsistent with standard jet models

Possible internal origin of the afterglow features, such as flares or plateaus

Abstract

GRB$\,$220831A is a gamma-ray burst (GRB) with a duration and spectral peak energy that places it at the interface between the distribution of long-soft and short-hard GRBs. In this paper, we present the multi-wavelength follow-up campaign to GRB$\,$220831A and its optical, near-infrared, X-ray and radio counterparts. Our deep optical and near-infrared observations do not reveal an underlying host galaxy, and establish that GRB$\,$220831A is observationally hostless to depth, $m_i\gtrsim26.6$ AB mag. Based on the Amati relation and the non-detection of an accompanying supernova, we find that this GRB is most likely to have originated from a collapsar at $z>2$, but it could also possibly be a compact object merger at $z<0.4$ with a large separation distance from its host galaxy. Regardless of its origin, we show that its optical and near-infrared counterpart departs from the evolution expected from a forward shock dominated synchrotron afterglow, exhibiting a steep post-break temporal powerlaw index of $-3.83^{+0.62}_{-0.79}$, too steep to be the jet-break. By analysing a range of models, we find that the observed steep departure from forward shock closure relations is likely due to an internal process producing either a flare or a plateau.