| Title: |
GRB 220831A: a hostless, intermediate gamma-ray burst with an unusual optical afterglow |
| Authors: |
Freeburn, J; O’Connor, B; Cooke, J; Dobie, D; Möller, A; Tejos, N; Zhang, J; Beniamini, P; Auchettl, K; DeLaunay, J; Dichiara, S; Fong, WF; Goode, S; Gordon, A; Kilpatrick, CD; Lien, A; Mihalenko, C; Ryan, G; Siellez, K; Suhr, M; Troja, E; Van Bemmel, N; Webb, S |
| Publisher Information: |
Oxford University Press (OUP) |
| Publication Year: |
2025 |
| Collection: |
The University of Melbourne: Digital Repository |
| Description: |
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 multiwavelength 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, $m_i\gtrsim 26.6$ 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 $z\gt 2$, but it could also possibly be a compact object merger at $z\lt 0.4$ 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 dominated synchrotron afterglow, exhibiting a steep post-break temporal power-law 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. |
| Document Type: |
article in journal/newspaper |
| Language: |
English |
| ISSN: |
0035-8711 |
| Relation: |
https://hdl.handle.net/11343/365341 |
| Availability: |
https://hdl.handle.net/11343/365341 |
| Rights: |
https://creativecommons.org/licenses/by/4.0/ ; CC-BY |
| Accession Number: |
edsbas.C8F9AB5 |
| Database: |
BASE |