Multi-scale variability in the optical afterglow of GRB 251013C from high-cadence LAST observations
Abstract
Context. Optical afterglows of gamma-ray bursts (GRBs) are often described by smooth power-law decays, but deviations such as flares and rebrightenings provide important constraints on the underlying emission processes. Aims. We investigate the temporal variability of the optical afterglow of GRB 251013C using high-cadence observations, and assess the origin of the observed variability. Methods. We analyze continuous optical observations obtained with the Large Array Survey Telescope (LAST), complemented by publicly available X-ray data from Swift-XRT. The optical light curve is modeled using a power-law decay with superposed variability components, and contemporaneous optical and X-ray measurements are used to constrain the broadband spectrum. Results. The optical light curve shows pronounced variability on multiple timescales, including broad rebrightening episodes with Δt ∼ t and superposed faster fluctuations with Δt/t < 1. The main rebrightening exhibits a structured rise with a clear steepening prior to the peak. The fast variability comprises asymmetric fast-rise, slow-decay features whose durations increase with peak time. The optical flux lies on the extrapolation of the X-ray spectrum, and the spectrum hardens by the same amount whether measured within the X-ray band or from optical to X-ray. Conclusions. We favor a refreshed external-shock interpretation for the main rebrightening. The broadband spectrum and post-peak decay are consistent with slow-cooling synchrotron emission in a wind-like medium. The spectral hardening requires a newly dominant, harder electron population, while the faster optical variability indicates structure within the refreshed ejecta or shocked region.
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BibTeX
@misc{konno2026multiscale,
title = {{Multi-scale variability in the optical afterglow of GRB 251013C from high-cadence LAST observations}},
author = {R. Konno and E. O. Ofek and S. Garrappa and O. Teboul and E. Waxman and S. Ben-Ami and D. Kovaleva and A. Krassilchtchikov and D. Polishook and E. Segre and E. A. Zimmerman},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.25342},
eprint = {2609.25342},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.25342},
}
RIS
TY - UNPB
TI - Multi-scale variability in the optical afterglow of GRB 251013C from high-cadence LAST observations
AU - R. Konno
AU - E. O. Ofek
AU - S. Garrappa
AU - O. Teboul
AU - E. Waxman
AU - S. Ben-Ami
AU - D. Kovaleva
AU - A. Krassilchtchikov
AU - D. Polishook
AU - E. Segre
AU - E. A. Zimmerman
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2609.25342
UR - https://arxiv.org/abs/2609.25342
AB - Context. Optical afterglows of gamma-ray bursts (GRBs) are often described by smooth power-law decays, but deviations such as flares and rebrightenings provide important constraints on the underlying emission processes. Aims. We investigate the temporal variability of the optical afterglow of GRB 251013C using high-cadence observations, and assess the origin of the observed variability. Methods. We analyze continuous optical observations obtained with the Large Array Survey Telescope (LAST), complemented by publicly available X-ray data from Swift-XRT. The optical light curve is modeled using a power-law decay with superposed variability components, and contemporaneous optical and X-ray measurements are used to constrain the broadband spectrum. Results. The optical light curve shows pronounced variability on multiple timescales, including broad rebrightening episodes with Δt ∼ t and superposed faster fluctuations with Δt/t < 1. The main rebrightening exhibits a structured rise with a clear steepening prior to the peak. The fast variability comprises asymmetric fast-rise, slow-decay features whose durations increase with peak time. The optical flux lies on the extrapolation of the X-ray spectrum, and the spectrum hardens by the same amount whether measured within the X-ray band or from optical to X-ray. Conclusions. We favor a refreshed external-shock interpretation for the main rebrightening. The broadband spectrum and post-peak decay are consistent with slow-cooling synchrotron emission in a wind-like medium. The spectral hardening requires a newly dominant, harder electron population, while the faster optical variability indicates structure within the refreshed ejecta or shocked region.
ER -