The Early-time Optical Properties of Gamma-Ray Burst Afterglows

TL;DR

This study analyzes early optical and X-ray afterglows of 63 gamma-ray bursts, classifying their light curves, and discusses challenges in modeling some afterglows within the standard framework, highlighting the role of central engine activity.

Contribution

It provides a comprehensive multiwavelength analysis of GRB afterglows, introducing a classification scheme based on temporal breaks and exploring the implications of central engine activity.

Findings

Optical emission detected in 24 out of 63 GRBs within 10 minutes.

Four main classes of light curves based on optical and X-ray temporal breaks.

Approximately 50% of GRBs are dark bursts with no early optical detection.

Abstract

We present a multiwavelength analysis of 63 Gamma-Ray Bursts observed with the world's three largest robotic optical telescopes, the Liverpool and Faulkes Telescopes (North and South). Optical emission was detected for 24 GRBs with brightnesses ranging from R = 10 to 22 mag in the first 10 minutes after the burst. By comparing optical and X-ray light curves from t = 100 to about 10^6 seconds, we introduce four main classes, defined by the presence or absence of temporal breaks at optical and/or X-ray wavelengths. While 15/24 GRBs can be modelled with the forward-shock model, explanation of the remaining nine is very challenging in the standard framework even with the introduction of energy injection or an ambient density gradient. Early X-ray afterglows, even segments of light curves described by a power-law, may be due to additional emission from the central engine. 39 GRBs in our sample were not detected and have deep upper limits (R < 22 mag) at early time. Of these, only ten were identified by other facilities, primarily at near infrared wavelengths, resulting in a dark burst fraction of about 50%. Additional emission in the early time X-ray afterglow due to late-time central engine activity may also explain some dark bursts by making the bursts brighter than expected in the X-ray band compared to the optical band.