A Correlated Study of Optical and X-ray Afterglows of GRBs

TL;DR

This study analyzes 87 GRBs with simultaneous optical and X-ray afterglow data, finding that most are consistent with the external shock model, with detailed insights into their environments and decay behaviors.

Contribution

It provides a comprehensive comparison of optical and X-ray afterglows, quantifies their consistency with the external shock model, and explores environmental and decay characteristics of GRBs.

Findings

62% of GRBs are consistent with the standard afterglow model.

Up to 91% of bursts may align with the external shock model when advanced modeling is used.

54% of X-ray and 40% of optical observations show the end of shallow decay coinciding with jet breaks.

Abstract

We study an extensive sample of 87 GRBs for which there are well sampled and simultaneous optical and X-ray light-curves. We extract the cleanest possible signal of the afterglow component, and compare the temporal behaviors of the X-ray light-curve, observed by Swift XRT, and optical data, observed by UVOT and ground-based telescopes for each individual burst. Overall we find 62\% GRBs that are consistent with the standard afterglow model. When more advanced modeling is invoked, up to 91\% of the bursts in our sample may be consistent with the external shock model. A large fraction of these bursts are consistent with occurring in a constant interstellar density medium (ISM) (61\%) while only 39\% of them occur in a wind-like medium. Only 9 cases have afterglow light-curves that exactly match the standard fireball model prediction, having a single power law decay in both energy bands which are observed during their entire duration. In particular, for the bursts with chromatic behavior additional model assumptions must be made over limited segments of the light-curves in order for these bursts to fully agree with the external shock model. Interestingly, for 54\% of the X-ray and 40\% of the optical band observations the end of the shallow decay ($t^{\sim-0.5}$) period coincides with the jet break ($t^{\sim-p}$) time, causing an abrupt change in decay slope. The fraction of the burst that consistent with the external shock model is independent of the observational epochs in the rest frame of GRBs. Moreover, no cases can be explained by the cooling frequency crossing the X-ray or optical band.