A correlation between intrinsic brightness and average decay rate of Swift UVOT GRB optical/UV light curves

TL;DR

This study finds a significant correlation between the intrinsic brightness at 200 seconds and the average decay rate of long GRB optical/UV light curves, suggesting an intrinsic property or viewing angle effect.

Contribution

It identifies and analyzes a new correlation in GRB afterglows, exploring its possible physical origins and ruling out selection effects.

Findings

Significant negative correlation between luminosity and decay rate (Spearman coefficient -0.58).

The correlation is intrinsic to long GRBs, not due to selection bias.

Two potential explanations: central engine properties or observer viewing angle.

Abstract

We examine a sample of 48 Swift/UVOT long Gamma-ray Burst light curves and find a correlation between the logarithmic luminosity at 200s and average decay rate determined from 200s onwards, with a Spearman rank coefficient of -0.58 at a significance of 99.998% (4.2 sigma). We discuss the causes of the log L_200s - alpha_>200s correlation, finding it to be an intrinsic property of long GRBs, and not resulting from the selection criteria. We find two ways to produce the correlation. One possibility is that there is some property of the central engine, outflow or external medium that affects the rate of energy release so that the bright afterglows release their energy more quickly and decay faster than the fainter afterglows. Alternatively, the correlation may be produced by variation of the observers viewing angle, with observers at large viewing angles observing fainter and slower decaying light curves.