A Correlation Between the Intrinsic Brightness and Average Decay Rate of Gamma-ray Burst X-ray Afterglow Light Curves

TL;DR

This paper identifies a correlation between the initial brightness and decay rate of gamma-ray burst X-ray afterglows, suggesting a link between luminosity and energy dissipation mechanisms.

Contribution

It introduces a new scale-independent correlation between early-time luminosity and decay rate of X-ray afterglows, enhancing understanding of gamma-ray burst physics.

Findings

More luminous afterglows decay faster on average.

The correlation is consistent across X-ray and optical observations.

Selection effects and biases were thoroughly analyzed.

Abstract

We present a correlation between the average temporal decay ({\alpha}X,avg,>200s) and early-time luminosity (LX,200s) of X-ray afterglows of gamma-ray bursts as observed by Swift-XRT. Both quantities are measured relative to a rest frame time of 200 s after the {\gamma}-ray trigger. The luminosity average decay correlation does not depend on specific temporal behavior and contains one scale independent quantity minimizing the role of selection effects. This is a complementary correlation to that discovered by Oates et al. (2012) in the optical light curves observed by Swift-UVOT. The correlation indicates that on average, more luminous X-ray afterglows decay faster than less luminous ones, indicating some relative mechanism for energy dissipation. The X-ray and optical correlations are entirely consistent once corrections are applied and contamination is removed. We explore the possible biases introduced by different light curve morphologies and observational selection effects, and how either geometrical effects or intrinsic properties of the central engine and jet could explain the observed correlation.