Evidence for a luminosity-decay correlation in GRB GeV light curves

TL;DR

This paper discovers a significant correlation in Fermi LAT long GRB light curves showing that brighter early afterglows tend to decay faster, providing insights into the central engine and jet physics.

Contribution

It reports the first evidence of a luminosity-decay correlation in GeV GRB light curves, linking early luminosity to decay rate and expanding understanding of GRB afterglow behavior.

Findings

A negative correlation coefficient of -0.74 with 99.6% confidence.

Brighter GRB afterglows decay faster than less luminous ones.

The observed slope is consistent with optical and X-ray correlations.

Abstract

Correlations between intrinsic properties of gamma-ray burst (GRB) light curves provide clues to the nature of the central engine, the jet, and a possible means to standardise GRBs for cosmological use. Here we report on the discovery of a correlation between the intrinsic early time luminosity, $L_{G,\rm 10s}$, measured at rest frame 10s, and the average decay rate measured from rest frame 10s onward, $\alpha_{G,\rm avg>10s}$, in a sample of 13 Fermi Large Array Telescope (LAT) long GRB light curves. We note that our selection criteria, in particular the requirement for a redshift to construct luminosity light curves, naturally limits our sample to energetic GRBs. A Spearman's rank correlation gives a coefficient of -0.74, corresponding to a confidence level of 99.6%, indicating that brighter afterglows decay faster than less luminous ones. Assuming a linear relation with $\log(L_{G,\rm 10s})$, we find $\alpha_{G,\rm avg>10s} = -0.31_{-0.09}^{+0.12}\log(L_{G,\rm 10s}) + 14.43_{-5.97}^{+4.55}$. The slope of -0.31 is consistent at $1\sigma$ with previously identified correlations in the optical/UV and X-ray light curves. We speculate that differences in the rate at which energy is released by the central engine or differences in observer viewing angle may be responsible for the correlation.