Selection biases in the gamma ray burst E$_{\rm iso}$ -- L$_{\rm opt,X}$ correlation

TL;DR

This study reveals that selection biases significantly affect the observed correlation between gamma-ray burst energies and afterglow luminosities, and after correction, the correlation is not statistically significant for long GRBs.

Contribution

The paper demonstrates that selection effects dominate the observed E$_{ m iso}$ -- L$_{ m opt,X}$ correlation in GRB afterglow data and provides a corrected analysis showing the true nature of this relationship.

Findings

Selection biases are strong in optical and X-ray afterglow data.

Correcting for biases weakens the E$_{ m iso}$ -- L$_{ m opt,X}$ correlation for long GRBs.

Tentative evidence suggests higher magnetic energy fractions in short GRBs.

Abstract

Gamma ray burst (GRB) optical and X-ray afterglow luminosity is expected to correlate with the GRB isotropic equivalent kinetic energy of the outflow in the standard synchrotron model for GRB afterglows. Previous studies, using prompt GRB isotropic equivalent energy ($E_{\rm iso}$) as a proxy for isotropic equivalent kinetic energy, have generally confirmed a correlation between X-ray and optical afterglow luminosities. Assuming that GRB afterglow luminosity does not evolve strongly with redshift, we identify a strong Malmquist bias in GRB optical and X-ray afterglow luminosity data. We show that selection effects dominate the observed E$_{\rm iso}$ -- L$_{\rm opt,X}$ correlations, and have likely been underestimated in other studies. The bias is strongest for a subset of optically faint bursts $m>24$ at 24 hr with $z>2$. After removing this optical selection bias, the E$_{\rm iso}$ -- L$_{\rm opt,X}$ correlation for long GRBs is not statistically significant, but combining both long and short GRB luminosity data the correlation is significant. Using the median of the $E_{\rm iso}$ and $L_{\rm opt,X}$ distributions, we apply the synchrotron model assuming the same power law index for short and long GRBs, but different microphysical parameter distributions. Comparing the ratio of optical and X-ray luminosities, we find tentative evidence that the fraction of post-shock energy in magnetic fields, $\epsilon_B$, could be systematically higher in SGRBs compared to LGRBs.