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Description
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_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_iso_-L_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 24h with z>2. After removing this optical selection bias, the E_iso_-L_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_iso_ and L_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 short GRBs compared to long GRBs.
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