Selection biases in the gammaray burst Eiso  Lopt, X correlation.
Abstract
Gammaray burst (GRB) optical and Xray 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 Xray 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 Xray 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 24 h 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 powerlaw index for short and long GRBs, but different microphysical parameter distributions. Comparing the ratio of optical and Xray luminosities, we find tentative evidence that the fraction of postshock energy in magnetic fields, ɛ_{B}, could be systematically higher in short GRBs compared to long GRBs.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 April 2015
 DOI:
 10.1093/mnrasl/slu192
 arXiv:
 arXiv:1411.6711
 Bibcode:
 2015MNRAS.449L...6C
 Keywords:

 methods: statistical;
 gammaray burst: general;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 6 pages, 3 figures, MNRAS Letter accepted, supplementary data to appear online