The true redshift distribution of Pre-SWIFT gamma-ray bursts
  SWIFT bursts appear to be more distant than previous bursts. We present the Boer & Gendre relation that link redshift and afterglow luminosities. Taking advantage of the XMM-Newton, Chandra and BeppoSAX catalogs, and using this relation, we have investigated the redshift distribution of GRBs. We find that XMM burst sources with unknown redshift appear to be more distant than those with a known redshift. We propose that this effect may be due to a selection effect of pre-SWIFT optical observations. 
B. Gendre (IASF-Roma/INAF) and M. Boer (Observatoire de Haute   Provence)