  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.
