  Instrumental selection effects can act upon the estimates of the peak energy
Ep, the fluence F and the peak flux P of GRBs. If this were the case, then the
correlations involving the corresponding rest frame quantities (i.e. Ep, Eiso
and the peak luminosity Liso) would be questioned. We estimated, as a function
of Ep, the minimum peak flux necessary to trigger a GRB and the minimum fluence
a burst must have to determine the value of Ep by considering different
instruments (BATSE, Swift, BeppoSAX). We find that the latter dominates over
the former. We then study the Ep-fluence (and flux) correlation in the observer
plane. GRBs with redshift show well defined Ep-F and Ep-P correlations: in this
planes the selection effects are present, but do not determine the found
orrelations. This is not true for Swift GRBs with redshift, for which the
spectral analysis threshold does affect their distribution in the observer
planes. Extending the sample to GRBs without z, we still find a significant
Ep-F correlation, although with a larger scatter than that defined by GRBs with
redshift. We find that 6% are outliers of the Amati correlation. The Ep-P
correlation of GRBs with or without redshift is the same and no outlier is
found among bursts without redshift.
