  We probe the existence of the E_peak-E_iso correlation in long GRBs using a
sample of 442 BATSE bursts with known E_peak and with redshift estimated
through the lag-luminosity correlation. This sample confirms that the rest
frame peak energy is correlated with the isotropic equivalent energy. The
distribution of the scatter of the points around the best fitting line is
similar to that obtained with the 27 bursts with spectroscopic redshifts. We
interpret the scatter in the E_peak-E_iso plane as due to the opening angle
distribution of GRB jets. By assuming that the collimation corrected energy
correlates with E_peak we can derive the observed distribution of the jet
opening angles, which turns out to be log-normal with a peak value of ~6.5
degrees.

Astrophysics