Multiple Pulsed Gamma-Ray Bursts Observed by HETE-2

It is known that there is a tight correlation between the peak energy of nu-F-nu spectrum (Ep) and the brightness (isotropic equivalent energy Eiso, collimated jet energy Egamma or 1-sec peak luminosity Lp) of GRBs. In most long GRBs, their light curves have many pulses and each pulse has a different spectrum. Using data of multiple pulsed GRBs observed by HETE-2, we found each pulse also follows the correlations of Ep-Eiso, Ep-Egamma and Ep-Lp. This may imply that each pulse behaves as a single burst.

Considering that the GRB are associated with death of massive stars, and therefore the GRB rate follow the star formation rate of the Universe, we should have already detected a number of distant GRBs without knowing their redshifts. Some methods are proposed to estimate a redshift by applying the above mentioned correlation such as Ep-Eiso to an observed GRB. If we treat each pulse as an individual event which has same redshift, the constraint of redshift inferences will be improved.

Taking advantage of the wide energy coverage of HETE-2 from 2 to 400 keV, we have studied the spectra of GRB021004, GRB020813, GRB030328, GRB030329, GRB041006, GRB050408 and GRB050709. Extending the previous studies, we applied the redshift inferences using Ep-Eiso, Ep-Egamma and Ep-Lp correlation in each pulse, estimated the most probable redshifts which satisfy spectra of all pulses, and indicated a possibility of improvement in redshift estimation.