Pulse-wise Amati correlation in Fermi GRBs

Abstract: We make a detailed pulse-wise study of gamma-ray bursts (GRBs) with known redshift detected by \emph{Fermi}/Gamma Ray Burst Monitor (GBM). The sample contains 19 GRBs with 43 pulses. We find that the average peak energy is correlated to the radiated energy (the Amati relation) for individual pulses with a correlation coefficient of 0.86, which is slightly better than the correlation for the full GRBs. As the present correlation holds within GRBs, it is a strong evidence supporting the reliability of such a correlation. We investigate several aspects of this correlation. (i) We divide our sample into redshift bins and study the evolution of the correlation. Though there is a marginal indication of evolution of the correlation, we can conclude that the present data is consistent with no evolution. (ii) We compare the correlation in the first or single pulses of these GRBs to that of the rest of the pulses, and confirm that the correlation is unaffected by the fact that first/single pulses are generally harder than the rest. Finally, we conclude that the pulse-wise Amati correlation is more robust and it has the potential of refining the correlation so that GRB study could be used as a cosmological tool.