Correlation between the isotropic energy and the peak energy at zero fluence for the individual pulses of gamma-ray bursts: toward a universal physical correlation for the prompt emission

Abstract

We find a strong correlation between the peak energy at zero fluence (E_{peak, 0}) and the isotropic energy (E _{γ, iso}) of the 22 pulses of nine gamma-ray bursts (GRBs) detected by the Fermi satellite. The correlation holds for the individual pulses of each GRB, which shows the reality of the correlation. The derived correlation (Spearman correlation coefficient, r, which is 0.96) is much stronger compared to the correlations using E_peak (in place of E _{peak, 0}) determined from the time-integrated spectrum (r = 0.8), the time-resolved spectrum without accounting for broad pulse structures (r = 0.37), or the pulsewise spectrum (r = 0.89). Though the improvement in the E_peak-E_{γ, iso} relation (the Amati relation) for a pulsewise analysis is known earlier, this is the first time a parameter derived from a joint spectral and timing fit to the data is shown to improve the correlation. We suggest that E_{peak, 0}, rather than E_peak, is intrinsic to a GRB pulse and a natural choice as the parameter in pulsewise correlation studies.