The statistical study of the spectral hardness evolution of tracking Gamma-ray Burst pulses

Abstract

We select a sample including 42 individual tracking pulses (here we defined tracking as the cases in which the hardness follows the same pattern as the flux or count rate time profile) within 36 Gamma‐ray Bursts (GRBs) to investigate the spectral hardness, Epeak, evolutionary characteristics. It is found that the overall characteristics of Epeak of our selected sample are: 1) the Epeak evolution in the rise phase always start on the high state (the values of Epeak are always higher than 50 keV); 2) the spectra of rise phase clearly start at higher energy (the median of Epeak are about 300 keV), whereas the spectra of decay phase end at much lower energy (the median of Epeak are about 200 keV); 3) the spectra of rise phase are harder than that of the decay phase and the duration of rise phase are much shorter than that of decay phase as well. In other words, for a complete pulse the initial Epeak is higher than the final one and the duration of initial phase (rise phase) are much shorter than the final pha...