The Temporal Evolution of the Hardness Ratio of Gamma-Ray Bursts

The temporal evolution of hardness ratio (HR)of gamma-ray bursts (GRBs)have been studied with a sample of 599 bursts observed by the BATSE on board of the CGRO. A wavelet package de-noising techniqie has been applied to substract noise from burst tempral profiles and a definition of time resolved hardness ratio has been proposed to investigate tempral evolutional features of the HR of GRBs. The results show that:(1)For most of the bursts in the sample (77%)the tempral evolutions of the HR are"hard to soft", while a small fraction of bursts in the sample (23%)are"soft to hard ",especial 3% of bursts in the sample are monotonous "soft to hard".(2)The temporal evolutional features of HRs in different groups of GRBs with different duration are quite different.Those bursts with duration between 23-35 sec.tend to have more obvious "hard to soft" feature than others, and those bursts with duration between 1-10 second tend to have more clear "soft to hard" feature than others.Those bursts with duration between 35-53 sec are almost constant.(3)The temporal evolutional features of HRs in different groups of GRBs with different peak fluxes are quite different too. For more intense bursts, the tempral evolutional features of the HR tend to be more "soft to hard". Based on the results, a brief discussion has been addressed.