The correlations between hardness ratios and fitting parameters in GRB spectrum

Abstract

In this paper, besides general definition of hardness ratio of gamma-ray bursts (GRBs), HR32, we also presented new definitions of hardness ratios, HRhl and HRll, then presented the results of correlation studies, examining the association between the hardness ratios and the spectral fitting parameters (E-0 or E-peak, alpha, beta) by using the GRB data observed by BATSE. The HRhl is defined as the fluence of channel 4 divided by the sum of the fluences of channels 1 to 3, and the HRll is defined as the fluence of channel 3 divided by the sum of the fluences of channel 1 and 2. We found that E-0 and E-peak are correlated with hardness ratios, and beta is only correlated with log (HRhl). The alpha is not correlated with any hardness ratios in the total sample of GRBs, while it correlated with log (HRll) and log (HR32) in the two subsets seperated at log (HRll) = 0.34 x alpha +0.66, respectively. These results show that a harder spectra tends to be steeper and has a higher E-peak or E-0; HRhl describes the spectral behaviors in high energy, while both of HRll and HR32 reflect the spectral characteristics in low energy; the spectral behaviors in low energy are different for the two subsets. We also presented a brief qualitative analysis discussion to the correlations and suggested that the correlations are caused by both of intrinsic and cosmological effects, but intrinsic effects are dominant.