2001A&A...369..537Q

We study correlations between various hardness ratios of gamma-ray bursts (GRBs) and investigate if there are any differences between the short- and long-duration classes of the objects in the distributions of the ratios. The results show that most of the hardness ratios are mutually correlated, but that the ratio defined at the lower energy bands is not correlated with those defined at the higher energy bands; the most significant correlations come from those ratios defined with both the lower and higher energy bands. It is also shown that the short-duration bursts tend to have higher values of hardness ratios. We reach these conclusions that the slope of the higher part of the spectrum of most GRBs is independent of that of the lower part; emissions at higher energy bands from the bursts of both short- and long-duration classes are significantly different for different sources, but radiations at lower energy bands are similar; the spectra of the short-duration bursts is harder than that of the long-duration bursts. A possible interpretation for these results involves Doppler boosting in the relativistic beaming model. In addition, the study reveals that hardness ratios of the long-duration class are more mutually correlated than those of the short-duration class; the data of hardness ratios for the long-duration class are much less scattered than those of the short-duration class. It is found that this difference is at least partially due to measurement errors.