Distributions of the hardness ratio of short and long gamma-ray bursts in different time intervals within the first two seconds

Abstract

In the present paper, we investigate the distribution of the hardness ratio (HR) for short and long gamma-ray bursts (GRBs) in different time-scales for the first two seconds. After including and subtracting the background count, we performed a Kolmogorov-Smirnov (K-S) test on the HR distributions of the two classes of GRBs in each time interval. Our analysis shows that the probabilities of the K-S test to the distributions are very small, suggesting that the two classes of bursts are unlikely to arise from the same HR distributions. The result indicates that the two kinds of bursts probably originate from different mechanisms or have different central engines. In addition, we found that the HR of short bursts within the time interval 0-0.96 s changes from hard to soft; the HR of long bursts does not. The two kinds of bursts have different characteristics in the first two seconds, which might be associated with different physical mechanisms.