Abstract
Gamma-ray bursts are usually classified into either short-duration or long-duration bursts. Going beyond the short-long classification scheme, it has been shown on statistical grounds that a third, intermediate population is needed in this classification scheme. We are looking for physical properties which discriminate the intermediate duration bursts from the other two classes. As the intermediate group is the softest, we argue that we have related them with X-ray flashes among the GRBs. We give a new, probabilistic definition for this class of events.
Highlights
To discern the physical properties of GRBs as a whole, we need to understand the number of physically different underlying classes of the phenomenon
We report on a significant difference in the peak-flux distribution between the intermediate and the short populations, and between the intermediate and long populations
For k components the number of free parameters is 6k − 1, since the sum of the weights is 1. We have applied this classification scheme on our sample, and found that the model with three components gives the best fit for the data in the Bayesian Information Criterion (BIC) sense, where the shape of the bivariate Gaussians is the same for each group, only their weights are different with no correlation, the best model has a value of BIC = −262.14
Summary
To discern the physical properties of GRBs as a whole, we need to understand the number of physically different underlying classes of the phenomenon. With the launch of the Swift satellite [1], a new perspective has opened up in the study of gamma-ray bursts and their afterglows. The intermediate GRB population in the studies [2,3,4,5] so far has always been the softest among the groups, meaning that intermediate GRBs emit the bulk of their energy in the low-energy gamma-rays. To find the fluences (SEmin,Emax ) we integrated the model spectrum in the usual Swift energy bands with 15 − 25 − 50 − 100 − 150 keV as their boundaries. We define the hardness ratio (Hij , where i and j mark the two energy intervals) as the ratio of the fluences in different channels for a given burst
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