Testing the bimodal distribution of long gamma-ray bursts in the cosmological rest-frame

Abstract

Among the several methods of classifying gamma-ray bursts (GRBs), the duration parameter has lead to the canonical classification of GRBs of long and shorts. However, the canonical classification of bursts has recently seen the emergence of a third type of GRB, which is present in a recent large burst sample from the Swift observatory. The high redshifts and the cosmological distances are directly confirmed for long bursts only, while for the short ones there is only indirect evidence for their cosmological origin. Cosmological objects should not only be redshifted in energy but also extended in time because of the expansion of the Universe. Meanwhile, an anticorrelation between the hardness and the duration is found for this subclass in contrast to the short and the long groups (Horvath et al. (2010)). Despite the differences among these three groups, it is not yet clear whether the third group represents a physically different phenomenon. In this scenario, we want to study the bimodal distribution of long bursts, focusing their temporal properties in the source location (burst frame). We have determined a temporal estimator in the cosmological rest-frame from a sample of 60 Swift's GRBs. If GRBs are at cosmological distances, then the burst profiles should be stretched in time due to cosmological time dilation by an amount proportional to the redshift, 1 + z (Chang (2001)). Complementary, we use the hardness ratio between the soft emission (15–50keV) and hard X-ray emission (50–150keV) in order to analyze the bimodal distribution of long bursts in the time-energy plane.