Are there Gravitationally Lensed Gamma-ray Bursts detected by GBM?

摘要

Gamma-ray Burst (GRB) sources are at cosmological distances. Hence it is possible that the GRB sources are gravitationally lensed by a foreground object like a galaxy or a cluster of galaxies. In such a case, two GRBs with the same location and with similar temporal as well as spectral properties could be the same burst imaged multiple times by a gravitational lens. Two GRBs lensed by a typical galaxy, for example, could be detected with a time separation of several months. We carry out a systematic search for pairs of GRBs from the first 2-year data from the Gamma-ray Burst Monitor (GBM) having the same location (within errors) and similar light curves using a cross-correlation analysis. We apply the following tests on a large number of possible pairs: (a) the Cross Correlation Function (CCF) between the light curves must be symmetric around zero lag, (b) the time integrated spectral fit parameters of the pair should be statistically consistent, (c) the variation of the maximum value of the CCF as a function of the temporal resolution of the light curves should increase smoothly and flatten, and (d) the ratios of the number of registered photons in corresponding bins of the burst light curves should be relatively constant. Finally, (e) the burst pairs should have same location within location errors. We have a total of 132870 possible pairs from a total of 515 GRBs detected by GBM during the first 2 years of operation. After applying all the above tests the number of qualifying pairs reduces to 1. It so happened that both the GRBs of this surviving pair has been detected by Swift which gives an angular separation of approx2 deg, ruling out the lensing possibility.