Fermi-GBM Observation of GRB 090717034: χ 2 Test Confirms Evidence of Gravitational Lensing by a Supermassive Black Hole with a Million Solar Mass

Abstract

Abstract Gravitational lensing of gamma-ray bursts (GRBs) can provide an opportunity to probe the massive compact objects in the universe at different redshifts. We have discovered two consecutive pulses in the light curve of GRB 090717034, with the same temporal profile and different count rate, separated by a time interval, which is identified as a gravitationally lensed candidate in the Fermi-GBM GRB catalog. Here, we use the χ 2 minimization method to investigate the similarity of the temporal profile variability of the two pulses as a gravitationally lensed GRB candidate. We find the magnification factor and the time delay between two pulses to minimize the χ 2 function. Then, we perform a Monte Carlo simulation on a sample of mock lensed GRBs and compare the χ 2 of the lensed GRB candidate with the simulation, which confirms this candidate with 1σ confidence level. Assuming that GRB 090717034 is lensed by a pointlike object, the redshifted lens mass is about M L (1 + z) = (4.839 ± 1.148) × 106 M ⊙. The lens of this GRB is a candidate for a supermassive black hole along the line of sight to the GRB.