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

TL;DR

This paper presents evidence that the gamma-ray burst GRB 090717034 was gravitationally lensed by a supermassive black hole, using a chi-squared test and Monte Carlo simulations to confirm the lensing hypothesis.

Contribution

It introduces a novel application of the chi-squared test combined with Monte Carlo simulations to confirm gravitational lensing of a GRB by a supermassive black hole.

Findings

Identification of two similar pulses in GRB 090717034 consistent with lensing

Estimated lens mass of approximately 4 million solar masses

Confirmation of lensing candidate at 1-sigma confidence level

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 gravitationally lensed candidate in Fermi/GBM GRB catalogue \citep{Kalantari}. Here, we use the $\chi^2$ method to investigate the similarity of the temporal profile variability of the two pulses as a gravitationally lensed candidate GRB. We find the magnification factor and the time delay between two pulses to correspond to minimising the $\chi^2$ function. Then, we perform a Monte Carlo simulation on a sample of mock lensed GRBs and compare the $\chi^2$ of the lensed GRB candidate with the simulation, which confirms this candidate with $1\sigma$ confidence level. Assuming that GRB 090717034 is lensed by a point-like object, the redshifted lens mass is about $M_L(1+z)=(4.220\pm 6.615) \times 10^6 M_{\odot}$. The lens of this GRB is a candidate for a super-massive black hole along the line of sight to the GRB.