GRB 200716C: Evidence for a Short Burst Being Lensed

TL;DR

This paper presents evidence that the long-duration GRB 200716C is actually a short GRB that has been gravitationally lensed by an intermediate-mass black hole, supported by analysis of its light curves and spectral properties.

Contribution

It provides the first candidate of a lensed short GRB, estimating the lens mass and supporting the existence of intermediate-mass black holes along the line of sight.

Findings

GRB 200716C shows repeated pulses consistent with lensing.

Spectral properties of the pulses follow the short GRB Amati relation.

Flux ratios between pulses are constant across channels.

Abstract

A tiny fraction of observed gamma-ray bursts (GRBs) may be lensed. The time delays induced by the gravitational lensing are milliseconds to seconds if the point lenses are intermediate-mass black holes. The prompt emission of the lensed GRBs, in principle, should have repeated pulses with identical light curves and spectra but different fluxes and slightly offset positions. In this work, we search for such candidates within the GRBs detected by Fermi/GBM, Swift/BAT, and HXMT/HE and report the identification of an attractive event GRB 200716C that consists of two pulses. Both the autocorrelation analysis and the Bayesian inference of the prompt emission light curve are in favor of the gravitational lensing scenario. Moreover, the spectral properties of the two pulses are rather similar and follow the so-called Amati relation of short GRBs rather than long duration bursts. The measured flux ratios between the two pulses are nearly constant in all channels, as expected from gravitational lensing. We therefore suggest that the long duration burst GRB 200716C was a short event being lensed. The redshifted mass of the lens was estimated to be $4.25^{+2.46}_{-1.36}$ $\times$ $10^5$ $M_{\odot}$ (90$\%$ credibility). If correct, this could point towards the existence of an intermediate-mass black hole along the line of sight of GRB 200716C.