Fermi-GBM Observations of GRB 210812A: Signatures of a Million Solar Mass Gravitational Lens

TL;DR

This paper reports the detection of a gravitational lensing signature in a gamma-ray burst, suggesting a lensing object of about one million solar masses, which is rare and provides insights into massive compact objects.

Contribution

It presents the first strong evidence of a gravitational lens in a GRB, with detailed temporal and spectral analysis supporting the lensing hypothesis.

Findings

Evidence of gravitational lensing in GRB 210812A

Estimated lens mass of approximately one million solar masses

Challenges to existing GRB models due to identical emission episodes

Abstract

Observing gravitationally lensed objects in the time domain is difficult, and well-observed time-varying sources are rare. Lensed gamma-ray bursts (GRBs) offer improved timing precision to this class of objects complementing observations of quasars and supernovae. The rate of lensed GRBs is highly uncertain, approximately 1 in 1000. The Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray Space Telescope has observed more than 3000 GRBs making it an ideal instrument to uncover lensed bursts. Here we present observations of GRB 210812A showing two emission episodes, separated by 33.3 s, and with flux ratio of about 4.5. An exhaustive temporal and spectral analysis shows that the two emission episodes have the same pulse and spectral shape, which poses challenges to GRB models. We report multiple lines of evidence for a gravitational lens origin. In particular, modeling the lightcurve using nested sampling we uncover strong evidence in favor of the lensing scenario. Assuming a point mass lens, the mass of the lensing object is about 1 million solar masses. High-resolution radio imaging is needed for future lens candidates to derive tighter constraints.