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

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 for this class of objects, complementing observations of quasars and supernovae. The rate of lensed GRBs is highly uncertain, approximately one in 1000. The Gamma-ray Burst Monitor on board 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 a 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.