Abstract
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 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.
Highlights
Strong gravitational lensing is a tool that serendipitously enhances our observing capabilities and offers new opportunities to study the Universe
The gamma-ray bursts (GRBs) location was behind the spacecraft, meaning most Gamma-ray Burst Monitor (GBM) detector normals have a large angle to the source
We explore a few properties of GRB 210812A that are not direct proofs of lensing, but they are necessary to any such claim
Summary
Strong gravitational lensing is a tool that serendipitously enhances our observing capabilities and offers new opportunities to study the Universe (see e.g. Oguri 2019). Called millilensing (Nemiroff et al 2001, because the expected separation between the images is on the order of milli-arcseconds), the gravitational lens signature is imprinted upon the lightcurve of a single trigger. In this case, we have, e.g., two emission episodes with similar lightcurve patterns which can be separated by timescales spanning from a fraction of a second to a few minutes. We present observations of the long duration GRB 210812A and show that it is consistent with a gravitational lensing scenario It is the first lensing claim with a flux ratio 3.
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