Stars lensed by the supermassive black hole in the center of the Milky Way: predictions for ELT, TMT, GMT, and JWST

TL;DR

Future extremely large telescopes like ELT, TMT, and GMT are predicted to resolve gravitational lensing signatures of background stars near the Milky Way's supermassive black hole, enabling precise tests of general relativity.

Contribution

This paper predicts the capabilities of upcoming telescopes to detect and resolve gravitational lensing of stars by the SMBH in the Milky Way, a novel application for testing general relativity.

Findings

ELT, TMT, and GMT can resolve lensing images of ~100 stars in the disk and ~30 in the bulge.

Detection requires limiting magnitudes of at least 24 mag for several stars.

Decade-long monitoring can reveal the rotation of lensed images.

Abstract

Gravitational lensing is an important prediction of general relativity, providing both its test and a tool to detect faint but amplified sources and to measure masses of lenses. For some applications (e.g. testing the theory), a point source lensed by a point-like lens would be more advantageous. However, until now only one gravitationally lensed star has been resolved. The future telescopes will resolve very small lensing signatures for stars orbiting the supermassive black hole (SMBH) in the center of the Milky Way. The lensing signatures should however be easier to detect for background stars. We predict that ELT, TMT, and GMT will resolve the lensed images of around 100 (60) stars in the disk and 30 (20) stars in the bulge in the background of the SMBH, down to 28 (27) mag (Vega) limits at K-band, requiring 5 (1) hr of integration. In order to detect several such stars one needs the limit of at least 24 mag. With a decade-long monitoring one can also detect the rotation of the lensed images. The detection of elongated images will not be possible, because this would require a nearly perfect source-lens alignment. JWST will likely be limited by the confusion caused by stars near the Galactic center. The detection of such lensed images will provide a very clean test of general relativity, when combined with the SMBH mass measurement from orbital motions of stars, and accurate measurements of the SMBH properties, because both the source and the lens can be considered point-like.