Gravitational lensing by the supermassive black hole in the center of M31

TL;DR

This paper investigates the potential to observe gravitational lensing effects caused by the supermassive black hole in M31, analyzing expected image brightness, positions, and event durations across different stellar populations.

Contribution

It provides the first detailed predictions of gravitational lensing signatures by M31's black hole, including expected numbers, positions, and durations of secondary images for various stellar sources.

Findings

Approximately 1-2 secondary images brighter than K=24 or V=27 expected at any time.

Secondary images typically appear within 0.1 to 1 milliarcsecond from the black hole.

Lensing events have durations ranging from days to months.

Abstract

We examine the possibility of observing gravitational lensing in the weak deflection regime by the supermassive black hole in the center of the galaxy M31. This black hole is significantly more massive than the black hole in the center of our Galaxy qualifying itself as a more effective lens. However, it is also more distant and the candidate stellar sources appear consequently fainter. As potential sources we separately consider stars belonging to the bulge, to the disk, to the triple nucleus formed by P1+P2 and by the recently discovered inner cluster P3. We calculate the number of simultaneously lensed stars at a given time as a function of the threshold magnitude required for the secondary image. For observations in the K-band we find 1.4 expected stars having secondary images brighter than K=24 and 182 brighter than K=30. For observations in the V-band we expect 1.3 secondary images brighter than V=27 and 271 brighter than V=33. The bulge stars have the highest chance to be lensed by the supermassive black hole, whereas the disk and the composite nucleus stars contribute by 10% each. The typical angular separation of the secondary images from the black hole range from 1 mas to 0.1''. For each population we also show the distribution of the lensed sources as a function of their distance and absolute magnitude, the expected angular positions and velocities of the generated secondary images, the rate and the typical duration of the lensing events.