Gravitational Lensing of Stars in the Central Arcsecond of Our Galaxy

In the neighborhood of Sgr A*, several stars (S2, S12, S14, S1, S8, S13)enjoy an accurate determination of their orbital parameters. General Relativitypredicts that the central black hole acts as a gravitational lens on thesestars, generating a secondary image and two infinite series of relativisticimages. For each of these six stars, we calculate the light curves for thesecondary and the first two relativistic images, in the Schwarzschild blackhole hypothesis, throughout their orbital periods. The curves are peaked aroundthe periapse epoch, but two subpeaks may arise in nearly edge-on orbits, whenthe source is behind or in front of Sgr A*. We show that for most of thesestars the secondary image should be observable during its brightness peak. Inparticular, S14 is the best candidate, since its secondary image reaches K=23.3with an angular separation of 0.125 mas from the apparent horizon of thecentral black hole. The detection of such images by future instruments couldrepresent the first observation of gravitational lensing beyond the weak fieldapproximation.Comment: 28 pages, 9 figures, in press on Ap