Interferometric pattern of the relativistic images of a Schwarzschild star

For a horizonless Schwarzschild star with a photon sphere, its strong deflection gravitational lensing can generate unique inner relativistic images of pointlike sources, which are absent for a Schwarzschild black hole. In order to understand the signatures of its outer and inner relativistic images, we generalize the strong deflection limit method for an ultracompact object by including the finite distance effect of a source, and analytically calculate the complex visibility of all the relativistic images of the Schwarzschild star. We show that the interferometric pattern of the relativistic images of the Schwarzschild star has richer features than the one of the Schwarzschild black hole and is more likely to be resolved by future space-borne very long baseline interferometry.