Evolving Morphology of Resolved Stellar Einstein Rings

Abstract

We consider strong gravitational lensing by nearby stars. Using our wave-optical treatment of lensing phenomena, we study Einstein rings that may form around nearby stellar lenses. It is remarkable that these rings are bright and large enough to be detected and resolved by existing instruments. Such lensing events have durations of hours or days, with peak light amplification lasting for several minutes. Many such events may be predicted using the Gaia astrometric catalog. Serendipitous discoveries are also possible. Fortuitous alignments can be used to confirm or discover and study exoplanets. For lenses that have dense stellar regions in their background, these events may occur several time a year, warranting their continuous or recurrent monitoring. Resolved imaging and spectroscopy of the evolving morphology of an Einstein ring offers knowledge about both the lens and the source. The angular size of the Einstein ring amounts to a direct measurement of the lens mass. The changing orientation of the major and minor images of the source offers astrometric information related to the mutual orientation of the objects. The event duration, when the full ring is present, helps to determine the source’s size. The sky position of planetary lensing events constrains the planet’s orbit. Spectroscopy of the ring allows for direct investigations of the source. The frequency and predictability of these events and the wealth of information that can be obtained by imaging motivate observational campaigns using existing facilities and/or construction of new instruments dedicated to the search and study of Einstein rings that are forming around nearby stars. As a specific example, we consider a predicted 2028 lensing of a red giant by α Centauri A and discuss the relevant science campaign.