Dubravko Horvat, Sasa Ilijic, Anamarija Kirin, Zoran Narancic
A static spherically symmetric asymptotically flat spacetime may allow for circular closed null-geodesics which are said to belong to a photon sphere. In the context of gravitational lensing in the strong deflection regime, the presence of a photon sphere leads to unbounded angle of deflection of light (multiple turns) and formation of relativistic images. In this paper we show that photon spheres may form in some configurations of boson stars constructed with free massive complex scalar field nonminimally coupled to gravity. Assuming the boson star is transparent to light, photon spheres would not only give raise to phenomena in the realm of strong gravitational lensing, but also to considerably increased photon flux in the central region of the star, relative to the flux in its surroundings.
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http://arxiv.org/abs/1302.4369
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