Strong Lensing and Nonminimally Coupled Electromagnetism

TL;DR

This paper investigates how nonminimal coupling between gravity and electromagnetism affects gravitational lensing by black holes, predicting measurable differences in Einstein ring separations that could constrain the coupling parameter.

Contribution

It introduces the concept of an effective geometry for photons in nonminimally coupled theories and calculates the impact on Einstein ring separations for the first time.

Findings

Effective photon sphere exists due to nonminimal coupling.

Angular separation of Einstein rings depends on the coupling parameter.

Potential to constrain coupling parameter with future telescopic observations.

Abstract

The lensing at large deflection angles caused by a Schwarzschild black hole for the case of a nonminimal coupling between gravitation and electromagnetism is examined. We show that photons follow an effective geometry, which displays an effective photon sphere. For the case in which the source, lens and observer are aligned, so that relativistic Einstein rings are formed, the dependence of the angular separation $\delta\theta$ between the first and second ring with the relevant coupling parameter is calculated. We argue that such a separation, which may be measured by telescopes that will be operative in the near future, may set an upper and a lower limit for the coupling parameter.