Abstract
Celebrating the centennial of its first experimental test, the theory of
General Relativity (GR) has successfully and consistently passed all subsequent
tests with flying colours. It is expected, however, that at certain scales new
physics, in particular in the form of quantum corrections, will emerge,
changing some of the predictions of GR, which is a classical theory. In this
respect, black holes (BHs) are natural configurations to explore the quantum
effects on strong gravitational fields. BH solutions in the low-energy
effective field theory description of the heterotic string theory, which is one
of the leading candidates to describe quantum gravity, have been the focus of
many studies in the last three decades. The recent interest in strong
gravitational lensing by BHs, in the wake of the Event Horizon Telescope
observations, suggests comparing the BH lensing in both GR and heterotic string
theory, in order to assess the phenomenological differences between these
models. In this work, we investigate the differences in the shadows of two
charged BH solutions with rotation: one arising in the context of GR, namely
the Kerr-Newman solution, and the other within the context of low-energy
heterotic string theory, the Kerr-Sen solution. We show and interpret, in
particular, that the stringy BH always has a larger shadow, for the same
physical parameters and observation conditions.
