A new method for shadow calculations: application to parameterised axisymmetric black holes

TL;DR

This paper introduces a new general method for calculating black hole shadows applicable to various axisymmetric metrics, aiding in the interpretation of potential future observational data from the Event Horizon Telescope.

Contribution

It presents a novel algorithm for ray-tracing in arbitrary metric theories of gravity, validated on extremal black hole solutions, enhancing the analysis of black hole shadows.

Findings

Algorithm is robust and rapidly convergent for extremal parameters

Applicable to a wide range of metric theories of gravity

Facilitates comparison of theoretical black hole models with observations

Abstract

Collaborative international efforts under the name of the Event Horizon Telescope project, using sub- mm very long baseline interferometry, are soon expected to provide the first images of the shadow cast by the candidate supermassive black hole in our Galactic center, Sagittarius A*. Observations of this shadow would provide direct evidence of the existence of astrophysical black holes. Although it is expected that astrophysical black holes are described by the axisymmetric Kerr solution, there also exist many other black hole solutions, both in general relativity and in other theories of gravity, which cannot presently be ruled out. To this end, we present calculations of black hole shadow images from various metric theories of gravity as described by our recent work on a general parameterisation of axisymmetric black holes [R. Konoplya, L. Rezzolla and A. Zhidenko, Phys. Rev. D 93, 064015 (2016)]. An algorithm to perform general ray-tracing calculations for any metric theory of gravity is first outlined and then employed to demonstrate that even for extremal metric deformation parameters of various black hole spacetimes, this parameterisation is both robust and rapidly convergent to the correct solution.