Observational signature of near-extremal Kerr-like black holes in a modified gravity theory at the Event Horizon Telescope

TL;DR

This paper investigates the shadows and images of near-extremal Kerr-like black holes in a modified gravity theory, revealing distinctive features that could be tested by the Event Horizon Telescope to distinguish from standard Kerr black holes.

Contribution

The study provides an analytical analysis of black hole shadows and images in Kerr-MOG black holes, highlighting how modified gravity parameters affect observable signatures.

Findings

Shadow size decreases with increased modified gravity parameter

Images of orbiting emitters show rich structures on the shadow

Fluxes of secondary images increase with the modified parameter

Abstract

We study the shadows cast by near-extremal Kerr-MOG black holes for different values of the parameter in modified gravity (MOG). In particular, we consider an isotropic emitter orbiting near such black holes and analytically compute the positions, fluxes and redshift factors of their images. The size of the shadow decreases when the modified parameter is increased. For each shadow, the images of the emitter appear on a special part of the shadow which has a rich structure. The primary image and secondary images are similar to those produced for the near-extremal (high spin) Kerr black hole, but the near-extremal Kerr-MOG black hole can have a spin ($\hat{J}/M^2_{\alpha}$) which is finitely lower than 1. When the modified parameter is varied, the typical positions of the corresponding images do not change, nor does the typical redshift factor associated with the primary image. However, another typical redshift factor associated with the secondary image increases when the modified parameter is increased. We also find that the fluxes increase in that case. These images appear periodically with period greater than that of Kerr. This provides an alternative signature away from the Kerr case which may be tested by the Event Horizon Telescope.