Implications of axionic hair on shadow of M87*

TL;DR

This paper investigates how axionic fields, originating from the Kalb-Ramond tensor, could influence the shadow of the M87* black hole, providing potential observational signatures of axions in strong gravity regimes.

Contribution

It introduces a novel analysis of axion effects on black hole shadows and compares theoretical predictions with M87* observations, suggesting axions may violate energy conditions.

Findings

Axion presence can alter black hole shadow shapes.

Observations favor axions that violate energy conditions.

Results provide a new way to test axion physics in astrophysics.

Abstract

Detection of axion field can unfold intriguing facets of our universe in several astrophysical and cosmological scenarios. In four dimensions, such a field owes its origin to the completely anti-symmetric Kalb-Ramond field strength tensor. Its invisibility in the solar system based tests compels one to look for its signatures in the strong field regime. The recent observation of the shadow of the supermassive black hole in the galaxy M87 ushers in a new opportunity to test for the footprints of axion in the near horizon region of black holes, where the gravity is expected to be strong. In this paper, we explore the impact of axion on the black hole shadow and compare the result with the available image of M87*. Our analysis indicates that axion which violates the energy condition seems to be favored by observations. The implications are discussed.