Implications of axionic hair on the shadow of M87*
Indrani Banerjee, Subhadip Sau, Soumitra SenGupta
Abstract
The detection of axion fields 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 antisymmetric 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 axions in the near-horizon regions of black holes, where the gravity is expected to be strong. In this paper, we explore the impact of axions on the black hole shadow and compare the result with the available image of M87*. Our analysis indicates that an axion which violates the energy condition seems to be favored by observations. The implications are discussed.