Implications of axionic hair on shadow of M87*

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.

Imprints of the Janis-Newman-Winicour spacetime on observations related to shadow and accretion

“…The photon sphere in an arbitrary spherically symmetric metric is therefore obtained by solving Eq. ( 18) for r. In the limit R = 1 we get back the known result rν = 2 [44].…”

“…From the above data, one can constrain the background spacetime created by the compact object M87* through the radius of its shadow. We have shown earlier [44,52] that given the mass and distance of M87*, a spacetime which creates a shadow larger than the Schwarzschild scenario explains the observations better. In the current situation since the presence of the scalar field (smaller γ) diminishes the shadow radius, the Schwarzschild scenario seems to be observationally more favored compared to the Winicour solution.…”

“…We have already explored the properties of accretion and shadow in the spacetime with the axionic charge [43,44]. In this work, we will explore the motion of both the massless and the massive particles around the Janis-Newman-Winicour spacetime.…”

“…However, in the case of the scalar field the solution of the gravitational field equations lead to an exact metric representing a naked singularity while in the other case the solution leads to a perturbative metric representing a black hole. We have explored in an earlier work [44] the dependence of the shadow radius on the axion parameter h and found that a negative h enhances, while a positive h diminishes the shadow compared to the Schwarzschild scenario. In the Winicour solution on the other hand, the shadow decreases with decrease in γ (or increase with the scalar charge) and its radius is always less than the Schwarzschild case.…”

Constraining fundamental physics with the event horizon telescope

“…In principle, sufficiently large deviations between the observed EHT picture and what is consistent with a vanilla black hole can be excluded. This type of logic has been used to constrain several theories of modified gravity, such as naked singularities [31,32], extra dimensions [33,34], scalar hair [35][36][37], and magnetically charged black holes [38]. The focus of this work is to follow [20] and show how to constrain all UV modifications of GR whose implications are localized near the horizon, and to use the EFT framework to quantify the theoretical error that is involved in making potentially-observable predictions.…”