Constraints on a tidal charge of the supermassive black hole in M87* with the EHT observations in April 2017

TL;DR

This paper uses Event Horizon Telescope observations of M87* to constrain the possible tidal charge of a black hole, comparing theoretical shadow sizes with observational data to explore deviations from standard black hole models.

Contribution

It introduces constraints on the tidal charge parameter of a black hole in M87* based on shadow size measurements, extending previous models to include tidal charge effects.

Findings

Tidal charge constraints derived from M87* shadow size.

Comparison of theoretical models with EHT observational data.

Implications for alternative black hole metrics like Reissner--Nordstr"om with tidal charge.

Abstract

More than two years ago the Event Horizon Telescope collaboration presented the first image reconstruction around the shadow for the supermassive black hole in M87*. It gives an opportunity to evaluate the shadow size. Recently, the Event Horizon Telescope collaboration constrained parameters ("charges") of spherical symmetrical metrics of black holes from an estimated allowed interval for shadow radius from observations of M87* in 2017. Earlier, analytical expressions for the shadow radius as a function of charge (including a tidal one) in the case of Reissner--Nordstr\"om metric have been obtained. Some time ago, Bin-Nun proposed to apply a Reissner--Nordstr\"om metric with a tidal charge as an alternative to the Schwarzschild metric in Sgr~A*. If we assume that a Reissner--Nordstr\"om black hole with a tidal charge exists in M87*, therefore, based on results of the shadow size evaluation for M87* done by the Event Horizon Telescope collaboration we constrain a tidal charge. Similarly, we evaluate a tidal charge from shadow size estimates for Sgr~A*.