Photon surfaces, shadows and accretion disks in gravity with minimally coupled scalar field

TL;DR

This study analyzes observable images of black hole simulators with scalar fields, comparing their shadows and accretion disk radiation to Kerr black holes, revealing high similarity for TSL and significant deviations for KL solutions.

Contribution

The paper introduces and analyzes two new rotating geometries with scalar fields, providing detailed comparisons of their observable features to Kerr black holes.

Findings

TSL closely mimics Kerr black hole shadows with less than 1% deviation.

Near-extreme solutions show reduced accretion disk luminosity.

Scalar charge significantly alters optical properties, creating complex shadow structures.

Abstract

In this article, we conduct a sequential study of possible observable images of black hole simulators described by two recently obtained rotating geometries in Einstein gravity, minimally coupled to a scalar field. One of them, "Kerr-like" (KL), can be seen as a legitimate alternative to the rotating Fisher-Janis-Newman-Winicour (FJNW) solution, and the other (TSL) is a scalar generalization of the Tomimatsu-Sato solution. Unlike the previous version of the rotating FJNW, these solutions do indeed satisfy the system's equations of motion. Our study includes both analytical and numerical calculations of equatorial circular orbits, photon regions, gravitational shadows, and radiation from thin accretion disks for various values of the object's angular momentum and scalar charge. The TSL solution was found to simulate Kerr for all valid parameter values with high accuracy. The maximum difference between the deviations of shadows from a circle for the Kerr and TSL cases does not exceed 1% and fits into the experimental observational data M87*. However, near-extreme objects show two times smaller peak values of the observed outflow luminosity of the accretion disk than for the Kerr black hole. The KL solution cannot be ruled out by the experimental data for small values of the scalar charge either. As the scalar charge increases, the optical properties change dramatically. The shadow can become multiply connected, strongly oblate, and the photon region does not hide the singularity, so it should be classified as a strong singularity.