Black Hole Images as Tests of General Relativity: Effects of Plasma Physics

Abstract

The horizon-scale images of black holes obtained with the Event Horizon Telescope have provided new probes of their metrics and tests of General Relativity. The images are characterized by a bright, near circular ring from the gravitationally lensed emission from the hot plasma and a deep central depression cast by the black hole. The metric tests rely on fact that the bright ring closely traces the boundary of the black hole shadow with a small displacement that has been quantified using simulations. In this paper we develop a self-consistent covariant analytic model of the accretion flow that spans a broad range of plasma conditions and black-hole properties to explore the general validity of this result. We show that, for any physical model of the accretion flow, the ring always encompasses the outline of the shadow and is not displaced by it by more than half the ring width. This result is a consequence of conservation laws and basic thermodynamic considerations and does not depend on the microphysics of the plasma or the details of the numerical simulations. We also present a quantitative measurement of the bias between the bright ring and the shadow radius based on the analytical models.

Distinguishing gravitational and emission physics in black-hole imaging: spherical symmetry

“…While this work was in preparation, some of these issues have recently been addressed in Bauer et al (2021), where the M (𝑎 1 )−RZ metric is also used, as well as in Ozel et al (2021) and Younsi et al (2021), where analytic models away from spherical symmetry are considered. In particular, it is worth noting that the range of the singleparameter |𝑎 1 | < 0.2 adopted by Bauer et al (2021) is considerably smaller than what is investigated here (see Table 2), leading to a suppression of the variation of the characteristic size of the emission ring around an RZ black hole with spacetime geometry (see Fig.…”

“…Our results are also in agreement with those in Sec. 2 of Ozel et al (2021), where spherical accretion is also considered. While Ozel et al (2021) and Younsi et al (2021) argue that the size of the bright emission ring is not disjoint from the shadow boundary using semi-analytic non-spherical accretion and emission models around Kerr and non-Kerr black holes respectively, we establish the same here by introducing an agnostic inner cut-off for the emission region, albeit within spherical symmetry, which has been a point of recent debate (see, e.g., Gralla 2021).…”

“…2 of Ozel et al (2021), where spherical accretion is also considered. While Ozel et al (2021) and Younsi et al (2021) argue that the size of the bright emission ring is not disjoint from the shadow boundary using semi-analytic non-spherical accretion and emission models around Kerr and non-Kerr black holes respectively, we establish the same here by introducing an agnostic inner cut-off for the emission region, albeit within spherical symmetry, which has been a point of recent debate (see, e.g., Gralla 2021). Here we show the redshifts 𝛾 ± experienced by photons that reach asymptotic observers, depending on where they were emitted from (𝑥-axis) and in which way they were moving initially, i.e., whether they were emitted toward the BH (−) or away from it (+).…”

Dark matter effect on the weak deflection angle by black holes at the center of Milky Way and M87 galaxies

“…In this paper, since we are interested in deriving the weak deflection angles by black holes at the center of a galaxy surrounded by dark matter, we will avoid such integrals in the expressions in Eqs. ( 3) and (6). Instead, we will focus on the calculation of the positional angles Ψ and longitudinal angle φ.…”

“…Until recently, the Event Horizon Telescope collaboration unveiled the first image of the shadow of a black hole in the electromagnetic regime [2][3][4], which again confirmed the correctness of Einstein's General Theory of Relativity [5] as a model for compact objects with an extreme gravitational field. With the only confirmation of black hole's physical existence, one can not underestimate the progress of theoretical research on the search for the most realistic model of a black hole, as well as its dynamical interactions to any astrophysical environments [6][7][8][9][10][11][12][13][14][15][16][17][18].…”