Horizonscale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strongfield regime. We consider a wide range of wellmotivated deviations from classical general relativity (GR) BH solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A
The groundbreaking image of the black hole at the center of the M87 galaxy has raised questions at the intersection of observational astronomy and black hole physics. How well can the radius of a black hole shadow be measured, and can this measurement be used to distinguish general relativity from other theories of gravity? We explore these questions using a simple spherical flow model in general relativity, scalar Gauss–Bonnet gravity, and the Rezzolla and Zhidenko parameterized metric. We assume an optically thin plasma with powerlaw emissivity in radius. Along the way we present a generalized Bondi flow, as well as a piecewise analytic model for the brightness profile of a cold inflow. We use the second moment of a synthetic image as a proxy for EHT observables and compute the ratio of the second moment to the radius of the black hole shadow. We show that corrections to this ratio from modifications to general relativity are subdominant compared to corrections to the critical impact parameter, and we argue that this is generally true. In our simplified model the astrophysical parameter uncertainty dominates the gravity theory parameter uncertainty, underlining the importance of understanding the accretion model if EHT is to be used to successfully test theories of gravity.
more » « less Award ID(s):
 2007936
 NSFPAR ID:
 10362338
 Publisher / Repository:
 DOI PREFIX: 10.3847
 Date Published:
 Journal Name:
 The Astrophysical Journal
 Volume:
 925
 Issue:
 2
 ISSN:
 0004637X
 Format(s):
 Medium: X Size: Article No. 119
 Size(s):
 ["Article No. 119"]
 Sponsoring Org:
 National Science Foundation
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