Photon Ring Astrometry for Superradiant Clouds

Yifan Chen, Xiao Xue, Richard Brito, Vitor Cardoso

Submitted on 7 November 2022


Gravitational atoms produced from the superradiant extraction of rotational energy of spinning black holes can reach energy densities significantly higher than that of dark matter, turning black holes into powerful potential detectors for ultralight bosons. These structures are formed by coherently oscillating bosons, which induce oscillating metric perturbations, deflecting photon geodesics passing through their interior. The deviation of nearby geodesics can be further amplified near critical bound photon orbits. We discuss the prospect of detecting this deflection using photon ring autocorrelations with the Event Horizon Telescope and its next generation upgrade, which can probe a large unexplored region of the cloud mass parameter space when compared with previous constraints.


Comment: 9 pages, 5 figures

Subjects: General Relativity and Quantum Cosmology; Astrophysics - Cosmology and Nongalactic Astrophysics; Astrophysics - High Energy Astrophysical Phenomena; High Energy Physics - Phenomenology


Examples of photon geodesics connecting a far-away observer to a Kerr BH surrounded by a bosonic cloud, that execute multiple orbits intersecting the equatorial plane (gray plane in top panel). We considered a BH with spin $a_J = 0.94$, viewed at an inclination angle $\theta_{\rm o} = 17^\circ$, with a strain normalization amplitude $\epsilon_T = 0.0007$ for a massive tensor with $\alpha = 0.2$. Top panel: white line shows the unperturbed geodesic, while different colors show perturbed geodesics $x_{(0)}^i + \epsilon_T\, x_{(1)}^i$ at different oscillation phases. Bottom panel: Deviation of the geodesics in Kerr-Schild coordinates in terms of the affine parameter $\lambda$. The initial value at the observer's location is set to be $\lambda_{\rm o} = 0, r_{\rm o} = 10^3\, r_g$. The dashed vertical line corresponds to the point where the unperturbed orbit $x_{(0)}^\mu$ first crosses the equatorial plane.