Toward determining the number of observable supermassive black hole shadows

Abstract

We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semi-analytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that in excess of a million SMBH shadows meeting these criteria are potentially accessible to interferometric observations with sufficient angular resolution and sensitivity. We then further decompose the shadow source counts into the number of black holes for which we could expect to observe the first-and second-order lensed photon rings. Our model predicts that with modest improvements to sensitivity, as many as ∼5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.

Analytical study of higher-order ring images of accretion disk around black hole

“…Higherorder rings in spherically symmetric metrics have been discussed by Wielgus [66] in a simple model of infinitely thin ring of given radius. Particular attention is paid to discussing the possibility of observing the next rings after the secondary image [62][63][64][65].…”

“…Observation of high-order rings in future projects is now actively discussed, e.g. [62][63][64][65].…”

“…After recent observation of black hole shadow in M87 [52][53][54][55][56][57][58][59], the attention has been drawn to investigation of higher-order rings which can arise around the black hole shadow and can be probably observed in the substructure of black hole image [60][61][62][63][64][65][66]. These rings are lensed images of luminous accreting matter of the black hole environment.…”

“…Images of thin accretion disk in case of lensing by spherically symmetric black hole and an observer located on the axis of symmetry. For every image we write the number n of half-orbits[62,[64][65][66] and the change of angular coordinate ∆ϕ of the light ray, see Section III for more details. The objects on the figure are not in scale.…”

First Space-VLBI Observations of Sagittarius A*

“…4. SUMMARY Space-VLBI is an exciting frontier for black hole astrophysics, with the potential to resolve the gravitationally lensed "photon rings" of nearby supermassive black holes (Johnson et al 2020), to measure the masses of thousands of supermassive black holes via their "shadow" diameters (Pesce et al 2021), and to track the orbits of many supermassive black hole binaries (D'Orazio & Loeb 2018). Proposed mission concepts to observe Sgr A * have focused on exploring configurations that allow for rapid baseline sampling to reconstruct movies (e.g., Palumbo et al 2019;Johnson et al 2019) as well as enabling observations at submillimeter wavelengths for which interstellar scattering is sharply reduced (e.g., Roelofs et al 2019;Kudriashov et al 2021).…”