Modeling Targets and Optimal Frequencies for Imaging the Shadows of Nearby Supermassive Black Holes

TL;DR

This paper evaluates the potential for imaging black hole shadows in nearby supermassive black holes using modeling and ray tracing, guiding future observational strategies for space-based VLBI.

Contribution

It introduces a covariant semi-analytic model for accretion flows and assesses their impact on imaging black hole shadows across a broad source population.

Findings

Identifies three classes of sources based on transparency at various frequencies.

Determines the critical frequency for optical thinness varies with black hole and flow parameters.

Provides guidelines for target selection and wavelength choice in future VLBI observations.

Abstract

Horizon-scale imaging of supermassive black holes has opened a new window onto the studies of strong-field gravity and plasma physics in low-luminosity accretion flows. As future efforts aim to image fainter and smaller angular-size targets, primarily through space-based very long baseline interferometry (VLBI), it is important to identify optimal sources and observing strategies for such studies. In this work, we assess the prospects for imaging black hole shadows in a broad population of nearby supermassive black holes by modeling their accretion flows using a covariant semi-analytic model for the flow and general relativistic ray tracing. We explore the influence of black hole and accretion flow parameters on spectra, image morphology, and the critical frequency at which the flows become optically thin. We identify three general classes of sources: those that become transparent at traditional imaging frequencies; those requiring higher frequencies; and those unlikely to be transparent down to the black hole shadow in the submillimeter band. Our results will inform target selection and wavelength optimization for future VLBI arrays, where both resolution and transparency are essential for resolving black hole shadows.