A simple model for extracting astrophysics from black hole images

Abstract

The Event Horizon Telescope (EHT) is providing unprecedented high-resolution images of supermassive black holes. These images are fundamentally related to properties of the luminous accretion disks, since black holes themselves produce no light. We develop a simple prescription to relate observational black hole image features to a toy model for the intensity profile of the associated accretion disk. We apply our model to both the original EHT image of M87* and the reanalyzed image from the PRIMO algorithm, providing generic, simultaneous constraints on the black hole mass and disk emission properties. While current images lack the resolution to confidently detect the photon ring, we use multiple observed features from the original EHT image to constrain M87*'s mass to $6.6^{+1.2}_{-1.0}\times 10^9 M_\odot$, and find emission may extend to the black hole horizon. Conversely, using constraints from the PRIMO image alongside brightness asymmetry constraints from the original EHT analysis yields a mass of $6.4^{+0.7}_{-0.7}\times 10^9 M_\odot$, with the disk's inner edge between $3M$ and $5.3M$. Both analyses rule out the disk's inner edge coinciding with the innermost stable circular orbit for a Schwarzschild black hole ($6M$), and our analysis with PRIMO confidently rules out significant emission extending to the horizon ($2M$). While this work restricts disks to Keplerian orbits outside the ISCO, we demonstrate that brightness asymmetry is especially sensitive to the disk velocity profile. Further assumptions on the mass of M87* and connections between the accretion disk cutoff and physical radii allow for rudimentary black hole spin estimates.