Photon Orbit Signatures in Spectra of Black Hole Accretion Disks

Abstract Light orbiting an accreting black hole may impact the disk or jet multiple times before escaping to the observer, at a variety of angles with respect to the local magnetic field. In this Letter, we characterize the imprints of these long path lengths and disparate magnetic field impacts in synchrotron spectra of hot accretion disks, as the strongly lensed “photon ring” exhibits a higher synchrotron turnover frequency in each lensed subimage. We apply tools of varying complexity: first, we develop a minimal, unlensed one-zone model that isolates the first two subimages of the accretion flow. By varying the magnetic field geometry encountered by each subimage, we show that distinctive spectral signatures emerge in both total intensity and fractional linear polarization. Second, we examine a semianalytic radiatively inefficient accretion flow model, in which we find that there is generally a frequency at which the first indirect image outshines the direct image even in total flux density. Lastly, we demonstrate that even general relativistic magnetohydrodynamic simulation snapshots show this spectral character. We find a typical correction to the unresolved spectrum of order 10% near the turnover frequency that grows with increasing viewing inclination, growing to order unity at higher frequencies. We predict sensitive spectral studies of the cores of Messier 87* and Sagittarius A* at frequencies exceeding 300 GHz to constrain the existence of the photon ring even without imaging, with prospects for photon ring detection even in other sources with unresolved shadows.