Photon Orbit Signatures in Spectra of Black Hole Accretion Disks

TL;DR

This paper investigates how photon orbits around black holes influence the spectra of accretion disks, revealing distinctive spectral signatures that can help detect photon rings without direct imaging.

Contribution

It introduces a minimal one-zone model, semi-analytic RIAF models, and analyzes GRMHD simulations to identify spectral signatures of photon rings in black hole accretion disks.

Findings

Photon rings cause higher synchrotron turnover frequencies in lensed sub-images.

Spectral signatures vary with magnetic field geometry and viewing angle.

Unresolved spectra show a ~10% correction near the turnover frequency, increasing at higher frequencies.

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 sub-image. We apply tools of varying complexity: first, we develop a minimal, unlensed one-zone model that isolates the first two sub-images of the accretion flow. By varying the magnetic field geometry encountered by each sub-image, we show that distinctive spectral signatures emerge in both total intensity and fractional linear polarization. Second, we examine a semi-analytic radiatively inefficient accretion flow (RIAF) 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 (GRMHD) 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.