Measuring Photon Rings with the ngEHT

TL;DR

This paper assesses the potential of the ngEHT to detect and measure black hole photon rings, highlighting the limitations of current EHT data and the challenges of hybrid imaging techniques in accurately identifying these features.

Contribution

It demonstrates that ngEHT's improved resolution can detect photon rings and critically evaluates the reliability of hybrid imaging methods for their measurement.

Findings

EHT 2017 and 2022 lack sufficient resolution for photon ring detection

ngEHT at 230 GHz and 345 GHz can detect photon rings in models

Hybrid imaging can produce false positives and misinterpret ring properties

Abstract

General relativity predicts that images of optically thin accretion flows around black holes should generically have a ``photon ring,'' composed of a series of increasingly sharp subrings that correspond to increasingly strongly lensed emission near the black hole. Because the effects of lensing are determined by the spacetime curvature, the photon ring provides a pathway to precise measurements of the black hole properties and tests of the Kerr metric. We explore the prospects for detecting and measuring the photon ring using very long baseline interferometry (VLBI) with the Event Horizon Telescope (EHT) and the next generation EHT (ngEHT). We present a series of tests using idealized self-fits to simple geometrical models and show that the EHT observations in 2017 and 2022 lack the angular resolution and sensitivity to detect the photon ring, while the improved coverage and angular resolution of ngEHT at 230 GHz and 345 GHz is sufficient for these models. We then analyze detection prospects using more realistic images from general relativistic magnetohydrodynamic simulations by applying ``hybrid imaging,'' which simultaneously models two components: a flexible raster image (to capture the direct emission) and a ring component. Using the Bayesian VLBI modeling package \comrade, we show that the results of hybrid imaging must be interpreted with extreme caution for both photon ring detection and measurement -- hybrid imaging readily produces false positives for a photon ring, and its ring measurements do not directly correspond to the properties of the photon ring.