Measuring Black Hole Light Echoes with Very Long Baseline Interferometry

TL;DR

This paper proposes a novel interferometric method to detect light echoes near black holes, which can reveal black hole parameters and test general relativity, using high-frequency correlations in simulated M87-like data.

Contribution

It introduces a new technique correlating total light curves with high spatial frequency signals to identify light echoes around black holes.

Findings

Method can infer echo delay period in simulated data.

Detection of echoes would confirm photons circling the black hole.

Feasibility shown for space-based very long baseline interferometry.

Abstract

Light passing near a black hole can follow multiple paths from an emission source to an observer due to strong gravitational lensing. Photons following different paths take different amounts of time to reach the observer, which produces an echo signature in the image. The characteristic echo delay is determined primarily by the mass of the black hole, but it is also influenced by the black hole spin and inclination to the observer. In the Kerr geometry, echo images are demagnified, rotated, and sheared copies of the direct image and lie within a restricted region of the image. Echo images have exponentially suppressed flux, and temporal correlations within the flow make it challenging to directly detect light echoes from the total light curve. In this paper, we propose a novel method to search for light echoes by correlating the total light curve with the interferometric signal at high spatial frequencies, which is a proxy for indirect emission. We explore the viability of our method using numerical general relativistic magnetohydrodynamic simulations of a near-face-on accretion system scaled to M87-like parameters. We demonstrate that our method can be used to directly infer the echo delay period in simulated data. An echo detection would be clear evidence that we have captured photons that have circled the black hole, and a high-fidelity echo measurement would provide an independent measure of fundamental black hole parameters. Our results suggest that detecting echoes may be achievable through interferometric observations with a modest space-based very long baseline interferometry mission.