Explanation for the absence of secondary peaks in black hole light curve autocorrelations

TL;DR

This paper explains why secondary peaks are absent in black hole light curve autocorrelations by showing that the source's correlation timescale exceeds the light echo delay, supported by models and observations of Sgr A*.

Contribution

The authors develop an analytical model demonstrating how source timescales suppress autocorrelation peaks, validated with simulations and applied to real black hole data.

Findings

Light echoes exist but do not produce autocorrelation peaks when source correlation timescale exceeds echo delay.

Model validated against simulated light curves from general-relativistic ray tracing.

Application to Sgr A* suggests its source timescale is greater than the echo delay, explaining observed autocorrelation features.

Abstract

The observed radiation from hot gas accreting onto a black hole depends on both the details of the flow and the spacetime geometry. The lensing behavior of a black hole produces a distinctive pattern of autocorrelations within its photon ring that encodes its mass, spin, and inclination. In particular, the time autocorrelation of the light curve is expected to display a series of peaks produced by light echoes of the source, with each peak delayed by the characteristic time lapse $\tau$ between light echoes. However, such peaks are absent from the light curves of observed black holes. Here, we develop an analytical model for such light curves that demonstrates how, even though light echoes always exist in the signal, they do not produce autocorrelation peaks if the characteristic correlation timescale $\lambda_0$ of the source is greater than $\tau$. We validate our model against simulated light curves of a stochastic accretion model ray traced with a general-relativistic code, and then fit the model to an observed light curve for Sgr A*. We infer that $\lambda_0>\tau$, providing an explanation for the absence of light echoes in the time autocorrelations of Sgr A* light curves. Our results highlight the importance for black hole parameter inference of spatially resolving the photon ring via future space-based interferometry.