Time Domain Filtering of Resolved Images of Sgr A*

TL;DR

This paper introduces a novel time-domain filtering technique for analyzing variable images of Sgr A* from the Event Horizon Telescope, revealing emission regions and model-dependent features related to black hole properties.

Contribution

The paper presents a new time-domain filtering method applied to simulated Sgr A* images, enabling extraction of emission radius information and model differentiation based on variability.

Findings

Filtered images correlate variability with emission radius.

Technique can distinguish models with different black hole spins.

Method reveals emission structure not accessible in time-averaged images.

Abstract

The goal of the Event Horizon Telescope (EHT) is to provide spatially resolved images of Sgr A*, the source associated with the Galactic Center black hole. Because Sgr A* varies on timescales short compared to an EHT observing campaign, it is interesting to ask whether variability contains information about the structure and dynamics of the accretion flow. In this paper, we introduce "time-domain filtering", a technique to filter time fluctuating images with specific temporal frequency ranges, and demonstrate the power and usage of the technique by applying it to mock millimeter wavelength images of Sgr A*. The mock image data is generated from General Relativistic Magnetohydrodynamic (GRMHD) simulation and general relativistic ray-tracing method. We show that the variability on each line of sight is tightly correlated with a typical radius of emission. This is because disk emissivity fluctuates on a timescale of order the local orbital period. Time-domain filtered images therefore reflect the model dependent emission radius distribution, which is not accessible in time-averaged images. We show that, in principle, filtered data have the power to distinguish between models with different black hole spins, different disk viewing angles, and different disk orientations in the sky.