How Spatially Resolved Polarimetry Informs Black Hole Accretion Flow Models

TL;DR

This paper reviews how spatially resolved polarimetry from the Event Horizon Telescope enhances understanding of black hole accretion flows and plasma properties through various observables, especially with upcoming technological improvements.

Contribution

It provides a comprehensive overview of how different polarimetric observables inform black hole and plasma models, emphasizing the potential of next-generation EHT capabilities.

Findings

Polarimetric data reveal plasma and spacetime properties.

Upcoming EHT improvements will significantly enhance observational data.

Polarimetry is effective even with sparse, long-baseline coverage.

Abstract

The Event Horizon Telescope (EHT) Collaboration has successfully produced images of two supermassive black holes, enabling novel tests of black holes and their accretion flows on horizon scales. The EHT has so far published total intensity and linear polarization images, while upcoming images may include circular polarization, rotation measure, and spectral index, each of which reveals different aspects of the plasma and space-time. The next-generation EHT (ngEHT) will greatly enhance these studies through wider recorded bandwidths and additional stations, leading to greater signal-to-noise, orders of magnitude improvement in dynamic range, multi-frequency observations, and horizon-scale movies. In this paper, we review how each of these different observables informs us about the underlying properties of the plasma and the spacetime, and we discuss why polarimetric studies are well-suited to measurements with sparse, long-baseline coverage.