Multifrequency Analysis of Favored Models for the Messier 87* Accretion Flow

TL;DR

This study analyzes favored models of the M87* black hole's accretion flow using multifrequency simulations to forecast VLBI observations and understand polarization and morphological features.

Contribution

It compares two magnetically arrested disk models with different spins, examining their polarization, spectra, and imaging predictions across multiple frequencies.

Findings

Polarization remains approximately symmetric even at low frequencies.

86 GHz images show a ring-like shape smaller than recent observations.

Photon ring becomes more prominent at higher frequencies, especially in the retrograde model.

Abstract

The polarized images of the supermassive black hole Messier 87* (M87*) produced by the Event Horizon Telescope (EHT) provide a direct view of the near-horizon emission from a black hole accretion and jet system. The EHT theoretical analysis of the polarized M87* images compared thousands of snapshots from numerical models with a variety of spins, magnetization states, viewing inclinations, and electron energy distributions, and found a small subset consistent with the observed image. In this article, we examine two models favored by EHT analyses: a magnetically arrested disk with moderate retrograde spin and a magnetically arrested disk with high prograde spin. Both have electron distribution functions which lead to strong depolarization by cold electrons. We ray trace five snapshots from each model at 22, 43, 86, 230, 345, and 690 GHz to forecast future VLBI observations and examine limitations in numerical models. We find that even at low frequencies where optical and Faraday rotation depths are large, approximately rotationally symmetric polarization persists, suggesting that shallow depths dominate the polarization signal. However, morphology and spectra suggest that the assumed thermal electron distribution is not adequate to describe emission from the jet. We find 86 GHz images show a ring-like shape determined by a combination of plasma and spacetime imprints, smaller in diameter than recent results from the Global mm-VLBI array. We find that the photon ring becomes more apparent with increasing frequency, and is more apparent in the retrograde model, leading to large differences between models in asymmetry and polarization structure.