High Resolution Linear Polarimetric Imaging for the Event Horizon Telescope

TL;DR

This paper develops an advanced polarimetric imaging method using Maximum Entropy Method (MEM) for high-resolution imaging of AGN with VLBI, demonstrating improved resolution and robustness over traditional algorithms.

Contribution

It introduces a novel polarimetric MEM algorithm that combines Stokes I and Q/U imaging techniques, enhancing image reconstruction for VLBI data, especially for EHT observations.

Findings

Polarimetric MEM outperforms CLEAN in resolution for smooth, compact sources.

The method is effective on real VLBA data and simulated EHT observations.

MEM's flexibility allows for various polarization constraints.

Abstract

Images of the linear polarization of synchrotron radiation around Active Galactic Nuclei (AGN) identify their projected magnetic field lines and provide key data for understanding the physics of accretion and outflow from supermassive black holes. The highest resolution polarimetric images of AGN are produced with Very Long Baseline Interferometry (VLBI). Because VLBI incompletely samples the Fourier transform of the source image, any image reconstruction that fills in unmeasured spatial frequencies will not be unique and reconstruction algorithms are required. In this paper, we explore extensions of the Maximum Entropy Method (MEM) to linear polarimetric VLBI imaging. In contrast to previous work, our polarimetric MEM algorithm combines a Stokes I imager that uses only bispectrum measurements that are immune to atmospheric phase corruption with a joint Stokes Q and U imager that operates on robust polarimetric ratios. We demonstrate the effectiveness of our technique on 7- and 3-mm wavelength quasar observations from the VLBA and simulated 1.3-mm Event Horizon Telescope observations of Sgr A* and M87. Consistent with past studies, we find that polarimetric MEM can produce superior resolution compared to the standard CLEAN algorithm when imaging smooth and compact source distributions. As an imaging framework, MEM is highly adaptable, allowing a range of constraints on polarization structure. Polarimetric MEM is thus an attractive choice for image reconstruction with the EHT.