First M87 Event Horizon Telescope Results. VII. Polarization of the Ring

TL;DR

The paper presents the first polarimetric images of the M87 black hole's event horizon region, revealing polarized emission patterns and temporal evolution of magnetic field structures near the black hole.

Contribution

This work provides the first linear polarization images of M87's black hole environment, demonstrating the magnetic field structure and its evolution over a week.

Findings

Polarized emission is concentrated in part of the ring with up to 15% polarization.

Polarization angles form a nearly azimuthal pattern around the ring.

Evidence of temporal evolution in the polarized structure over one week.

Abstract

In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon region around the supermassive black hole at the core of the M87 galaxy. These 1.3 mm wavelength observations revealed a compact asymmetric ring-like source morphology. This structure originates from synchrotron emission produced by relativistic plasma located in the immediate vicinity of the black hole. Here we present the corresponding linear-polarimetric EHT images of the center of M87. We find that only a part of the ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of about 15%. The polarization position angles are arranged in a nearly azimuthal pattern. We perform quantitative measurements of relevant polarimetric properties of the compact emission and find evidence for the temporal evolution of the polarized source structure over one week of EHT observations. The details of the polarimetric data reduction and calibration methodology are provided. We carry out the data analysis using multiple independent imaging and modeling techniques, each of which is validated against a suite of synthetic data sets. The gross polarimetric structure and its apparent evolution with time are insensitive to the method used to reconstruct the image. These polarimetric images carry information about the structure of the magnetic fields responsible for the synchrotron emission. Their physical interpretation is discussed in an accompanying publication.