High-resolution Linear Polarimetric Imaging for the Event Horizon Telescope

Citation:

Andrew A. Chael, Michael D. Johnson, Ramesh Narayan, Sheperd S. Doeleman, John F. C. Wardle, and Katherine L. Bouman. 2016. “High-resolution Linear Polarimetric Imaging for the Event Horizon Telescope.” The Astrophysical Journal, 829.

Abstract:

Images of the linear polarizations of synchrotron radiation aroundactive galactic nuclei (AGNs) highlight their projected magnetic fieldlines and provide key data for understanding the physics of accretionand outflow from supermassive black holes. The highest-resolutionpolarimetric images of AGNs are produced with Very Long BaselineInterferometry (VLBI). Because VLBI incompletely samples the Fouriertransform of the source image, any image reconstruction that fills inunmeasured spatial frequencies will not be unique and reconstructionalgorithms are required. In this paper, we explore some extensions ofthe Maximum Entropy Method (MEM) to linear polarimetric VLBI imaging. Incontrast to previous work, our polarimetric MEM algorithm combines aStokes I imager that only uses bispectrum measurements that are immuneto atmospheric phase corruption, with a joint Stokes Q and U imager thatoperates on robust polarimetric ratios. We demonstrate the effectivenessof our technique on 7 and 3 mm wavelength quasar observations from theVLBA and simulated 1.3 mm Event Horizon Telescope observations of Sgr A*and M87. Consistent with past studies, we find that polarimetric MEM canproduce superior resolution compared to the standard CLEAN algorithm,when imaging smooth and compact source distributions. As an imagingframework, MEM is highly adaptable, allowing a range of constraints onpolarization structure. Polarimetric MEM is thus an attractive choicefor image reconstruction with the EHT.

Recent Publications

The Event Horizon Telescope Collaboration. 4/10/2019. “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole.” ApJL, 875, L6, Pp. 1-44. Publisher's Version Abstract

We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42 ± 3 μas and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc2 = 3.8 ± 0.4 μas. Folding in a distance measurement of ${16.8}_{-0.7}^{+0.8}\,\mathrm{Mpc}$ gives a black hole mass of $M=6.5\pm 0.2{| }_{\mathrm{stat}}\pm 0.7{| }_{\mathrm{sys}}\times {10}^{9}\hspace{2pt}{M}_{\odot }$ . This measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.

The Event Horizon Telescope Collaboration. 4/10/2019. “First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring.” ApJL, 875, L5, Pp. 1-31. Publisher's Version Abstract

The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.

Sheperd S. Doeleman

Smithsonian Astrophysicist and Harvard Senior Research Fellow

Citation:

Abstract:

Recent Publications