In 1977, Blandford and Znajek showed that the electromagnetic field surrounding a rotating black hole can harvest its spin energy and use it to power a collimated astrophysical jet, such as the one launched from the center of the elliptical galaxy M87. Today, interferometric observations with the Event Horizon Telescope (EHT) are delivering high-resolution, event-horizon-scale, polarimetric images of the supermassive black hole M87* at the jet launching point. These polarimetric images offer an unprecedented window into the electromagnetic field structure around a black hole. In this paper, we show that a simple polarimetric observable -- the phase $\angleβ_2$ of the second azimuthal Fourier mode of the linear polarization in a near-horizon image -- depends on the sign of the electromagnetic energy flux and therefore provides a direct probe of black hole energy extraction. In Boyer-Lindquist coordinates, the Poynting flux for axisymmetric electromagnetic fields is proportional to the product $B^ϕB^r$. The phase $\angleβ_2$ likewise depends on the ratio $B^ϕ/B^r$, thereby enabling an observer to experimentally determine the direction of electromagnetic energy flow in the near-horizon environment. Data from the 2017 EHT observations of M87* are consistent with electromagnetic energy outflow. Currently envisioned multi-frequency observations of M87* will achieve higher dynamic range and angular resolution, and hence deliver measurements of $\angleβ_2$ closer to the event horizon as well as better constraints on Faraday rotation. Such observations will enable a definitive test for energy extraction from the black hole M87*.
Black Hole Polarimetry: Universal Polarization of Synchrotron Radiation at the Horizon
Niels Bohr International Academy, Niels Bohr Institute（尼尔斯·玻尔国际学院，尼尔斯·玻尔研究所）；Department of Physics & Astronomy, Vanderbilt University（范德堡大学物理与天文学系）；Princeton Gravity Initiative, Princeton University（普林顿引力倡议，普林斯顿大学）；School of Natural Sciences, Institute for Advanced Study（高等研究院自然科学院）；Department of Physics, Princeton University（普林斯顿大学物理系）；Department of Astrophysical Sciences, Princeton University（普林斯顿大学天体科学系）
Stanford University（斯坦福大学）；Department of Physics, Stanford University（斯坦福大学物理系）；Kavli Institute for Particle Astrophysics and Cosmology, Stanford University（斯坦福大学卡弗里粒子天体物理学与宇宙学研究所）
