A supermassive black hole’s magnetically perturbed event horizon

Two years ago the Event Horizon Telescope (EHT) collaboration enthralled the world when it released the first-ever image of hot plasma swirling around the event horizon of the supermassive black hole at the center of a galaxy, Messier 87. Now the EHT collaboration has reported the results of measuring the polarization of the millimeter-wave radiation emitted by the plasma—both with EHT and the Atacama Large Millimeter Array (ALMA) in Chile.

The resolving power of a telescope is proportional to the observation wavelength divided by the telescope’s diameter. In the case of the EHT, whose constituent telescopes span the globe to form a giant array, its operating wavelength of 1.3 mm yields an angular resolution of 20 microarcseconds. That’s equivalent to resolving a golfball on the surface of the Moon, and it’s sufficient to image a supermassive black hole in a not-too-distant galaxy. The telescopes that make up ALMA are at most a few kilometers apart. They can’t resolve extragalactic black holes. But when they are brought to bear on M87, they provide complementary data.

Magnetic fields pervade the plasma that envelops black holes. As the plasma’s electrons gyrate round the field lines, they lose energy via synchrotron radiation. The radiation is linearly polarized in the plane of the gyration. As the radiation makes its way through the plasma along the observer’s line of sight, another electromagnetic process, Faraday rotation, comes into play. If all the magnetic field lines around M87 were neatly aligned, the radiation’s polarization would be strong and rotated by the same amount. But if instead the field lines were a tangled mess, whatever polarization the radiation originally had would be wiped out.

As resolved and mapped by EHT, the fractional polarization of the radiation from M87’s event horizon is at most 20%. The ALMA observations, which average the polarization over a larger region, yield an upper limit of around 3%. The observed pattern is consistent with poloidal magnetic field—that is, a field with north and south poles like Earth’s. Such a field could be generated by plasma orbiting the black hole in an accretion disk.

In particular, the observed polarization is consistent with what astrophysicists call a magnetically arrested disk (MAD) scenario. As the magnetic field builds up near the event horizon, it becomes so intense that it pushes back against the plasma being drawn into the black hole. The scenario forces the magnetic fields to orient themselves perpendicular to the accretion disk (along the north and south poles of the spinning black hole), and this is what gives the polarized image the “twist.”

M87 is a giant elliptical galaxy of trillions of stars at the center of the Virgo supercluster. A jet of hot plasma emanates at relativistic speeds from the galaxy’s core (see accompanying image). In 1977 Roger Blandford and Roman Znajek proposed a way by which a rotating black hole could extract energy from surrounding plasma and convert it into a jet. Can the Blandford–Znajek mechanism account for M87’s jet, given the values for field strength, temperature, and density inferred from the EHT and ALMA observations?

To find out, the collaboration created a set of jet models and then simulated how their associated radiation would appear if observed by EHT at the distance of M87—4900 light-years. The model that best matched the observations validated an assumption that Blandford and Znajek had made: that magnetic fields in the vicinity of the event horizon are dynamically important for jet production. (K. Akiyama et al., Event Horizon Telescope collaboration, Astrophys. J. Lett. 910, L13, 2021 .)