A strong magnetic field in the jet base of a supermassive black hole

TL;DR

This paper reports the detection of a strong magnetic field near the base of a jet in a supermassive black hole, using polarization measurements from ALMA, indicating magnetic fields of at least tens of Gauss within light-day scales.

Contribution

First direct measurement of magnetic fields at the jet base of a supermassive black hole using Faraday rotation with ALMA.

Findings

Detected high Faraday rotation measures indicating strong magnetic fields.

Estimated magnetic field strength of at least tens of Gauss near the black hole.

Magnetic fields are critical in jet formation and acceleration processes.

Abstract

Active galactic nuclei (AGN) host some of the most energetic phenomena in the Universe. AGN are thought to be powered by accretion of matter onto a rotating disk that surrounds a supermassive black hole. Jet streams can be boosted in energy near the event horizon of the black hole and then flow outward along the rotation axis of the disk. The mechanism that forms such a jet and guides it over scales from a few light-days up to millions of light-years remains uncertain, but magnetic fields are thought to play a critical role. Using the Atacama large mm/submm array (ALMA), we have detected a polarization signal (Faraday rotation) related to the strong magnetic field at the jet base of a distant AGN, PKS1830-211. The amount of Faraday rotation (rotation measure) is proportional to the magnetic field strength along the line of sight times the density of electrons. Although it is impossible to precisely infer the magnetic fields in the region of Faraday rotation, the high rotation measures derived suggest magnetic fields of at least tens of Gauss (and possibly considerably higher) on scales of the order of light days (0.01 pc) from the black hole.