How can a black hole swallow enormous quantities of matter while launching a jet almost as fast as light?
A black hole attracts gas and dust that pass too close to it. Before falling in, this matter often orbits it, forming a disk. At the same time, some black holes project two very powerful jets into space, pointing in opposite directions. What puzzles astronomers is that the biggest feeders should not necessarily produce the strongest jets.
Artist's view of a black hole surrounded by an accretion disk and projecting a plasma jet. This diagram illustrates the mechanisms studied for the galaxy 3C 120, without representing this particular object.
Illustration: NASA/JPL-Caltech, public domain.
To understand this paradox, an international team observed the black hole located at the center of the galaxy 3C 120. This object rapidly swallows matter and emits a spectacular jet. The researchers combined measurements from the XRISM space X-ray telescope with those from several networks of radio telescopes spread across Earth.
The X-rays allowed them to examine the gas orbiting the black hole. This gas contains iron in particular, whose light bears a recognizable signature. Gravity and very rapid movements distort this signature. By studying its shape, the researchers can estimate how far the disk of matter extends.
The result shows that this relatively cool and flattened disk does not extend all the way to the immediate vicinity of the black hole. It seems to stop short, giving way to a region where the gas is much hotter and less flattened. This gas, a plasma, can reach several hundred million degrees.
Radio telescopes reveal the other half of the story. They made it possible to track the shape of the jet at its point of origin, very close to this scorching region. The two sets of measurements therefore indicate that the jet could be born within this very hot gas, rather than in the cooler disk surrounding it.
The researchers think that the hot gas can sustain powerful magnetic fields near the black hole. These fields would guide some of the matter outward and could draw on the black hole's rotational energy to power the jet. Nothing emerges from inside the black hole: the jet forms in its vicinity.
The researchers now want to examine other galaxies to see whether this mechanism is reproduced elsewhere.