In the 2010s, astronomers noticed something remarkable: supermassive black holes at the centers of galaxies, despite vast differences in size and brightness, fall onto a single straight line when you plot their millimeter emission against X-ray emission and mass. This line was dubbed the "millimeter fundamental plane of black hole accretion."
Now, a team of researchers has tested whether stellar-mass black holes, those born from the collapse of massive stars, follow the same rule. They took data from five X-ray binary systems, where a black hole pulls matter from a companion star, and found that these objects also land on the same plane. It's as if all black holes in the universe, from the tiniest to the most gigantic, obey one universal law.
The scientists also tested theoretical models describing matter falling onto a black hole. Both leading models (one with a hot, rarefied flow and the other with narrow plasma jets) predict the existence of such a plane for black holes of any mass. However, the accuracy of mass prediction from this plane differs between the models, opening the door for further research.
as written by the authors arXiv:2601.04007
Recent work revealed the existence of a galaxy "millimetre fundamental plane of black hole accretion", a tight correlation between nuclear $$1$$mm luminosity, intrinsic $$2$$ - $$10$$keV X-ray luminosity and supermassive black hole mass, originally discovered for nearby low- and high-luminosity active galactic nuclei. Here we use mm and X-ray data of $$5$$ X-ray binaries (XRBs) to demonstrate that these stellar-mass black holes also lie on the mm fundamental plane, as they do at radio wavelengths. One source for which we have multi-epoch observations shows evidence of deviations from the plane after a state change, suggesting that the plane only applies to XRBs in the hard state, as is true again at radio wavelengths. We show that both advection-dominated accretion flows and compact jet models predict the existence of the plane across the entire range of black hole masses, although these models vary in their ability to accurately predict the XRB black hole masses.
Millimeter emission from black holes can be observed with radio telescopes like ALMA in Chile, which can see through the dust clouds that hide the galactic center.