Supermassive black holes at the centers of galaxies sometimes eject narrow streams of matter — jets that travel at nearly the speed of light. For a long time, astronomers could not say precisely how fast these jets are for different black holes. In the new work, scientists collected data on hundreds of such jets and applied a method previously used in other fields of science.

The method is called "likelihood-free inference." Instead of trying to compute the probability of each scenario directly, the computer learns from many simulated examples and then compares them to real observations. This allowed accounting for distortions introduced by brightness limitations: faint jets are simply not visible, and if this is not considered, speed estimates are biased. The new approach showed that previous errors were greatly underestimated, and the speed distribution does not resemble a simple curve.

The result: most jets have speeds close to 99% of the speed of light, but there are also slower ones. The speed distribution turned out to be similar to that observed for stellar-mass black holes: those that form from the explosions of massive stars. This suggests that the mechanism for launching jets may be the same for black holes of any mass.