We’ve long known that black holes can produce powerful jets of ionized gas. These jets stream away from the black hole at nearly the speed of light. Jets produced by supermassive black holes are so powerful they are seen as quasars from billions of light-years away. But when you think about it, jets are a bit counterintuitive. Black holes trap and consume material through their tremendous gravity, so how can they push streams of material away? A recent study in Publications of the Astronomical Society of Japan shows how it works.
In this study, the team looked for data on stellar mass black holes. Though tiny in comparison to their supermassive cousins that power quasars, stellar-mass black holes are driven by much the same physics. They also evolve on a much shorter timescale due to their smaller size. Some black holes actively produce jets for long periods of time, while others are quiet. But there are also black holes that cycle between active and quiet periods. The team focused on one such black hole, known as XTE J1859+226.
This particular stellar-mass black hole is part of a binary system. Its companion is a Sun-like star. As the two orbit each other, the black hole captures some of its companion’s material, and as a result, the black hole produces jets every 3–4 days. So the team analyzed X-ray and radio observations of the black hole gathered in 1999 and 2000.
The radio observations allowed the team to determine when jets form, since each lobe of a jet is bright at radio wavelengths. The X-ray observations allowed the team to understand the superheated disk of material around the black hole. The study shows that just before jets appear, the X-rays from the black hole “soften,” meaning that they become less high-energy and fluctuate less. Astronomers have known this for a while, but the team was able to show that the softening is caused by a rapid inward movement of accretion material. The material moves inward to a line known as the innermost stable circular orbit (ISCO). Any closer and the material is essentially doomed to fall into the black hole.
Through their work, the team found that two conditions must be met for a jet to form: The inner edge of a black hole’s accretion disk must rapidly shrink inward, and the edge must reach the ISCO. It’s kind of like how you can squirt water at someone while in a pool. Cup your hands around a bit of water and squeeze quickly. When there is no more gap between the water and your hands, the water squirts out in a rapid stream.
Supermassive black holes likely create their jets through a similar process. The next goal of the team is to study quasar data to see if they can prove that to be true.
Reference: Yamaoka, Kazutaka, et al. “X-ray spectral and timing properties of the black hole binary XTE J1859+ 226 and their relation to jets.” Publications of the Astronomical Society of Japan (2025): psae113.