Black holes are known for pulling in matter, but they can also eject powerful jets of particles that travel through space at almost the speed of light. These emissions arise in the extreme conditions around a black hole, where gas heats up as it spirals inward in the accretion disk (the rotating disk of matter falling onto the black hole) and emits X-rays. Predicting such events is still difficult.
A team of researchers from Japan led by Professor Toshihiro Kawaguchi (University of Toyama) and Dr. Kazutaka Yamaoka (Nagoya University) studied observations of four black hole systems. Their work was published in The Astrophysical Journal on November 5, 2026. The scientists looked for changes preceding jet emissions.
In a previous 2025 study, the same group suggested that a jet is launched when the inner edge of the accretion disk (the inner radius, R_in) rapidly contracts toward the innermost stable circular orbit (ISCO), the orbit closest to the black hole on which matter can still safely circulate. To test this, the team analyzed archival X-ray and radio data for three more systems: XTE J1550-564, GX 339-4 and MAXI J1820+070. They tracked R_in, changes in X-ray variability (fluctuations in the radiation flux) and the rate of gas flow toward the black hole, comparing these with the times of radio flares indicating a jet emission.
It turned out that in all four systems, roughly 1-10 days before the jet emission, a decrease in R_in and in X-ray variability was observed, while the gas flow rate showed no sharp jumps. Moreover, in two systems the jets arose at flow rates below the theoretical Eddington limit, the threshold at which radiation pressure balances gravity. This calls into question models in which a jet emission requires a high accretion rate.
According to Kawaguchi, monitoring R_in is suitable for predicting the timing of emissions. The researchers propose an alert system: when R_in and X-ray variability continue to decrease and R_in approaches ISCO, a warning is sent to astronomers. This will provide a window of several hours to several days to decide whether to carry out observations. "Developing a 'jet forecast' will allow observatories to focus on the expected time of emissions and will accelerate understanding of jet physics," Kawaguchi adds.
The authors note that the early signs they found help predict these hard-to-catch events. More frequent observations will make it possible to study the processes underlying the most energetic phenomena in the Universe and possibly linked to the history of our planet. "Jets produce high-energy photons and particles such as neutrinos, and cosmic rays have long influenced the evolution of living beings on Earth, including humans," Kawaguchi concludes.