X-Ray Signals Point to an Early Warning for Black Hole Jets

For the new paper, the team examined archived X-ray and radio observations of three additional systems:

• XTE J1550-564
• GX 339-4
• MAXI J1820+070

Using those data, the researchers tracked Rᵢₙ, X-ray variability, meaning changes in the brightness of the X-ray emission, and the rate of gas flow toward each black hole. They compared those measurements with the timing of radio flares, bright radio emissions used to denote jet ejections.

What changed before each jet?

Across all four systems, Rᵢₙ and X-ray variability decreased approximately 1 to 10 days before an ejection. The gas flow rate showed no comparable large jump.

In two of the four systems, jets formed at flow rates below the Eddington limit, the theoretical threshold at which the outward push of radiation balances gravity. This makes jet-ejection models that depend on high flow rates unlikely to explain the observations.

The timing points to the disk’s changing geometry, meaning its size and shape, rather than a surge in incoming gas. The 2025 analysis had already linked rapid contraction toward the ISCO with a subsequent ejection; the new results show that sequence across a broader set of systems.

Since astronomers discovered jet ejections more than a century ago, they have noticed changes in the X-ray spectrum, the range of X-ray energies emitted, and decreases in X-ray variability before some ejections. Those signs have not proved reliable enough to predict when a jet will emerge, which has made advance coordination difficult.

How could this become a forecast?

The team proposes an alert when Rᵢₙ and X-ray variability continue to decrease and Rᵢₙ approaches the ISCO. The warning would give astronomers several hours to several days to decide whether to observe and to coordinate facilities.

Kawaguchi added: “The development of a ‘jet forecast’ would enable us to concentrate observing facilities on the expected timing of jet ejections, thereby accelerating our understanding of jet physics.”

For an observatory, the alert would resemble a launch window rather than an exact appointment: a period in which instruments and teams could prepare, without a promise that a jet would emerge.

What remains uncertain?

The evidence still comes from four systems and archived observations, so the rule remains a candidate rather than a validated forecasting method. Known precursor signs have not been reliable enough for dependable advance warning. More frequent observations will help astronomers investigate the physical processes behind discrete ejections.

The researchers situate these ejections among the universe’s most energetic phenomena, which are ultimately linked to Earth’s history. Kawaguchi noted that ejections produce high-energy photons and particles, such as neutrinos, or subatomic particles, while cosmic rays, high-energy particles from space, have long influenced the evolution of living creatures on Earth, including humans.

The next test is whether repeated X-ray monitoring can convert those weak, early signals into accurately timed radio observations of jets.