A black hole’s jet bent by its stellar partner reveals its true power

Black holes are often depicted as cosmic vacuum cleaners, but they are also powerful engines capable of launching jets: extremely fast streams of matter and energy shot out at nearly the speed of light. These jets strongly influence their surroundings, from nearby stars to entire galaxies. Yet one key question has remained unanswered: how powerful are these jets at any given moment?

A study published in Nature Astronomy has now achieved this long‑sought measurement by observing a jet that is quite literally bent sideways, and changing direction along the orbit.

The research focuses on the microquasar Cygnus X‑1, one of the most famous black holes in our Galaxy, located about 7,000 light‑years from Earth. The black hole orbits a massive, hot companion star that produces a powerful stellar wind — a constant stream of gas moving at thousands of kilometres per second.

Using almost 20 years of ultra‑sharp radio observations, the team discovered that this stellar wind pushes against the black hole’s jet, bending it as it travels through space. By modelling this interaction along the orbit, the researchers were able to directly calculate the jet’s power.

“The stellar wind acts like a natural probe,” explains Valentí Bosch‑Ramon, researcher at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and co‑author of the study. “By measuring how much the jet bends and changes direction with time, we can determine how strong it really is.”

Reading a jet from its bend

An everyday analogy helps: a strong stream of water from a hose remains straight on a calm day but bends if the wind blows hard enough. In Cygnus X‑1, the same principle applies on cosmic scales.

Using a technique called Very Long Baseline Interferometry (VLBI) (which combines radio telescopes across the Earth) astronomers obtained images sharp enough to detect tiny changes in the jet’s direction during the black hole’s orbit. The jet always bends away from the companion star, leaving no doubt that the stellar wind is responsible.

From this bending, the team measured a jet power of about 10³⁷ ergs per second.

This is an enormous amount of energy, comparable to the system’s total X‑ray output and, over the age of the system, similar to the energy released by a supernova explosion.

(An “erg” is a unit of energy used in astrophysics; 10³⁷ ergs per second is trillions of trillions of times more powerful than human technologies can produce.)

Although Cygnus X‑1 hosts a relatively small black hole (about 20 times the mass of the Sun), the same physics applies to supermassive black holes at the centres of galaxies. Their jets are thought to regulate how galaxies grow — a process known as black‑hole feedback.

“This measurement gives strong observational support to assumptions used in galaxy‑formation models,” says Bosch‑Ramon. “Understanding a nearby system like Cygnus X‑1 helps us better understand the role of black holes across the Universe.”

Beyond this specific system, the study introduces a new way to measure jet power directly, turning a complex interaction into a powerful scientific tool.

“What used to be considered a complication for modelling this system,” Bosch‑Ramon adds, “has become a unique opportunity to measure one of the most extreme phenomena in astrophysics.”

Reference
Prabu, S. et al. A jet bent by a stellar wind in the black hole X‑ray binary Cygnus X‑1. Nature Astronomy (2026).
https://doi.org/10.1038/s41550-026-02828-3