Black hole jets ‘dance’ in the wind from a massive companion star

Black holes are among the most extreme objects in the universe. They can fling material outwards at speeds close to that of light, in powerful beams of plasma known as jets. These jets are thought to be among the most energetic phenomena in the cosmos.

Our new work, published today in Nature Astronomy, challenges this intuition. We find that something as seemingly ordinary as the “wind” from a star can rival – and even shape – the behaviour of these powerful jets.

A cosmic waltz

The Cygnus X-1 system is a cosmic waltz between a black hole and a massive star.

The black hole is the first ever discovered. It weighs about 21 times the mass of our Sun, compressed into a region roughly 100 kilometres across. It’s in what’s known as a binary system with a much larger companion star that’s almost 40 times as massive as the Sun. The black hole and the star whirl around each other in their orbit once every 5.6 days.

For about 20,000 years, the black hole has been feeding on material from this star. It does so by capturing the star’s powerful stellar wind, using its intense gravitational pull.

Some of this material disappears into the black hole, crossing the point of no return (known as the event horizon) in a one-way journey. The swirling magnetic fields dragged in with the gas lead to the launching of jets, moving at nearly the speed of light.

The jets carry energy away from close to the black hole out to a distance a trillion times greater, 16 light years away.

Their action over the past 20,000 years has inflated a giant bubble of hot gas in the surrounding interstellar space. But despite their importance, measuring the instantaneous power of these jets has remained a major challenge until now.

The power couple

Stellar winds are streams of particles blown off the surface of a star by the outward pressure of the light. When the solar wind from our Sun is particularly intense, it causes auroras when the particles slam into Earth’s magnetic field.

The companion star in Cygnus X-1 is so massive and so bright that it loses 100 million times as much mass in its wind as the Sun does, and accelerates it to speeds three times as high.

In our study, we made very high resolution images of the jets by combining telescopes separated by thousands of kilometres. This is the same technique used by the Event Horizon Telescope to make the first image of a black hole.

We found that the wind from the companion star in Cygnus X-1 is strong enough to bend the jets launched by the black hole. This shows just how powerful the winds of massive stars can be.

As the black hole orbits the star, the stellar wind continuously pushes against the jets, blowing them away from the star. This causes them to change direction, just like wind on Earth can blow around the water in a fountain.

From our point of view, the jets appear to “dance” in step with the orbital motion of the system. By modelling this cosmic dance, we were able to measure the instantaneous power of the jets for the first time, and find it to be equivalent to 10,000 suns.

The calorie deficit of a black hole diet

Understanding how black holes use their energy tells us about how galaxies evolve.

When matter falls towards a black hole, part of it contributes to the growth of the black hole itself. But a significant fraction can be redirected into jets, which inject energy back into their surroundings. For the most massive black holes at the centres of galaxies, the jets can shape their host galaxies and influence even larger cosmic structures.

We can measure how fast a black hole is feeding from the X-rays produced by the material falling inwards. However, until now, we have not had a direct way to measure how much energy goes into these jets at any given moment.

Our measurement of the jet power in Cygnus X-1 provides a new way to “balance the energy budget” of black holes. By comparing how fast a black hole is feeding and how much energy is carried away by the jets, we can fine-tune computer simulations of the universe. This tells us about how black holes influence the universe on the largest scales.

This cosmic dance of a black hole and a massive star reveals more than just a bent jet. It shows how even the most energetic phenomena, such as jets, are shaped by their surroundings. By watching the dancing jets in Cygnus X-1, we have improved our understanding of how black holes influence the evolution of the cosmos itself.