Scientists discover how stellar-mass black holes emit powerful plasma jets

Black holes are fundamental to the structure of galaxies and critical in our understanding of gravity, space, and time. A stellar mass black hole is a type of black hole that forms from the gravitational collapse of a massive star at the end of its life cycle. These black holes typically have masses ranging from about 3 to 20 times the mass of our Sun.

Sometimes black holes generate beams of ionized gas (plasma) that shoot outward at nearly light speed. Although discovered more than a century ago, how and why jets occur has remained a mystery, described as one of the “wonders of physics”. 

Prof. Kazutaka Yamaoka from Nagoya University in Japan, along with his colleagues from the University of Toyama and other international institutes, have discovered key conditions needed for a stellar black hole to create plasma jets. Their findings, published in Publications of the Astronomical Society of Japan, show that when superheated gas material experiences a rapid shrinkage towards the black hole, jet formation occurs.

Swirling disks of cosmic matter

Understanding jet ejection in black holes is crucial because it sheds light on galaxy evolution, energy distribution in the universe, and the properties of black holes themselves. Jets influence star formation, distribute energy across vast distances, and serve as cosmic beacons for locating distant black holes. Additionally, they provide insights into the fundamental physics of black holes. 

Material such as dust and gas gets pulled toward black holes because of their strong gravity. This material rotates around the black hole in a thin disk, called an accretion disk, which is needed to form a jet.

The scientists studied a black hole system consisting of a stellar-mass black hole and a sun-like star orbiting each other. In this system, 5 or 6 jets occur over a period of about 20 days, making it ideal to study this phenomenon. By analyzing X-ray and radio observation data from 1999 to 2000, they could track how quickly X-ray emissions near the black hole were changing over time and measure the total amount of energy produced by the jets.

Causes of jet formation

Results showed that the jets occur when the inner radius of the accretion disk suddenly decreases and reaches the innermost stable circular orbit (ISCO), the closest that matter can orbit without falling in. 

The researchers observed that initially the inner radius of the gas disk was located further away from the black hole. When the inner radius of the disk shrinks rapidly and reaches the ISCO, the jet erupts. The jet continues to erupt for a while; however, when the shrinking movement of the inner edge of the disk stops, the jet itself ceases.

From this, they identified two key conditions needed for a stellar black hole to create jets: the inner edge of the gas disk surrounding the black hole must rapidly move closer to the black hole and this movement must reach the ISCO.

Scientists already knew that when a black hole jet erupts, X-rays become "softer" (more low-energy X-rays compared to high-energy ones) and show fewer rapid fluctuations in a short time scale. This study discovered that these X-ray changes happen because the inner edge of the gas disk is rapidly moving closer to the black hole, which is the actual trigger for jet formation. As this inner edge shrinks, it produces more soft X-rays with less variability compared with highly variable hard X-rays. This explains why the X-ray patterns change right before jets form.

This study reveals that jets form under changing, dynamic conditions rather than stable, static ones, as many theoretical models have assumed. Now scientists can better predict the occurrence of plasma jets and study the mechanisms behind them in real-time. 

"Our discovery about jet formation in stellar-mass black holes may provide a universal key to understanding these phenomena. While these binary systems – where a black hole orbits a normal star – differ significantly from the supermassive black holes located at the center of a galaxy, we believe similar physical mechanisms operate across all black hole scales,” Prof. Yamaoka explained. 

“Though challenging due to their slower time evolution and difficulty measuring their inner structures, applying our findings to supermassive black holes is our next step,” he added.