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Black holes play a crucial role in the framework of galaxies and are essential to our grasp of gravitational forces, as well as the concepts of space and time. Stellar mass black holes emerge from the gravitational collapse of enormous stars as they reach the twilight of their life cycles, typically possessing masses between three and twenty times that of our Sun.
Occasionally, black holes emit beams of ionized gas, or plasma, that can propel outward at speeds approaching that of light. Though these jets have been known for over a century, the precise mechanisms behind their formation remain enigmatic, earning them a place among the “wonders of physics.”
Recent research led by Prof. Kazutaka Yamaoka from Nagoya University, in collaboration with scholars from the University of Toyama and several international institutions, has illuminated the critical conditions necessary for a stellar black hole to generate these plasma jets. Their findings, which appeared in Publications of the Astronomical Society of Japan, indicate that the formation of jets occurs when superheated gas experiences a rapid contraction towards the black hole.
Swirling disks of cosmic matter
Understanding the mechanism of jet ejection from black holes is vital, as it offers insights into how galaxies evolve, how energy is distributed throughout the universe, and the inherent properties of black holes. These jets can affect star formation, facilitate energy dispersion across immense distances, and serve as vital markers for identifying distant black holes. Moreover, they contribute to the comprehension of the fundamental physics underpinning black holes.
Material such as gas and dust is drawn toward black holes due to their intense gravitational pull. This material typically forms a rotating disk around the black hole, known as an accretion disk, which is integral to the formation of jets.
The research team investigated a binary system comprising a stellar-mass black hole and a sun-like star in orbit. Within this system, multiple jets appeared over approximately twenty days, providing a unique opportunity to study this occurrence. They analyzed X-ray and radio data collected from 1999 to 2000, which allowed them to monitor the rapid variations in X-ray emissions near the black hole and quantify the total energy produced by the jets.
Causes of jet formation
The study revealed that jet formation is initiated when the inner radius of the accretion disk suddenly decreases, reaching the innermost stable circular orbit (ISCO), which represents the closest distance at which matter can orbit the black hole without falling in.
The scientists observed that the inner radius of the accretion disk initially resided at a greater distance from the black hole; however, as this inner radius contracted swiftly and encroached upon the ISCO, a jet would emerge. This jet could persist for a time, but once the rapid contraction of the inner disk ceased, the jet would also stop.
From these observations, two crucial conditions for jet production by a stellar black hole were identified: the inner edge of the surrounding gas disk must rapidly move closer to the black hole, and this movement must reach the ISCO.
Previously established notions indicated that when a black hole jet is ejected, the X-rays emitted become “softer,” exhibiting more low-energy X-rays compared to their high-energy counterparts and displaying decreased fluctuations over short time scales. This investigation revealed that such X-ray alterations occur due to the rapid inward movement of the gas disk’s inner boundary, which acts as the catalyst for jet formation. As the inner edge contracts, it yields increased soft X-rays with less variability, in contrast to the more erratic patterns observed in high-energy X-rays. This finding elucidates the shifts in X-ray emissions that precede jet formation.
The research highlights that jet formation is contingent upon dynamic and fluctuating conditions rather than stable, unchanging ones, as previously posited by numerous theoretical models. This advancement paves the way for scientists to better predict and study plasma jet occurrences in real-time.
“Our findings regarding jet formation in stellar-mass black holes may provide a universal framework for understanding these phenomena. Even though these binary systems — where a black hole orbits a normal star — are notably different from the supermassive black holes found at galactic centers, we believe similar physical processes may operate across various scales of black holes,” Prof. Yamaoka noted.
“While examining supermassive black holes poses challenges due to their slower evolutionary timescales and the difficulties associated with measuring their internal structures, applying our discoveries in that context will be our next objective,” he added.
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