(ORDO NEWS) — Fluctuations in light from the black hole, observed over the course of 15 years, have allowed us to learn more about how these mysterious objects feed.
First, a structure called a corona forms around the outside of the event horizon. Powerful jets of plasma then erupt from the poles, knocking material out of the corona and into interstellar space at a speed close to the speed of light in a vacuum.
This discovery, which can be compared to the rhythmic beating of a “heart”, solves a long-standing question in the science of black holes.
“It sounds logical, but for twenty years there has been a debate about whether the corona and the jet are just the same,” explains astrophysicist Mariano Mendez of the University of Groningen in the Netherlands.
“Now we see that they arise one after another and that the jet follows the corona.”
The black hole in question is part of the star GRS 1915+105, located at a distance of about 36,000 light-years from the Sun. This is what we call a microquasar – a stellar-mass black hole locked in a tight binary system with and feeding off another object; in the case of GRS 1915+105, this is an ordinary star.
Because the two objects are so close to each other, the black hole is robbing material from the star; this material forms a disk around the black hole, which gradually feeds it.
We see the same thing on a large scale in quasars, which are galactic nuclei containing an active supermassive black hole with a mass millions and billions of times greater than the mass of the Sun.
The mass of the black hole GRS 1915+105 is only 12 times the mass of the Sun, therefore, it is a microquasar; despite this, it is one of the most massive stellar-mass black holes known in the Milky Way.
This process generates a lot of light due to the heating of the disk and the complex environment around the black hole. One of the light generating structures is the corona located between the inner edge of the accretion disk and the event horizon.
This is a region of scalding hot electrons that are believed to be fed by the black hole’s magnetic field, which acts like a synchrotron, accelerating the electrons to such high energies that they shine brightly in the X-rays.
In addition, there are jets. They are thought to be composed of material accelerated along magnetic field lines beyond the black hole’s event horizon to the polar regions, where they are launched into space at high speed, emitting radio light.
At least that’s what scientists think. The space around black holes is so extreme that it is difficult to understand the processes taking place there.
Mendes and his colleagues wanted to learn more about how jets accelerate and launch. They collected X-ray and radio data on the microquasar collected between 1996 and 2012 and carefully studied it for clues.
Their final sample consisted of 410 simultaneous X-ray and radio observations of GRS 1915+105. This meant that they could observe changes in both types of light at the same time. They found that when the X-ray emission is strong, the radio emission is weak, and vice versa; and that the jets are strongest when the corona is at its smallest.
This suggests that the energy feeding the microquasar system can be directed either to the X-ray corona or to the relativistic jet. Complementing the models of light fluctuations from the system, the researchers concluded that, at least in GRS 1915+105, it looks like the corona is turning into a jet.
“It was quite difficult to demonstrate this consistent character,” Mendez said. “We had to compare years of data with seconds, and very high energy data with very low energy data.”
Next, the team will have to try to explain some of the oddities that their observations revealed. The X-ray corona is brighter than temperature alone can explain. This means there might be something else in the game. The team believes the magnetic field may be the cause.
The differential rotation of the black hole and the accretion disk can lead to entanglement and randomization of the magnetic field. When the magnetic field is chaotic, the team suggests, the corona heats up; when it returns to order, the material can escape, and thus the jets are launched.
And this process should depend on the mass of the black hole, which can help us understand how massive quasars behave.
“In principle,” the researchers write, “the same direction of energy into the jet and corona should occur in supermassive black holes, and therefore should be applicable to the entire range of black hole masses in the fundamental plane of black hole activity.”
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