Black holes shine in x-ray light from super-heated gas

Black holes produce x-rays far more powerful and energetic than current theories would suggest were possible. Now we know how these high-energy waves are produced.

Astrophysicists from John Hopkins University, the Rochester Institute of Technology (RIT) and NASA studied what happens when matter falls into a black hole. What they found is that gas circling into such a massive collapsed star inevitably gives off x-rays.

As particles of gas spiral into a black hole from the accretion disk surrounding it, they heat up to 18 million degrees Fahrenheit. This large amount of heating produces soft x-rays. When astronomers observe black holes, however, they also see more powerful hard x-rays, which can have up to 100 times the energy of soft x-rays. Temperatures of billions of degrees would be needed to produce these x-rays, far more than produced in accretion disks.

Recent observations have determined that the majority of x-rays around a black hole are created not in the accretion disk itself, but in a corona which surrounds the collapsed star. Our Sun is also surrounded by a corona, which is much hotter than the surface of our local star. Similarly, the corona around black holes is much more energetic than the accretion disk, providing the extra energy needed to create the observed hard x-rays. The research confirms the modern observations.

To reach their conclusion, the team used both modern and classic techniques. They ran simulations on a supercomputer named Ranger that took more than 600 hours to complete, and they also did some calculations by hand, on paper.

"Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions and gravity exhibiting the full weirdness of general relativity. But our calculations show we can understand a lot about them using only standard physics principles," Julian Krolik, physics and astronomy professor at the Zanvyl Krieger School of Arts and Sciences, said.

As the gas spirals inward, higher density and velocities mean that the magnetic fields between the atoms becomes more pronounced, and starts to exert it's on force on the material. Calculating the complex interactions in such a system created a need for a supercomputer like Ranger.

"We're accurately representing the real object and calculating the light an astronomer would actually see. This is a first-of-a-kind calculation where we actually carry out all the pieces together. We start with the equations we expect the system to follow, and we solve those full equations on a supercomputer. That gives us the data with which we can then make the predictions of the X-ray spectrum," Scott Noble, of RIT's Center for Computational Relativity and Gravitation, said.

Research surrounding the study was published in the Astrophysical Journal.

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