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Mystery solved: X-ray light emitted from black holes

Exactly how do black holes produce so many high-power X-rays? The answer has remained a mystery to scientists for decades – until now. Supported by 40 years of theoretical progress, astrophysicists have conducted research that finally bridges the gap between theory and observation, demonstrating that gas spiraling toward a black hole inevitably results in X-ray emissions.

Published in May in The Astrophysical Journal, the study reveals that gas spiraling toward a black hole through an accretion disk (formed by material in orbit, typically around a star) heats up to roughly 10 million degrees Celsius. The main body of the disk is roughly 2,000 times hotter than the sun, and emits low-energy or “soft” X-rays. However, observations also detect “hard” X-rays, which produce up to 100 times higher energy levels. The collaborators showed for the first time that high-energy light emission is an inevitable outcome of gas being drawn into a black hole.

Simulated stellar-mass black hole. Video courtesy NASA Goddard Space Flight Center.

As the quality and quantity of high-energy light observations improved over the years, increasing evidence showed that photons are created in a hot, tenuous region called the corona. This corona, boiling violently above the comparatively cool accretion disk, is similar to the corona surrounding the sun, which is responsible for much of the ultra-violet and X-ray luminosity seen in the solar spectrum.

Magnetohydrodynamics (MHD) – the study of the dynamics of electrically conducting fluids – simulations have steadily improved in resolution and physical accuracy, helping scientists comprehend accreting black holes. However, while it is widely accepted that “hard” X-rays come from the inverse Compton scattering of seed photons from the disk through a hot corona, scientists still lack details on the corona itself.

“Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions and gravity exhibiting the full weirdness of general relativity,” Krolik says. “But our calculations show we can understand a lot about them using only standard physics principles.”

Advances in numerical simulation methods now enable an approach founded directly on disk dynamics – a critical step in bridging the gap between theory and observation. Researchers have shown that MHD turbulence in an accretion disk can lead to dissipation outside the disk's photosphere strong enough to power “hard” X-rays emissions. The emissions are comparable in luminosity to the disk's thermal luminosity.

Collaborators on the study include Julian Krolik, professor of physics and astronomy at Johns Hopkins University in Maryland, US, Jeremy Schnittman, lead author and research astrophysicist at the NASA Goddard Space Flight Center in Maryland, US, and Scott Noble, an associate research scientist at the Center for Computational Relativity and Gravitation at Rochester Institute of Technology in New York, US.

Advanced technology's role in these discoveries should not be overlooked. Ranger, a supercomputer at the Texas Advanced Computing Center (TACC) at The University of Texas, US, worked over the course of about 27 days – over 600 hours – to solve the equations. Noble developed the computer simulation that solved all of the equations governing the complex motion of inflowing gas and its associated magnetic fields near an accreting black hole. Stampede, sixteen times more powerful and the newest supercomputer at TACC, has since taken over for Ranger.

The rising temperature, density, and speed of the inflowing gas dramatically amplify magnetic fields threading through the disk, which then exert additional influence on the gas. The result is a turbulent froth, which orbits the black hole at speeds approaching the speed of light. The calculations simultaneously track the fluid, electrical, and magnetic properties of the gas – while also taking into account Einstein's theory of relativity.

“In some ways, we had to wait for technology to catch up with us,” Krolik said. “It's the numerical simulations going on at this level of quality and resolution that make the results credible.”

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