June 14, 2013
Scientists Solve Mystery Of How Black Holes Produce Hard X-rays
Lee Rannals for redOrbit.com - Your Universe Online
Astronomers from several institutions have confirmed theories about how stellar-mass black holes produce their highest-energy light known as hard X-rays.Researchers from NASA, Johns Hopkins University and the Rochester Institute of Technology wrote in the Astrophysical Journal about how they were able to use supercomputer simulations of gas flowing into a back hole to reproduce a range of important X-ray features found in active black holes. Their study bridges the gap between theory and observation, showing both hard and soft X-rays arise from gas spiraling towards a black hole.
"Our work traces the complex motions, particle interactions and turbulent magnetic fields in billion-degree gas on the threshold of a black hole, one of the most extreme physical environments in the universe," said lead researcher Jeremy Schnittman, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Before gas is consumed by a black hole, it initially orbits around it and then accumulates into a flattened disk. The gas stored in this disk gradually spirals towards the black hole and becomes compressed and heated as it nears the center. Eventually the gas reaches temperatures up to 20 million degrees Fahrenheit, making it shine brightly in low-energy or soft X-rays.
Observations have also shown black holes produce considerable amounts of "hard" X-rays, which is light with energy tens to hundred of times greater than that of soft X-rays. This high-energy light implies the presence of hotter gas. Their study was able to use models that match the hard evidence collected through observation, which is important to confirming theories.
“This is very encouraging because it says we actually understand what´s going on. If we made all the correct steps and we saw a totally different answer, we´d have to rethink what our model is," said Scott Noble, associate research scientist in RITs Center for Computational Relativity and Gravitation.
The researchers' findings confirm hard X-rays originate in a hot, tenuous corona above the disk, which is a structure similar to the hot corona that surrounds the Sun.
“Astronomers also expected that the disk supported strong magnetic fields and hoped that these fields might bubble up out of it, creating the corona,” Noble says. “But no one knew for sure if this really happened and, if it did, whether the X-rays produced would match what we observe.”
They were able to develop tools to track how X-rays were emitted, absorbed and scattered throughout both the accretion disk and the corona region. The team demonstrated a direct connection between magnetic turbulence in the disk, the formation of a billion-degree corona, and the production of hard X-rays around an actively "feeding" black hole.
“Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions and gravity exhibiting the full weirdness of general relativity,” says Julian Krolik, a professor at Johns Hopkins. “But our calculations show we can understand a lot about them using only standard physics principles.”
He said the numerical simulations going on at this level of quality and resolution helps make the results credible.
“In some ways, we had to wait for technology to catch up with us,” Krolik said.
The black hole in the center of our galaxy, Sagittarius A, is getting ready to devour its own hot molecular gas soon. Scientists using the European Space Agency's (ESA) Herschel spacecraft were able look at the temperature of the gas cloud and determine that although it isn't emitting those high-energy X-rays yet, it could be soon as it continues speeding towards the center of Sagittarius A.