Tuesday, February 18, 2014

Simulations re-create X-rays emerging from the neighbourhood of black holes

Black holes may be dark, but the areas around them definitely are not. These dense, spinning behemoths twist up gas and matter just outside their event horizon, and generate heat and energy that gets radiated, in part, as light and when black holes merge, they produce a bright intergalactic burst that may act as a beacon for their collision.

Karl Schwarzschild
Astrophysicists became deeply interested in black holes in the 1960s, but the idea of their event horizon was first intimated in a paper by Karl Schwarzschild published after Einstein introduced general relativity in 1915.

Knowledge about black holes—these still-unseen objects—has grown tremendously in recent years.

Part of this growth comes from researchers' ability to use detailed numerical models and powerful supercomputers to simulate the complex dynamics near a black hole.

This is no trivial matter. Warped spacetime, gas pressure, ionizing radiation, magnetized plasma—the list of phenomena that must be included in an accurate simulation goes on and on.

Scott Noble
"It's not something that you want to do with a paper and pencil," said Scott Noble, an astrophysicist at the Rochester Institute of Technology (RIT).

Working with Jeremy Schnittman of Goddard Space Flight Center and Julian Krolik of Johns Hopkins University, Noble and his colleagues created a new tool that predicts the light that an accreting black hole would produce.

They did so by modeling how photons hit gas particles in the disk around the black hole (also known as an accretion disk), generating light—specifically light in the X-ray spectrum—and producing signals detected with today's most powerful telescopes.

In their June 2013 paper in the Astrophysical Journal, the researchers presented the results of a new global radiation transport code coupled to a relativistic simulation of an accreting, non-rotating black hole.

For the first time, they were able to re-create and explain nearly all the components seen in the X-ray spectra of stellar-mass black holes.

The ability to generate realistic light signals from a black hole simulation is a first and brings with it the possibility of explaining a whole host of observations taken with multiple X-ray satellites during the past 40 years.

"We felt excited and also incredibly lucky, like we'd turned up ten heads in a row," Noble said. "The simulations are very challenging and if you don't get it just right, it won't give you an accurate answer."

"This was the first time that people have put all of the pieces together from first principles in such a thorough way."

More information: "X-Ray Spectra from Magnetohydrodynamic Simulations of Accreting Black Holes." Jeremy D. Schnittman, Julian H. Krolik, Scott C. Noble. 2013 ApJ 769 156. DOI: 10.1088/0004-637X/769/2/156

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