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April 16, 2014

Supercomputers Help Capture Rare Black Hole Events

Tiffany Trader
A galaxy about 2.7 billion light years from Earth with a supermassive black hole at its center.

When an orbiting star gets too close to a galaxy’s central supermassive black hole, it eventually gets torn apart by the immense gravitational forces, a phenomenon known as a “tidal disruption.” Although black holes cannot be seen directly, since their dense mass means that not even light can escape, the inhaled star produces a brief burst of light. It’s difficult, however, to distinguish these very rare events from other bright light events, like supernovas.

Now with the help of comprehensive sky surveys and powerful supercomputers, astrophysicists are closer to understanding and even predicting this amazing phenomenon. Researchers from Georgia Institute of Technology and the Max Planck Institute in Germany are using a mix of computational and theoretical models to describe the dynamics of black hole events, such as tidal disruptions or the merging of two supermassive black holes.

A major part of the challenge of finding a star-sucking black hole is knowing where to look in the vast universe with its billions of galaxies. Through a multitude of astronomical surveys performed over the years, a more precise picture of the universe is emerging.

A turning point came when astrophysicists noticed that some galaxies thought to be inactive would suddenly light up at the center.

“This flare of light was found to have a characteristic behavior as a function of time,” said Tamara Bogdanovic, Assistant Professor of Physics at the Georgia Institute of Technology, a lead researcher on the study. “It starts very bright and its luminosity then decreases in time in a particular way. Astronomers have identified those as galaxies where a central black hole just disrupted and ‘ate’ a star. It’s like a black hole putting up a sign that says: ‘Here I am.’”

Bogdanovic and her colleagues employ a range of techniques – including observational, theoretical and computational methods – to understand these distant events. Simulations are especially valuable to decoding the signatures of tidal disruptions, explains Bogdanovic. Where before it was thought that this was a fairly uniform class of events, new evidence and better data suggest a diversity in candidate appearance.

The computational work for the project has been carried out on National Science Foundation-funded supercomputers like Stampede at the Texas Advanced Computing Center and Kraken at the National Institute for Computational Sciences – both part of the XSEDE environment. Georgia Tech’s Keeneland supercomputer was also involved.

Scientists know that tidal disruptions are rare cosmic occurrences – calculations show that galaxies like the Milky Way would experience a disruption like this once every 10,000 years. The telltale light flare, however, lasts only a few years. Both the rarity of the event and the discrepancy in timescale underscore the challenge of spotting a tidal disruption, and illustrate the need for further multi-galaxy sky surveys.

“Calculating the messy interplay between hydrodynamics and gravity is feasible on a human timescale only with a supercomputer. Because we have control over this virtual experiment and can repeat it, fast forward, or rewind as needed, we can examine the tidal disruption process from many perspectives. This in turn allows us to determine and quantify the most important physical processes at play,” says Roseanne Cheng of the Center for Relativistic Astrophysics at Georgia Tech, who coauthored a paper on black holes with Bogdanovic.

Astrophysicists are on the verge of a breakthrough in better techniques, of which supercomputing is a major part, that will lead to the ability to verify more bright light events as tidal disruptions. Currently, only a few dozen flares have been singled out as potential black hole event candidates. As more data from astronomical surveys flow in, cases are expected to rise sharply, perhaps to hundreds per year.

“[The] huge difference…means that we will be able to build a varied sample of stars of different types being disrupted by supermassive black holes,” states Bogdanovic.