Astronomers have identified 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.
Black Hole Caught Red-handed in a Stellar Homicide. [Credit: NASA; S. Gezari (The Johns Hopkins University); and J. Guillochon (University of California, Santa Cruz)]
Somewhere in the cosmos an ordinary galaxy spins, seemingly at slumber. Then all of a sudden, WHAM! A flash of light explodes from the galaxy’s center. A star orbiting too close to the event horizon of the galaxy’s central supermassive black hole is torn apart by the force of gravity, heating up its gas and sending out a beacon to the far reaches of the universe.
In a universe with tens of billions of galaxies, how would we see it? What would such a beacon look like? How would we distinguish it from other bright, monumental intergalactic events, like supernovas?
“Black holes by themselves do not emit light,” said Tamara Bogdanovic, Assistant Professor of Physics at the Georgia Institute of Technology. “Our best chance to discover them in distant galaxies is if they interact with stars and gas that are around them.”
In recent decades, with improved telescopes and observational techniques designed to repeatedly survey the vast numbers of galaxies on the sky, scientists noticed that some galaxies that previously looked inactive would suddenly light up at their very center.
“This flare of light was found to have a characteristic behavior as a function of time,” Bogdanovic explained. “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 relies on National Science Foundation-funded supercomputers like Stampede at the Texas Advanced Computing Center and Kraken at the National Institute for Computational Sciences. Using these systems, she and her collaborators recently simulated the dynamics of these super powerful forces and charted their behavior using numerical models. Stampede and Kraken are part of the Extreme Science and Engineering Discovery Environment (XSEDE), a single virtual system that scientists use to interactively share computing resources, data and expertise.
Using a mix of theoretical and computational approaches, Bogdanovic tries to predict the observational signatures of events like the black-hole-devouring-star scenario described above, also known as a “tidal disruption”— or two supermassive black holes merging, another of her interests. Such events would have a distinct signature to someone analyzing data from a ground-based or a space-based observatory.