article image

Supercomputers reveal how black holes feast on stars

Jenny Winder, News Writer
Apr 16, 2014, 17:00 UTC

Sen—A team of researchers from  Georgia Institute of Technology are using supercomputers to understand and predict how black holes swallow stars.

Scientists have noticed that some galaxies that previously looked inactive would suddenly light up as 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. The clash heats up the star's gas, sending out a beacon to the far reaches of the universe.

"This flare of light was found to have a characteristic behaviour as a function of time," Tamara Bogdanovic, Assistant Professor of Physics at the Georgia Institute of Technology, 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'."

Using 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.

Astrophysicists have calculated that a Milky Way-like galaxy stages the disruption of a star once in about 10,000 years, while the luminous flare of light can fade in only a few years. This difference in timescale highlights the observational challenge in pinpointing such events in the sky.

With data from PanSTARRS, Galex, the Palomar Transient Factory and other upcoming astronomical surveys becoming available to scientists, Bogdanovic believes this scarcity will change dramatically. Astrophysicists' understanding of black holes and the stars around them would advance by leaps and bounds, helping determine some key aspects of galactic physics.

"A diversity in the type of disrupted stars tells us something about the makeup of the star clusters in the centres of galaxies," Bodganovic said. "It may give us an idea about how many main sequence stars, how many red giants, or white dwarf stars are there on average.

"We use these observations as a window of opportunity to learn important things about the black holes and their host galaxies. Once the tidal disruption flare dims below some threshold luminosity that can be seen in observations, the window closes for that particular galaxy."

Bogdanovic added: "There are many situations in astrophysics where we cannot get insight into a sequence of events that played out without simulations. We cannot stand next to the black hole and look at how it accretes gas. So we use simulations to learn about these distant and extreme environments.

"The new, better quality data indicates that there is a great diversity among the tidal disruption candidates. This is contrary to our perception, based on earlier epochs of observation, that they are a relatively uniform class of events. We are yet to understand what causes these differences in observational appearance and computer simulations are guaranteed to be an important part of this journey."