SUPERCOMPUTERS PREDICT SIGNS OF BLACK HOLES CONSUMING STARS

Right:  Black Hole Caught in a Stellar Homicide.  This computer-simulated image shows gas from a star that is ripped apart by tidal forces as it falls into a black hole. Some of the gas also is being ejected at high speeds into space.  Image Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz).

FROM:  NATIONAL SCIENCE FOUNDATION 

Cosmic slurp

Georgia Tech researchers use supercomputers to understand and predict signs of black holes swallowing stars

April 14, 2014

Somewhere out 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 has been 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? And how would we distinguish it from other bright, monumental intergalactic events, such as supernovas?

"Black holes by themselves do not emit light," said Tamara Bogdanovic, an assistant professor of physics at the Georgia Institute of Technology. "Our best chance to discover them in distant galaxies is if they interact with the 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 in 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. It starts very bright and its luminosity then decreases in time in a particular way," she explained. "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 a mix of theoretical and computer-based approaches, Bogdanovic tries to predict the dynamics of events such as the black-hole-devouring-star scenario described above, also known as a "tidal disruption." Such events would have a distinct signature to someone analyzing data from a ground-based or space-based observatory.

Using National Science Foundation-funded supercomputers at the Texas Advanced Computing Center (Stampede) and the National Institute for Computational Sciences (Kraken), Bogdanovic and her collaborators recently simulated the dynamics of these super powerful forces and charted their behavior using numerical models.

Tidal disruptions are relatively rare cosmic occurrences. Astrophysicists have calculated that a Milky Way-like galaxy stages the disruption of a star only once in about 10,000 years. The luminous flare of light, on the other hand, can fade away in only a few years. Because it is such a challenge to pinpoint tidal disruptions in the sky, astronomical surveys that monitor vast numbers of galaxies simultaneously are crucial.

Huge difference

So far, only a few dozen of these characteristic flare signatures have been observed and deemed "candidates" for tidal disruptions. But with data from PanSTARRS, Galex, the Palomar Transient Factory and other upcoming astronomical surveys becoming available to scientists, Bogdanovic believes this situation will change dramatically.

"As opposed to a few dozen that have been found over the past 10 years, now imagine hundreds per year--that's a huge difference!" she said. "It means that we will be able to build a varied sample of stars of different types being disrupted by supermassive black holes."

With hundreds of such events to explore, 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 centers 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."

Tidal disruptions also tell us something about the population and properties of supermassive black holes that are doing the disrupting.

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

Role of supercomputer

In a recent paper submitted to the Astrophysical Journal, Bogdanovic, working with Roseanne Cheng (Center for Relativistic Astrophysics at Georgia Tech) and Pau Amaro-Seoane (Albert Einstein Institute in Potsdam, Germany), considered the tidal disruption of a red giant star by a supermassive black hole using computer modeling.

The paper comes on the heels of the discovery of a tidal disruption event in which a black hole disrupted a helium-rich stellar core, thought to be a remnant of a red giant star, named PS1-10jh, 2.7 billion light years from Earth.

The sequence of events they described aims to explain some unusual aspects of the observational signatures associated with this event, such as the absence of the hydrogen emission lines from the spectrum of PS1-10jh.

As a follow-up to this theoretical study, the team has been running simulations on Kraken and Stampede, as well as Georgia Tech's Keeneland supercomputer. The simulations reconstruct the chain of events by which a stellar core, similar to the remnant of a tidally disrupted red giant star, might evolve under the gravitational tides of a massive black hole.

"Calculating the messy interplay between hydrodynamics and gravity is feasible on a human timescale only with a supercomputer," Cheng said. "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."

The research shows how supercomputer simulations complement and constrain theory and observation.

"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," Bogdanovic said.

One of Bogdanovic's goals is to use the knowledge gained from simulations to decode the signatures of observed tidal disruption events.

"The most recent data on tidal disruption events is already outpacing theoretical understanding and calling for the development of a new generation of models," she explained. "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 have yet to understand what causes these differences in observational appearance, and computer simulations are guaranteed to be an important part of this journey."

Investigators
Roseanne Cheng
Pau Amaro-Seoane
Tamara Bogdanovic

SUPERCOMPUTER SIMULATIONS RECREATE X-RAYS FROM AREA OF A BLACK HOLE

FROM:  NATIONAL SCIENCE FOUNDATION 
Let there be light
Simulations on NSF-supported supercomputer re-create X-rays emerging from the neighborhood of black holes
February 18, 2014

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.

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.

"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."

The simulations are the combined results of two computational codes. One, Harm3d, re-creates the three-dimensional dynamics of a black hole accreting gas, including its magnetohydrodynamics (MHD), which charts the interplay of electrically conducting fluids like plasmas and a powerful magnetic field.

"The magnetic field is important in the area outside the black hole because it whips the gas around and can dictate its dynamics," Noble said. "Also, the movement of the field can lead to it kinking and trigger a reconnection event that produces an explosive burst of energy, turning magnetic field energy into heat."

Though the MHD forces are critical near the black hole, it is the X-rays these forces generate that can be observed. The second component, a radiative transport code called Pandurata, simulates what real photons do.

"They bounce around inside the gas, they reflect off the disk's surface, and their wavelengths change along the way," he explained. "Eventually, they reach some distant light collector--a numerically approximated observer--which provides the predicted light output of our simulation."

The researchers' simulations were run on the Ranger supercomputer at the Texas Advanced Computing Center, built with support from the National Science Foundation, which also funded the group's research.

The simulations were the highest resolution thin disk simulations ever performed, with the most points and the smallest length-scales for numerical cells, allowing the researchers to resolve very small features. Varying only the rate at which the black holes accrete gas, they were able to reproduce the wide range of X-ray states seen in observations of most galactic black hole sources.

With each passing year, the significance of black holes--and their role in shaping the cosmos--grows.

Nearly every good-sized galaxy has a supermassive black hole at its center, said Julian Krolik, a professor of physics and astronomy at Johns Hopkins University. For periods of a few to tens of million years at a time, black holes accrete incredible amounts of gas ultimately released as huge amounts of energy--as much as a hundred times the power output of all the stars in a black hole host galaxy put together.

"Some of that energy can travel out into their surrounding galaxies as ionizing light or fast-moving jets of ionized gas," Krolik continued. "As a result, so much heat can be deposited in the gas orbiting around in those galaxies that it dramatically alters the way they make new stars. It's widely thought that processes like this are largely responsible for regulating how many stars big galaxies hold."

In this way black holes may act as cosmic regulators--all the more reason to use numerical simulations to uncover further clues about how black holes interact with gas, stars and other supermassive black holes.

Said Noble: "To see that it works and reproduces the observational data when the observational data is so complicated...it's really remarkable."

-- Aaron Dubrow, NSF
Investigators
Scott Noble
Julian Krolik
Jeremy Schnittman
John Boisseau
Karl Schulz
Omar Ghattas
Tommy Minyard
Yosef Zlochower
Manuela Campanelli
Related Institutions/Organizations
Rochester Institute of Tech
Johns Hopkins University
Goddard Space Flight Center
University of Texas at Austin