Simulations uncover “flashy” secrets of merging black holes

According to Einstein, whenever massive objects interact, they produce gravitational waves -- distortions in the very fabric of space and time -- that ripple outward across the universe at the speed of light.

While astronomers have found indirect evidence of these disturbances, the waves have so far eluded direct detection. Ground-based observatories designed to find them are on the verge of achieving greater sensitivities, and many scientists think that this discovery is just a few years away.

Catching gravitational waves from some of the strongest sources -- colliding black holes with millions of times the Sun’s mass -- will take a little longer. These waves undulate so slowly that they won’t be detectable by ground-based facilities.

A team led by University of Colorado Boulder researcher Bruno Giacomazzo that includes astrophysicists at NASA’s Goddard Space Flight Center in Greenbelt, Md., is using computational models to explore the mergers of supersized black holes. Their most recent work investigates what kind of “flash” might be seen by telescopes when astronomers ultimately find gravitational signals from such an event.

Studying gravitational waves will give astrophysicists an unprecedented opportunity to witness the universe’s most extreme phenomena, leading to new insights into the fundamental laws of physics, the death of stars, the birth of black holes and perhaps the earliest moments of the universe, said Giacomazzo, a research associate at JILA, a joint institute of CU-Boulder and the National Institute of Standards and Technology.

A black hole is an object so massive that nothing, not even light, can escape its gravitational grip. Most big galaxies, including our own Milky Way, contain a central black hole weighing millions of times the Sun’s mass, and when two galaxies collide, their monster black holes settle into a close binary system.

Close to these titanic, rapidly moving masses, space and time become repeatedly flexed and warped. Just as a disturbance forms ripples on the surface of a pond, drives seismic waves through Earth, or puts the jiggle in a bowl of Jell-O, the cyclic flexing of space-time near binary black holes produces waves of distortion that race across the universe.

While gravitational waves promise to tell astronomers many things about the bodies that created them, they cannot provide one crucial piece of information -- the precise position of the source. So to really understand a merger event, researchers need an accompanying electromagnetic signal -- a flash of light, ranging from radio waves to X-rays -- that will allow telescopes to pinpoint the merger’s host galaxy, said Giacomazzo.

Since 2010, numerous studies indicate that mergers could produce a burst of light, but no one knew how commonly this occurred or whether the emission would be strong enough to be detectable from Earth.

To explore the problem in greater detail, Giacomazzo and his colleagues developed computer simulations that show what happens in the magnetized gas in the last stages of a black hole merger. “The black holes orbit each other and lose orbital energy by emitting strong gravitational waves, and this causes their orbits to shrink,” said Goddard astrophysicist John Baker. “The black holes spiral toward each other and eventually merge.”

Both of the simulations reported in the study were run on the Pleiades supercomputer at NASA’s Ames Research Center in Moffett Field, Calif. “What’s striking in the magnetic simulation is that the disk’s initial magnetic field is rapidly intensified by about 100 times, and the merged black hole is surrounded by a hotter, denser, thinner accretion disk than in the un-magnetized case,” said Giacomazzo.

The most interesting outcome of the magnetic simulation is the development of a funnel-like structure -- a cleared-out zone that extends up out of the accretion disk near the merged black hole. “This is exactly the type of structure needed to drive the particle jets we see from the centers of black-hole-powered active galaxies,” Giacomazzo said.


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