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Simulations Help Astronomers Understand The Secrets Of Merging Black Holes

Einstein predicted gravitational waves almost 100 years ago. According to his theory, whenever massive objects interact, they produce distortions in the very fabric of space and time that ripple outward across the universe at the speed of light.
 
Modern astronomers have found indirect evidence of these waves, but so far they have eluded direct detection. Greater sensitivities are just around the corner for ground-based observatories designed to find these waves, and many scientists think that this discovery is just a few years away.
 
It seems counterintuitive, but catching waves from some of the strongest sources, namely colliding black holes with millions of times the Sun’s mass, can’t be done with ground-based facilities. These waves undulate so slowly that scientists will need much larger space-based instruments, such as the proposed Laser Interferometer Space Antenna. The astronomical community has endorsed the Interferometer as a high-priority future project.
 
A research team that includes astrophysicists at NASA’s Goddard Space Flight Center is using computational models to explore the mergers of supersized black holes in anticipation of the Interferometer’s development. Most recently, their work investigates the type of “flash” that might be seen by telescopes when astronomers find gravitational signals from such an event.
 
Gravitational waves give astrophysicists an opportunity to witness the universe’s most extreme phenomena that is unprecedented. Studying these phenomena will lead to new insights about the fundamental laws of physics, the death of stars, the birth of black holes and perhaps even the earliest moments of the universe.
 
Black holes are so massive that nothing can escape their gravitational pull, not even light. Most large galaxies, including our own Milky Way, have a central black hole weighing millions of times the sun’s mass. When two galaxies collide, their monster black holes settle into a close binary system.
 
“The black holes orbit each other and lose orbital energy by emitting strong gravitational waves, and this causes their orbits to shrink. The black holes spiral toward each other and eventually merge,” said Goddard astrophysicist John Baker.
 
Space and time become repeatedly flexed and warped in proximity to these gigantic, rapidly moving masses. Like the ripples formed on a pond when you throw a stone, the cyclic flexing of space-time near binary black holes produces waves of distortion that race across the universe.
 
Gravitational waves promise to teach us an enormous amount about how the universe works, but they cannot provide one crucial bit of information – the precise location of the source of the waves. 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.
 
Understanding these electromagnetic events that might accompany a merger involves the task of tracking the complex interactions between black holes and the disks of hot, magnetized gas that surround them. Since they can be moving at more than half the speed of light in the last few orbits, this is a daunting task. Since 2010, many studies using simplified assumptions have found that mergers could produce a burst of light, but still no one knew how commonly this occurs or whether the emission would be strong enough to be detectable from Earth.
 
Scientists from the University of Colorado, Boulder, teamed up with Baker to develop computer simulations that show what happens in the plasma in last stages of a black hole merger. The results of this study were recently published in the June 10 issue of the Astrophysical Journal Letters.
 
The simulations require the use of advanced numerical codes and fast supercomputers to follow the complex electrical and magnetic interactions in the ionized gas – known as magnetohydrodynamics – within the extreme gravitational environment determined by the equations of Einstein’s general relativity.
 
The study used two simulations run on the Pleiades supercomputer at NASA’s Ames Research Center where they followed the black holes over their last three orbits and subsequent merger using models both with and without a magnetic field in the gas disk.
 
Additional simulations were run on the Ranger supercomputer at the University of Texas at Austin and the Discover supercomputer at the NASA Center for Climate Simulation in order to investigate the effects of different initial conditions, fewer orbits and other variations.
 
“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 unmagnetized case,” Bruno Giacomazzo from the University of Colorado explained.
 
The magnetic field intensifies as it becomes twisted and compressed near the merging black holes. According to the team, running the simulation for additional orbits would result in even greater amplification.
 
The most interesting outcome of the magnetic simulations is the development of a funnel-like structure. It seems to be 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.
 
The brightness of the merger’s flash is the most important aspect of the study. The magnetic model produces beamed emission that is some 10,000 times brighter than those seen in previous studies which ignored the plasma effects in the merging disks.
 
“We need gravitational waves to confirm that a black hole merger has occurred, but if we can understand the electromagnetic signatures from mergers well enough, perhaps we can search for candidate events even before we have a space-based gravitational wave observatory,” Baker said.
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