Computer Simulation Sheds Light on Supermassive Black Hole Mergers

For the first time, a new computer simulation that incorporates the physical effects of Einstein’s general theory of relativity shows that gas in supermassive binary black hole systems approaching merger will glow predominantly in UV and X-ray light. An analysis of the new simulation appears in the Astrophysical Journal.

Just about every galaxy the size of our own Milky Way Galaxy or larger contains a supermassive black hole at its center. Observations show galaxy mergers occur frequently in the Universe, but so far no one has seen a merger of these giant black holes.

“We know galaxies with central supermassive black holes combine all the time in the Universe, yet we only see a small fraction of galaxies with two of them near their centers,” said study co-author Dr. Scott Noble, an astrophysicist at NASA’s Goddard Space Flight Center.

NSF’s Laser Interferometer Gravitational-Wave Observatory (LIGO) recently detected merging stellar-mass black holes. Supermassive mergers will be much more difficult to find than their smaller cousins.

“Supermassive binaries result from galaxy mergers,” the researchers said.

“Each supermassive black hole brings along an entourage of gas and dust clouds, stars and planets.”

“We think a galaxy collision propels much of this material toward the central black holes, which consume it on a time scale similar to that needed for the binary to merge.”

“As the black holes near, magnetic and gravitational forces heat the remaining gas, producing light astronomers should be able to see.”

“It’s very important to proceed on two tracks,” said study co-author Dr. Manuela Campanelli, from the Rochester Institute of Technology.

“Modeling these events requires sophisticated computational tools that include all the physical effects produced by two supermassive black holes orbiting each other at a fraction of the speed of light.”

“Knowing what light signals to expect from these events will help modern observations identify them.”

“Modeling and observations will then feed into each other, helping us better understand what is happening at the hearts of most galaxies.”

The team’s computer simulation shows three orbits of a pair of supermassive black holes only 40 orbits from merging.

The models reveal the light emitted at this stage of the process may be dominated by UV light with some high-energy X-rays, similar to what’s seen in any galaxy with a well-fed supermassive black hole.

Three regions of light-emitting gas glow as the black holes merge, all connected by streams of hot gas: a large ring encircling the entire system, called the circumbinary disk, and two smaller ones around each black hole, called mini disks. All these objects emit predominantly UV light.

When gas flows into a mini disk at a high rate, the disk’s UV light interacts with each black hole’s corona, a region of high-energy subatomic particles above and below the disk. This interaction produces X-rays.

When the accretion rate is lower, UV light dims relative to the X-rays.

Based on the simulation, the scientists expect X-rays emitted by a near-merger will be brighter and more variable than X-rays seen from single supermassive black holes.

The pace of the changes links to both the orbital speed of gas located at the inner edge of the circumbinary disk as well as that of the merging black holes.

“The way both black holes deflect light gives rise to complex lensing effects, as seen in the movie when one black hole passes in front of the other,” said study lead author Stéphane d’Ascoli, a doctoral student at École Normale Supérieure, France.

“Some exotic features came as a surprise, such as the eyebrow-shaped shadows one black hole occasionally creates near the horizon of the other.”

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Stéphane d’Ascoli et al. 2018. Electromagnetic Emission from Supermassive Binary Black Holes Approaching Merger. ApJ 865, 140; doi: 10.3847/1538-4357/aad8b4