Repeating Light Signal May Be Two Supermassive Black Holes Set To Collide as Galaxies Merge

Photo credit: An artist's conception of a black hole binary in a heart of a quasar, with the data showing the periodic variability superposed / Santiago Lombeyda/Caltech Center for Data-Driven Discovery

Excerpt from 

An unusual, repeating light signal in the distance may be coming from the final stages of a merger between two supermassive black holes. At just a few hundredths of a light-year apart, they could be merging in a mere one million years. An event like this has been predicted based on theory, but has never been observed before, according to a new study published in Nature this week. 

The supermassive black holes at the center of most large galaxies (including ours) appear to co-evolve with their host galaxies: As galaxies merge, their black holes grow more massive too. Since we can’t actually see black holes, researchers look for their surrounding bands of material called accretion disks, which are produced by the intense pull of the black hole’s gravity. The disks of supermassive black holes can release vast amounts of heat, X-rays, and gamma rays that result in a quasar—one of the most luminous objects in the universe.  

Caltech’s Matthew Graham and colleagues noticed the light signal coming from quasar PG 1302-102 while studying variability in quasar brightness using data from the Catalina Real-Time Transient Survey, which continuously monitored 500 million celestial light sources across 80 percent of the sky with three ground telescopes. 

The team noticed 20 quasars emitting periodic optical signals, which was unexpected since the light curves of quasars are usually chaotic. (That’s because material from the accretion disk spiral randomly into the black hole.) And of these, PG 1302-102’s clean, strong signal, which repeated every five years or so, stood out. "It has a really nice smooth up-and-down signal, similar to a sine wave, and that just hasn't been seen before in a quasar," Graham explains in a news release. (See illustration above.)

Quasars typically have one emission line that’s viewed as a symmetric curve. "But with this quasar, it was necessary to add a second emission line with a slightly different speed than the first one in order to fit the data," says study co-author Eilat Glikman of Middlebury College. "That suggests something else, such as a second black hole, is perturbing this system." A supermassive black hole binary was the most likely explanation: Any object that’s less dense than a secondary black hole would be disrupted by the gravity of the primary black hole. 

"The end stages of the merger of these supermassive black hole systems are very poorly understood," Graham says. "The discovery of a system that seems to be at this late stage of its evolution means we now have an observational handle on what is going on." 

Study co-author Daniel Stern of JPL adds: "The black holes in PG 1302-102 are, at most, a few hundredths of a light-year apart and could merge in about a million years or less.” And when that happens, The New York Times reports, it’ll release as much energy as 100 million supernova explosions.