Ever since Einstein first published his theories of relativity, astronomers have come up with various ways of validating their predictions. They have, for the most part, been successful. However, there are still a few loose ends—perhaps the most famous of which is that of the gravitational wave. In the simplest possible terms, the theory of general relativity predicts that when two massive, dense objects come within close proximity to each other, they unleash ripples in the fabric of spacetime that propagate outward from the source.
There are a few combinations that could theoretically unleash gravity waves, but the most practical deals with black holes, specifically black holes that wander so close, they essentially become bound together by gravity—forming a black hole binary system. However, configurations are believed to be rare.
Now, researchers from the University of Maryland have published a study that points to the existence of a pulsating quasar. This, to them, amounts to the very first true confirmation of black hole binary systems (which may not be entirely accurate). At the least,
"We believe we have observed two supermassive black holes in closer proximity than ever before," said Suvi Gezari, assistant professor of astronomy at the University of Maryland and a co-author of the study. "This pair of black holes may be so close together that they are emitting gravitational waves, which were predicted by Einstein's theory of general relativity."
Firstly, all quasars are believed to be black holes by nature (but it should be noted that not all black holes are quasars). When supermassive black holes—typically those found within the galactic nuclei of young, gas and dust-filled galaxies—have more infalling matter than they can consume, rather than falling into the event horizon, the material accumulates into something called accretion disks. Friction and tidal stresses see the material heated up to such high temperatures, the quasars themselves outshine their host galaxies. However, given just how much material a black hole needs to transform into a quasar, they have only been found in very distant galaxies, which formed when the universe was only a fraction of its current age.
Then, when a second supermassive black hole—or quasar, in this case— is involved, it's predicted that, because of how these systems absorb energy, its presence can be deduced by looking for periodic variations in the apparent luminosity of its companion.
Working with this hunch, the researchers scoured data from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) Medium Deep Survey, looking for telltale signs of variable quasars. Their work involved tediously monitoring one section of the sky every three days over the course of 4 years. Eventually, they happened upon a quasar now called PSO J334.2028+01.4075.
PSO J334.2028+01.4075 (we'll call it PSO for short) boasts a central supermassive black hole with the mass of 10 billion Suns. Over the course of their observations, they found that its optical luminosity changed every 542 days, whereas the light curves of most quasars change at random.
After running numerous simulations and calculations, they looked at its spectral signature as recorded by the FIRST Bright Quasar Survey (FBQS), and the photometric data from the Catalina Real-Time Transient Survey (a three-telescope tool used to make note of fluctuations in an object's electromagnetic radiation emission). All of which support the assertion that a nearby companion is to blame for the systematic brightening and dimming of PSO.
Of course, more research needs to be carried out. Unfortunately, we might have to wait until the Large Synoptic Survey Telescope goes live in 2023 to say definitively. It may also aid in the discovery of other variable quasars, and binary black hole systems.
"The discovery of a compact binary candidate supermassive black hole system like PSO J334.2028+01.4075, which appears to be at such close orbital separation, adds to our limited knowledge of the end stages of the merger between supermassive black holes," said Tingting Liu, lead author.
"These telescopes allow us to watch a movie of how these systems evolve." Additionally,"What's really cool is that we may be able to watch the orbital separation of these supermassive black holes get smaller and smaller until they merge," he finished.
Their research was published on April 14, 2015, in an online edition of the Astrophysical Journal Letters,