Scientests may have identified and observed evidence a new type of celestial body: a binary black hole. Suvi Gezari, an assistant professor of astronomy at the University of Maryland and a co-author of a new study in the April 14, 2015, in the Astrophysical Journal Letters entitled A PERIODICALLY VARYING LUMINOUS QUASAR AT z = 2 FROM THE PAN-STARRS1 MEDIUM DEEP SURVEY: A CANDIDATE SUPERMASSIVE BLACK HOLE BINARY IN THE GRAVITATIONAL WAVE-DRIVEN REGIME says, "We believe we have observed two supermassive black holes in closer proximity than ever before. ... 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."
How does this sort of phenomenon occur? First, some important facts: in case you didn't know, scientists are fairly certain that most galaxies have super-massive black holes at the center of them. And sometimes, over the course of billions of years, some galaxies merge. In the final stages of these mergers, scientists have theorized that the black holes at the core of each galaxy come into such close proximity that they begin to orbit one another to form a "binary" black hole.
What does this mean? Usually, as black holes draw in and swallow matter, the matter heats up, causing it to emit electromagnetic energy and intense light. These phenomena are called quasars. Gezari and her team's insight was that when two black holes orbit one another as a binary, instead of absorbing matter at a constant rate, the way a single black hole would, they should absorb it cyclically, as they orbit one another. This should cause them to brighten and dim in a regular pattern. Seeking this phenomenon, Gezari and her team used the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) Medium Deep Survey to conduct a systematic search for these variable quasars. Over the course of four years, the team observed the same patch of sky once every three days and collected hundreds of data points for each object. And finally, they found what they were looking for: quasar PSO J334.2028+01.4075. It is a black hole of almost 10 billion solar masses which, unlike most quasars, emits a regular optical signal that repeats every 542 days. This is unusual, as most quasars give off an irregular signal.
Once they found this one quasar with a regular signal, the team decided to look for more. They examined additional data, including photometric data from the Catalina Real-Time Transient Survey and spectroscopic data from the FIRST Bright Quasar Survey. "The discovery of a compact binary candidate supermassive black hole system like PSO J334.2028+01.4075," says Tingting Liu, a UMD astronomy graduate student, and the paper's first author, "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."
Now that the team has found such tantalizing data, they plan to continue searching for new quasars with regular emissions. Starting in 2023, they may get access to the Large Synoptic Survey Telescope. This would allow the team to survey a much larger area, potentially locating thousands of merging supermassive black holes. "These telescopes allow us to watch a movie of how these systems evolve," Liu says. "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."