Most galaxies have a supermassive black hole at their center and some of these black holes are actively accreting, a process that releases a stupendous amount of energy into the space between galaxies. It's thought that a small subsection of these galaxies has not just one but two active supermassive black holes, and researchers think they have found the telltale signs to identify these special galaxies.
Most of the energy released by the supermassive black holes is in the form of X-rays and gamma-rays. The key to detecting these binary black hole galaxies appears to be in the periodicity of gamma-rays. The team suggests one of the reasons for the periodicity is two black holes rotating around each other. As reported in The Astrophysical Journal, they discovered 11 systems out of more than 2,000 observed galaxies that show a repetition of gamma-ray signals. On average, the emissions repeated every two years.
"Identifying regular patterns in their gamma-ray emission is like looking at the stormy sea and searching for the tiny regular set of waves caused by, say, the passage of a small boat," lead author Pablo Peñil, graduate researcher at Universidad Complutense de Madrid in Spain, said in a statement. "It becomes very challenging very quickly."
Supermassive black hole binaries are believed to be a common feature across the universe, although currently, they are still theoretical. Galaxies merge with each other across their lifetimes, which is a process that is both spectacular and extremely long, in the order of billions of years. Our own galaxy, the Milky Way, is set to collide with the Andromeda galaxy in about 4.5 billion years' time.
For most of the duration of the merger, the black holes are thousands of light-years away, but when they get close enough they can begin to influence each other in a more concrete way. Periodicity in gamma-ray emissions could be the result of such proximity interactions, according to the team. The analysis comes from nine years of data from the Large Area Telescope (LAT) and NASA’s Fermi space observatory.
"Previously only two blazars were known to show periodic changes in their gamma-ray brightness. Thanks to our study, we can confidently say that this behavior is present in 11 other sources," added co-author Sara Buson, a professor at the University of Würzburg in Germany. "In addition, our study found 13 other galaxies with hints of cyclical emission. But to confidently confirm this, we need to wait for Fermi-LAT to collect even more data."
The team is now planning an observation campaign to better understand if its truly black hole binaries behind these signals.
“The next step will be the preparation of observational campaigns with other telescopes to closely follow up on these galaxies and hopefully unravel the reasons behind these compelling observations,” said co-author Marco Ajello, an associate professor in the College of Science’s department of physics and astronomy at Clemson University. “We have a few possibilities in mind – from lighthouse effects produced by the jets to modulations in the flow of matter to the black hole – but one very interesting solution would be that periodicity is produced by a pair of supermassive black holes rotating around each other. Understanding the relation of these black holes with their environment will be essential for a complete picture of galaxy formation.”