Astronomers investigating gamma ray emissions have discovered that certain active galaxies seem to be giving off bursts in regular patterns. This, the team says, could be an indication of galaxies harboring two supermassive black holes in their centers.
Conventional thinking says that lurking at the heart of most galaxies is a supermassive black hole. The Milky Way is the perfect example – Sagittarius A* lies about 26,000 light-years from Earth and has a mass about 4 million times that of the Sun.
While it’s generally thought that galaxies would only host one supermassive black hole, the idea that some could have two has been theoretically possible. And now, an international team of researchers has found what could be the first evidence of this scenario.
The team was studying gamma ray emissions from active galactic nuclei (AGN). These energetic objects lie at the center of galaxies, throwing off huge amounts of light as the black holes there gobble up matter.
Most of the time, these emissions happen at random intervals. But the team discovered some that repeat with predictable patterns, suggesting there’s some novel mechanism behind them.
The researchers used a decade of data gathered by the Large Area Telescope instrument, onboard NASA’s Fermi Gamma-ray Space Telescope. Looking back that long allowed the team to identify longer-term patterns in gamma ray signals which would be missed in shorter observation runs.
In doing so, the researchers noted 11 AGN with cyclic gamma ray signals, which repeated every two years on average. That’s a small number considering over 2,000 galaxies were studied.
“Previously only two blazars were known to show periodic changes in their gamma-ray brightness,” says Sara Buson, co-author of the study. “Thanks to our study, we can confidently say that this behavior is present in 11 other sources. 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.”
So what exactly is causing these signals to repeat like clockwork? There are several theories to investigate in future work.
“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,” says Marco Ajello, co-author of the study. “Understanding the relation of these black holes with their environment will be essential for a complete picture of galaxy formation.”
As the instrument gathers more data over the coming years, the answers may come to light.
The research was published in The Astrophysical Journal.
Source: Clemson University