Evidence that this supermassive black hole may have a companion comes from observations by radio telescopes on Earth. Black holes don’t emit light, but their gravity can gather disks of hot gas around them and eject some of that material into space. These jets can stretch for millions of light-years. A jet pointed toward Earth appears far brighter than a jet pointed away from Earth. Astronomers call supermassive black holes with jets oriented toward Earth blazars, and a blazar named PKS 2131-021 is at the heart of this recent paper.
Located about 9 billion light-years from Earth, PKS 2131-021 is one of 1,800 blazars that a group of researchers at Caltech in Pasadena has been monitoring with the Owens Valley Radio Observatory in Northern California for 13 years as part of a general study of blazar behavior. But this particular blazar exhibits a strange behavior: Its brightness shows regular ups and downs as predictably as the ticking of a clock.
Researchers now think this regular variation is the result of a second black hole tugging on the first as they orbit each other about every two years. Each of the two black holes in PKS 2131-021 is estimated to be a few hundred million times the mass of our Sun. To confirm the finding, scientists will try to detect gravitational waves – ripples in space – coming from the system. The first detection of gravitational waves from black hole binaries was announced in 2016.
To confirm that the oscillations weren’t random or the cause of a temporary effect around the black hole, the team had to look beyond the decade (2008 to 2019) of data from the Owens Valley Observatory. After learning that two other radio telescopes had also studied this system – the University of Michigan Radio Observatory (1980 to 2012) and the Haystack Observatory (1975 to 1983) – they dug into the additional data and found that it matched predictions for how the blazar’s brightness should change over time.
“This work is a testament to the importance of perseverance,” said Lazio. “It took 45 years of radio observations to produce this result. Small teams, at different observatories across the country, took data week in and week out, month in and month out, to make this possible.”
To learn more, read the news release from Caltech.