An international research team led by Silke Britzen of the Max Planck Institute for Radio Astronomy has found direct observational evidence of two supermassive black holes orbiting each other at the center of the galaxy Mrk 501. The findings will appear in Monthly Notices of the Royal Astronomical Society.
Astronomers tracked a second jet of particles shooting from the system over 23 years of radio observations at multiple frequencies. The newly discovered jet moves in a different direction from the previously known jet and sweeps counterclockwise around the larger black hole, revealing regular orbital motion.
"We searched for it for so long, and then it came as a complete surprise that we could not only see a second jet, but even track its movement," Britzen said.
The team calculated that the two black holes complete an orbit around each other approximately every 121 days. Their separation is estimated at 250 to 540 times the Earth-Sun distance. At this rate, they could merge in as little as 100 years, depending on the black holes' actual masses.
The researchers also identified an Einstein ring in June 2022, when radiation from the system formed a ringlike appearance. This suggests gravitational lensing bent light from the second jet behind the foreground black hole.
Because Mrk 501 is too distant for current instruments to resolve the two black holes separately, astronomers cannot directly watch them spiral inward. However, pulsar timing arrays could detect extremely low-frequency gravitational waves produced by the orbiting pair and measure how their orbit evolves over time.
"If gravitational waves are detected, we may even see their frequency steadily rise as the two giants spiral toward collision, offering a rare chance to watch a supermassive black hole merger unfold," notes co-author Héctor Olivares.