Researchers led by Silke Britzen at the Max Planck Institute for Radio Astronomy have found a previously unknown second jet at the core of galaxy Mrk 501. It points away from Earth, which is why it went unnoticed.

Analysis of about 23 years of radio observations shows the jet's position shifts over time, suggesting it orbits a hidden companion black hole roughly every 121 days.

The black holes are estimated at 100 million to a billion solar masses and could merge within about 100 years. The work appears in Monthly Notices of the Royal Astronomical Society.

Source: ScienceDaily and other published reports — rewritten in our own words.