Astronomers have detected a pattern in a mysterious fast radio burst coming from space for the second time.

Fast radio bursts, or FRBs, are millisecond-long bursts of radio waves in space, and astronomers have been able to trace some radio bursts back to their home galaxies.

Now, they have detected a pattern in a second repeating fast radio burst, known as FRB 121102. During this cyclical pattern, radio bursts are emitted during a 90-day window, followed by a silent period of 67 days. This pattern repeats every 157 days.

FRB 121102 has been known as a repeating fast radio burst since 2016. Now, they know it has a pattern.

“Until now, only one other repeating FRB was known to show such a pattern in its bursting activity,” said Kaustubh Rajwade, lead study author and postdoctoral researcher in astronomy at the University of Manchester.

“Finding such a pattern reveals important clues as to what could [be] the progenitor of FRBs. A periodicity tell us that the object that is producing FRBs is probably in an orbit with another astrophysical body.”

The study published in the journal Monthly Notices of the Royal Astronomical Society.

FRB 121102 was the first repeating fast radio burst to be traced back to its source, linked back to a small dwarf galaxy more than 3 billion light-years away in 2017.

The fact that this repeating fast radio burst pattern is at least 10 times longer than the one repeating every 16.4 days shows the potential large range for such activity, the researchers said.

Individual radio bursts emit once and don’t repeat. But repeating fast radio bursts are known to send out short, energetic radio waves multiple times.

Previous observations showed that usually when they repeat, it’s sporadic or in a cluster.

That all changed earlier this year when astronomers found that FRB 180916.J0158+65 had a pattern in bursts occurring every 16.35 days. Over the course of four days, the signal would release a burst or two each hour. Then, it would go silent for another 12 days.