(CNN)For the second time ever, astronomers have detected a pattern in a mysterious fast radio burst coming from space.
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.
They have yet to determine the actual cause of the bursts.
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.
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, in an email. "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 Sunday 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.
What's behind the burst pattern?
So what could be the cause of FRB 121102's extended pattern? Researchers believed these powerful bursts could be due to the orbit of a massive star, a black hole or a dense neutron star.
One potential explanation for repeating fast radio bursts has been the precession, or wobbling top motion, of a highly magnetized neutron star's axis. But that may not explain what astronomers are seeing for this particular burst because it lasts so long, the researchers said. That model may be more suited to bursts that repeat over a few weeks.
Moving forward, the researchers want to find other repeating fast radio bursts, determine if they also have patterns and see if these two represent the range of patterns. They also want to observe FRB 121102 more and see if the patterns change over time.
"Answering these questions will take us closer to the true source of FRBs," Rajwade said.
The burst pattern in this study was detected while using the Lovell Telescope at the Jodrell Bank Observatory in the United Kingdom over four years. The telescope is sensitive to faint radio signals and capable of regularly monitoring repeating fast radio bursts that have already been identified.
Fast radio bursts were only discovered in 2007, followed by the discovery that some of them can repeat in 2016. Now, researchers know they can have patterns as well.
"This exciting discovery highlights how little we know about the origin of FRBs," said Duncan Lorimer, study coauthor, associate dean for research and professor of physics and astronomy at West Virginia University. "Further observations of a larger number of FRBs will be needed in order to obtain a clearer picture about these periodic sources and elucidate their origin."
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