A mysterious radio source located in a galaxy 500 million light years from Earth is pulsing on a 16-day cycle, like clockwork, according to a new study. This marks the first time that scientists have ever detected periodicity in these signals, which are known as fast radio bursts (FRBs), and is a major step toward unmasking their sources.

FRBs are one of the most tantalizing puzzles that the universe has thrown at scientists in recent years. First spotted in 2007, these powerful radio bursts are produced by energetic sources, though nobody is sure what those might be. FRBs are also mystifying because they can be either one-offs or “repeaters,” meaning some bursts appear only once in a certain part of the sky, while others emit multiple flashes to Earth.

Pulses from these repeat bursts have, so far, seemed somewhat random and discordant in their timing. But that changed last year, when the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB), a group dedicated to observing and studying FRBs, discovered that a repeater called FRB 180916.J0158+65 had a regular cadence.

The CHIME/FRB team kept tabs on the repeating burst between September 2018 and October 2019 using the CHIME radio telescope in British Columbia. During that period, the bursts were clustered into a period of four days, and then seemed to switch off for the next 12 days, for a total cycle of about 16 days. Some cycles did not produce any visible bursts, but those that did were all synced up to the same 16-day intervals.

“We conclude that this is the first detected periodicity of any kind in an FRB source,” the team said in a paper published on the preprint server arXiv in late January. “The discovery of a 16.35-day periodicity in a repeating FRB source is an important clue to the nature of this object.”

Scientists recently tracked down this particular FRB to a galaxy called SDSS J015800.28+654253.0, which is a half a billion light years from Earth. That may seem like a huge distance, but FRB 180916.J0158+65 is actually the closest FRB ever detected.

But while we know where it is, we still don’t know what it is. To that point, the beat of the FRB suggests that it might be modulated by its surroundings. If the source of the FRB is orbiting a compact object, such as a black hole, then it might only flash its signals toward Earth at a certain point in its orbital period. That scenario could potentially match this recognizable 16-day cycle.

It’s also possible that we are witnessing a binary system containing a massive star and a super-dense stellar core known as a neutron star, according to a study published on arXiv on Wednesday by a separate team that looked at the same data. In that model, the neutron star would emit radio bursts, but those signals would be periodically eclipsed by opaque outflowing winds from its giant companion.

Another scenario is that the FRB rhythm isn’t tempered by another object, and is sending out the pulses directly from the source. Scientists have previously suggested that flares from highly magnetized neutron stars, called magnetars, might be the source of some FRBs. But since magnetars tend to rotate every few seconds, a 16-day cycle does not match the expected profile of a magnetar-based FRB.

Ultimately, the CHIME/FRB team hopes to find similar patterns in the handful of known repeating bursts to see if these cycles are common. The researchers also plan to keep a careful eye on FRB 180916.J0158+6 while it is active in order to spot any other details that might point to its identity.