Scientists have discovered the first fast radio burst that beats at a steady rhythm, and the mysterious repeating signal is coming from the outskirts of another galaxy.

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.

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