Radio pulsations from a neutron star within the gamma-ray binary LS I +61° 303

LS I +61° 303 is one of the rare gamma-ray binaries 1 that emit most of their luminosity in photons with energies beyond 100 MeV (ref. 2 ). It is well characterized—the ~26.5 day orbital period is clearly detected at many wavelengths 2 – 4 —and other aspects of its multifrequency behaviour make it the most interesting example of its class. The morphology of high-resolution radio images changes with orbital phase, displaying a cometary tail pointing away from the high-mass star component 5 and LS I +61° 303 also shows superorbital variability 3 , 6 – 9 . A couple of energetic (~10 37 erg s −1 ), short, magnetar-like bursts have been plausibly ascribed to it 10 – 13 . Although the phenomenology of LS I +61° 303 has been the subject of theoretical scrutiny for decades, there has been a lack of certainty regarding the nature of the compact object in the binary that has hampered our understanding of the source. Here, using observations with the Five-hundred-meter Aperture Spherical radio Telescope, we report the existence of transient radio pulsations from the direction of LS I +61° 303 with a period P = 269.15508 ± 0.00016 ms at a significance of >20 σ . These pulsations strongly argue for the existence of a rotating neutron star within LS I +61° 303.