Magnetars, highly magnetised neutron stars, are thought to be the most likely progenitors for fast radio bursts (FRBs). Freely precessing magnetars are further invoked to explain the repeating FRBs. We report here on new high-cadence polarimetric radio observations of the magnetar XTE J1810−197 recorded shortly after an outburst in late 2018. We interpret the rapid polarisation variations of the magnetar radio emission as strong evidence for the magnetar undergoing free precession following the X-ray outburst and damped on a timescale of months. The observations of precession being damped argue against the scenario of freely precessing magnetars as the origin of repeating FRBs. Using a free precession model based on crust-core coupling with relaxing ellipticity, we find the magnetar ellipticity to be in good agreement with theoretical predictions from nuclear physics. Our precise measurement of the magnetar’s geometry can also further help in refining the modelling of X-ray light curves and constrain the star’s compactness.
