2020ApJ...892L..34S

Many gamma-ray bursts are followed by periods of extended emission. At least in some cases, the burst afterglow may be powered by a rapidly rotating, highly magnetized neutron star, which spins down due to electromagnetic and gravitational-wave emission. Such a remnant is likely to strongly precess in the early stages of its life, which would lead to modulations in the X-ray luminosity as the triaxiality of the system evolves over time. Using a radiation profile appropriate for a precessing, oblique rotator, we find that Swift-XRT data of a long (080602) and a short (090510) burst matches the model with significantly higher accuracy (mean-square residuals dropping by >=200% in the early stages of the extended emission) than for an orthogonal rotator. We interpret this as evidence for precession in newborn magnetars.