2022MNRAS.513L..89Z

The observed early X-ray plateau in the afterglow lightcurves of some gamma-ray bursts (GRBs) is attributed to the dipole radiations (DRs) of a newborn magnetar. A quasi-periodic oscillation (QPO) signal in the plateau would be strong evidence of the magnetar precession motion. By making a time-frequency domain analysis for the X-ray afterglow lightcurve of GRB 180620A, we find a QPO signal of ∼650 s in its early X-ray plateau. We fit the lightcurve with a magnetar precession model by adopting the Markov chain Monte Carlo algorithm. The observed lightcurve and the QPO signal are well represented with our model. The derived magnetic field strength of the magnetar is B_p= (1.02^+0.59_–0.61) ×10¹⁵ G. It rapidly spins down with angular velocity evolving as Ω_s ∝ (1 + t/τ_sd)^–0.96, where τ_sd = 9430 s. Its precession velocity evolution is even faster than Ω_s, i.e. Ω_p ∝ (1 + t/τ_p)^-2.18 ± 0.11^, where τ_p = 2239 ± 206 s. The inferred braking index is n = 2.04. We argue that the extra energy loss via the magnetospheric processes results in its rapid spin-down, low braking index, and strong precession motion of the magnetar.