Early evolution of a newborn magnetar with strong precession motion in GRB 180620A

Abstract

ABSTRACT 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 $B_{\rm p}= (1.02^{+0.59}_{-0.61})\times 10^{15}$ G. It rapidly spins down with angular velocity evolving as Ωs ∝ (1 + t/τsd)−0.96, where τsd = 9430 s. Its precession velocity evolution is even faster than Ωs, i.e. Ωp ∝ (1 + t/τp)−2.18 ± 0.11, where τp = 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.