Early Evolution of a Newborn Magnetar with Strong Precession Motion in GRB 180620A

TL;DR

This paper analyzes the early X-ray afterglow of GRB 180620A, providing evidence for a precessing newborn magnetar through QPO detection and modeling its spin-down and precession evolution.

Contribution

It presents the first detection of a QPO signal indicating magnetar precession in GRB afterglow and models the magnetar's spin-down and precession dynamics.

Findings

Detected a ~650s QPO in the X-ray plateau of GRB 180620A.

Estimated the magnetar's magnetic field strength as ~10^15 G.

Found the magnetar's spin-down and precession evolve rapidly with specific timescales.

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 $\sim650$ seconds 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 $B_{\rm p}= (1.02^{+0.59}_{-0.61})\times10^{15}$~G. It rapidly spins down with angular velocity evolving as $\Omega_{s} \propto(1+t/\tau_{\rm sd})^{-0.96}$, where $\tau_{\rm sd}=9430$~s. Its precession velocity evolution is even faster than $\Omega_s$, i.e. $\Omega_{ p}\propto (1+t/\tau_{p})^{-2.18\pm0.11}$, where $\tau_{p}=2239\pm206$~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, a low braking index of the magnetar, and the strong precession motion.