Quasi-periodic Oscillations in GRB 210514A: a Case of a Newborn Supra-Massive Precessing Magnetar Collapsing into a Black Hole?

Magnetar is proposed as one of the possible central engines for a gamma-ray burst (GRB). Recent studies show that if a magnetar has a rotational axis misaligned from the magnetic one, a periodic lightcurve pattern is expected with a period of seconds to minutes. Inspired by this unique feature, in this paper, we search for the quasi-periodic oscillation (QPO) signals in the {\it Swift} observations of GRBs. Using the Lomb-Scargle periodogram and the weighted wavelet Z-transform algorithms, we find that the {\it Swift}-BAT data of GRB 210514A has a QPO signal with a period $\sim 11\,{\rm s}$. The estimated confidence level of the signal is over $3 \sigma$. The global lightcurve of this GRB exhibits a double-plateau structure with a sharp decay segment between plateaus. The lightcurve feature resembles those of GRBs that were reported to have internal plateaus. We explain the observations of GRB 210514A with a supra-massive magnetar (SMM) model, where the QPO signal in the first plateau is produced via the dipole radiation of the SMM experiencing a precession motion, the sharp decay is due to the collapse of the SMM into a black hole (BH), and the second plateau could be produced via the fall-back accretion of the newborn BH. We fit the precession model to the observations using the Bayesian statistic and the best-fit magnetar parameters are discussed. Alternative models concerning a BH central engine may also provide reasonable explanations for this burst, only in this case the QPO signal could merely be a coincidence.