Prompt Periodicity in the GRB 211211A Precursor: Black-hole or magnetar engine?

Abstract

The merger origin long GRB 211211A was a class (re-)defining event. A precursor was identified with a $\sim 1$ s separation from the main burst, as well as a claimed candidate quasi-periodic oscillation (QPO) with a frequency $\sim 20$ Hz. Here, we explore the implications of the precursor, assuming the quasi-periodicity is real. The precursor variability timescale requires relativistic motion with a Lorentz factor $\hat{I}“\gtrsim 80$, and implies an engine driven jetted outflow. The declining amplitude of the consecutive pulses requires an episodic engine with an `on/off' cycle consistent with the QPO. For a black-hole central engine, the QPO can have its origin in Lense-Thirring precession of the inner disk at $\sim 6-9$ $r_g$ (gravitational radii) for a mass $M_{\bullet }\le 4.5$ $M_{\odot }$, and $\lesssim 7$ $r_g$ for $M_{\bullet }>4.5$ $M_{\odot }$ and dimensionless spin $\ddot{I}‡\sim 0.3-0.9$. Alternatively, at a disk density of $\sim 10^{8-12}$ g cm${}^{-3}$, the required magnetic field strength for a QPO via magnetohydrodynamic effects will be on the order $B\sim 10^{12-14}$ G. If the central engine is a short lived magnetar or hypermassive neutron star, then a low-frequency QPO can be produced via instabilities within the disk at a radius of $\sim 20-70$ km, for a disk density $\sim 10^{9-12}$ g cm${}^{-3}$ and magnetic field $\gtrsim 10^{13-14}$ G. The QPO cannot be coupled to the neutron star spin, as the co-rotation radius is beyond the scale of the disk. Neither engine can be ruled out -- however, we favour an origin for the precursor candidate QPO as early jet-disk coupling for a neutron star -- black hole merger remnant with mass $M_{\bullet }>4.5$ $M_{\odot }$.

18 pages, 14 figures, accepted for publication in MNRAS