Prompt periodicity in the GRB 211211A precursor: black-hole or magnetar engine?

<p dir="ltr">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 time-scale requires relativistic motion with a Lorentz factor $\Gamma \gtrsim 80$, 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 disc 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 $\chi \sim 0.3 \!-\! 0.9$. Alternatively, at a disc density of $\sim 10^{8 - 12}$ g cm$^{-3}$, the required magnetic field strength for a QPO via magnetohydrodynamic effects will be of the order of $B\sim 10^{12 - 14}$ 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 disc at a radius of $\sim 20 \!-\! 70$ km, for a disc 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 disc. Neither engine can be ruled out – however, we favour an origin for the precursor candidate QPO as early jet–disc coupling for a neutron star–black hole merger remnant with mass $M_\bullet >4.5$ M$_{\odot }$.</p>