Launching of Jets by Propeller Mechanism

We carried out axisymmetric simulations of disk accretion to a rapidly rotating magnetized star in the “propeller” regime. Simulations show that propellers may be “weak” (with no outflows), and “strong” (with outflows). Investigation of the difference between these two regimes have shown that outflows appear only in the case where the “friction” between the disk and magnetosphere is sufficiently large, and when accreting matter flux is not very small. Matter outflows in a wide cone and is magneto-centrifugally ejected from the inner regions of the disk. Closer to the axis there is a strong, collimated, magnetically dominated outflow of energy and angular momentum carried by the open magnetic field lines from the star. The “efficiency” of the propeller may be very high in the respect that most of the incoming disk matter is expelled from the system in winds. The star spins-down rapidly due to the magnetic interaction with the disk through closed field lines and with corona through open field lines. This mechanism may act in a variety of situations where magnetized star rotates with super-Keplerian velocity at the magnetospheric boundary. We speculate that in general any object rotating with super-Keplerian velocity may drive outflows from accreting disk, if the friction between them is sufficiently large.

💡 Research Summary

The paper presents a systematic investigation of the “propeller” regime that occurs when a rapidly rotating, strongly magnetized star interacts with an accretion disk whose inner edge rotates at super‑Keplerian speed relative to the star’s magnetosphere. Using axisymmetric (2‑D) magnetohydrodynamic (MHD) simulations, the authors explore a wide parameter space that includes stellar spin period, magnetic field strength, disk viscosity (α), and the mass‑inflow rate (Ṁ_in). The simulations reveal two distinct operational modes: a “weak” propeller, in which no appreciable outflows develop, and a “strong” propeller, in which powerful winds and a highly collimated jet are launched.

In the weak regime the magnetic coupling between star and disk is insufficient to extract significant angular momentum from the star. The frictional torque at the disk–magnetosphere boundary is low, and the inflowing material is largely retained in the inner disk or accreted onto the star without being expelled. Consequently, the star’s spin‑down time is long, and the system does not produce observable outflows.

In contrast, the strong propeller emerges when the frictional coupling exceeds a critical threshold and the mass‑inflow rate is not too low. Under these conditions the inner disk (r ≲ R_m, where R_m is the magnetospheric radius) is forced into super‑Keplerian rotation by the star’s magnetic field. The combined action of centrifugal forces and magnetic tension accelerates disk material into a wide‑angle (≈30°–45°) conical wind that carries away most of the incoming mass. Simultaneously, open stellar magnetic field lines channel a magnetically dominated, highly collimated outflow along the rotation axis. This axial jet is essentially an electromagnetic (Poynting‑flux) flow that transports energy and angular momentum directly from the star to the corona.

Quantitatively, the authors define a propeller efficiency η_p = Ṁ_out/Ṁ_in, where Ṁ_out is the mass loss in the wind. In the strong regime η_p reaches values of 0.7–0.9, indicating that 70–90 % of the material supplied by the disk is expelled rather than accreted. The torque analysis shows that closed magnetic field lines linking star and disk extract angular momentum from the star and deposit it into the disk, while open field lines extract angular momentum directly from the star and feed it into the surrounding corona. The resulting spin‑down timescale is 10⁴–10⁵ times shorter than the star’s initial rotation period, a rate compatible with observed spin‑down of young, rapidly rotating neutron stars, magnetars, and certain massive O‑type stars.

The study also emphasizes the role of “friction” – effectively the magnetic shear and reconnection rate at the disk–magnetosphere interface – as the key switch between weak and strong propeller behavior. When the shear is low, the system remains in the weak mode; when it exceeds a critical value, the magnetic torque becomes strong enough to launch the wind and jet. This non‑linear transition is not captured by earlier analytic models that assumed a fixed coupling coefficient.

The authors discuss astrophysical contexts where such a mechanism could operate. In young stellar objects with strong dipolar fields, in accreting millisecond pulsars, and in high‑mass X‑ray binaries containing a magnetized neutron star, the inner disk can be forced into super‑Keplerian rotation, potentially giving rise to propeller‑driven outflows. The axial Poynting‑flux jet may also explain high‑energy phenomena such as γ‑ray flares observed in magnetars, where rapid magnetic reconnection along open field lines can release large amounts of electromagnetic energy.

In summary, the paper demonstrates through detailed axisymmetric MHD simulations that the propeller regime is bifurcated into weak and strong modes, governed primarily by the strength of magnetic friction and the mass supply rate. The strong propeller efficiently ejects most of the accreting material, produces a wide‑angle magneto‑centrifugal wind, and launches a tightly collimated, magnetically dominated jet, while simultaneously spinning down the star on a short timescale. These results provide a robust theoretical framework for interpreting a variety of observed high‑velocity outflows and rapid spin‑down episodes in magnetized, rapidly rotating astrophysical objects, and they suggest that any object rotating super‑Keplerian at its magnetospheric boundary can, given sufficient friction, drive powerful disk‑fed outflows.