Confirm the transition to the strong-coupling regime

Establish whether 4U 0114+65 has undergone a transition to the strong-coupling regime between the accreting matter and the rotating neutron-star magnetosphere, characterized by a toroidal magnetic-field component comparable to the poloidal component.

Background

The observed rapid spin-down is close to the maximum rate predicted for the strong-coupling regime of quasi-spherical settling accretion. In that regime, turbulent motions above the magnetosphere can amplify the toroidal magnetic field until it is comparable to the poloidal field. The authors interpret the observations as possible evidence for a transition to this regime, but explicitly characterize the interpretation as tentative and unresolved.

References

Although this interpretation remains tentative, if confirmed it would represent the first observational evidence of a transition to the strong-coupling regime in an accreting X-ray pulsar.

— Spin-down of the accreting magnetar candidate 4U 0114+65: possible first evidence for a strong coupling regime  (2609.11479 - Sanjurjo-Ferrín et al., 10 Sep 2026) in Section 5, Discussion, subsection “Toward centrifugal inhibition?”; Section 6, Summary and conclusions