Spin-down of the accreting magnetar candidate 4U 0114+65: Possible first evidence for a strong coupling regime
Year: 2026
Type: article
Abstract: 4U 0114+65 is a high-mass X-ray binary composed of the B1 Ia supergiant V* V662 Cas and one of the slowest known accreting neutron stars (NSs), with a spin period of ∼9.4 ks. In 2025, the source entered an unusual state in which its long X-ray pulsations became undetectable in the Swift /BAT monitoring for at least several months, motivating a Director’s Discretionary Time observation with XMM-Newton . We compared this 2025 observation with a previous XMM-Newton observation obtained in 2015, when the source was brighter and clearly pulsed. We also analyzed the long-term evolution of the spin period using Swift /BAT data. The spectra were studied through average and pulse-phase-resolved analyses using the same phenomenological model adopted in previous work. We find that the 2025 XMM-Newton observation still reveals weak pulsations, with a period of about 9.3 ks, despite their non-detection in the Swift /BAT monitoring. The overall spectral shape remains similar in both epochs, but the source luminosity decreases by about one order of magnitude, mainly because of strong suppression of the bulk-motion Comptonization component. Although the absorbing column is higher in 2025, the inferred wind properties remain broadly compatible with those obtained in 2015, suggesting that no major global change in the donor wind is required. Instead, the results suggest a substantial reduction in the accretion efficiency close to the NS magnetosphere. We propose that 4U 0114+65 may be evolving toward a regime of partial centrifugal inhibition in the strong-coupling regime, when the toroidal magnetic-field component is comparable to the poloidal one. If true, this phenomenon has been observed for the first time. In this state, accretion becomes progressively less efficient and more intermittent without reaching a fully developed propeller regime. In this scenario, the apparent vanishing of the pulse in long-term hard X-ray monitoring would result from reduced luminosity and lower absolute pulsed flux, rather than a complete disappearance of the underlying spin modulation.
Source: Astronomy and Astrophysics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Awards: G3480869486, G7084143925
OpenAlex ID: W7212308573