Propeller effect in action: Unveiling quenched accretion in the transient X-ray pulsar 4U 0115+63

¹ Department of Physics and Astronomy, FI-20014 University of Turku, Finland
² Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany
³ Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁴ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, PR China
⁵ CSST Science Center for the Guangdong-Hong Kong-Macau Greater Bay Area, DaXue Road 2, 519082 Zhuhai, PR China

^⋆ Corresponding author: hua.h.xiao@utu.fi

Received: 21 July 2025
Accepted: 10 September 2025

Abstract

The Be/X-ray pulsar 4U 0115+63 underwent a type II outburst in 2023. After the outburst, similar to the outbursts in 2015 and 2017, the source decayed into a quiescent state. Two out of three XMM-Newton observations conducted after the 2023 outburst confirmed the source to be in a low-luminosity state at a level of L_X ∼ 10³³ erg s⁻¹. X-ray pulsations were detected at ≈0.277 Hz in both observations with a pulsed fraction exceeding 50%. The power density spectra show no significant low-frequency red noise in either observation, suggesting that the radiation is not driven by accretion. The energy spectra in this state can be described by a single blackbody component, with an emitting area smaller than the typical size of the polar caps during the accretion phase. Based on the timing and spectral properties, we suggest that the propeller effect is active during the quiescent state, resulting in a total quenching of accretion. We discuss possible mechanisms for the generation of pulsations in this regime and consider the scenario of neutron star crust cooling.