X-ray Pulsar 4U 1626-67 Returns to Spin-Down: New Torque Reversal

Abstract: 4U 1626−67 is an accretion-powered X-ray pulsar in an ultra-compact binary. Since its discovery, this X-ray pulsar has shown several unique timing and spectral features associated with systematic changes in the inner accretion flow. The most prominent characteristic of 4U 1626−67 is its steady spin-up and spin-down episodes of several years with torque reversals in between. It has experienced multiple torque states, transitioning from spin-up during 1977–1990 to spin-down during 1990–2008, and again spin-up until 2023. From NuStar and AstroSat observations of 2023, we have investigated the timing and spectral properties of this pulsar during its recent spin-down phase, while also comparing them with previous spin-up and spin-down states. A significant flux drop by a factor of ∼3 in the spin-down was observed. For energies up to 8 keV, a distinct bi-horned pulse profile was observed during the spin-up phase, while several substructures emerged during spin-down. Beyond 8 keV, a broad asymmetric peak was consistently observed across all torque states. The pulse fraction during the spin-down phase was higher than that during the spin-up phase. A prominent ∼46.8 mHz quasi-periodic oscillation has been exclusively detected during the spin-down phase. The broad-band spectrum during the spin-down phase is described by the empirical NPEX model, cyclotron absorption feature and its first harmonic. The spectrum during the spin-up phase requires an additional blackbody component and asymmetry in the cyclotron absorption line. I will discuss the implications of the results obtained from our study.