4U 1626-67 Returns to Spin-Down: Timing features toe the line

TL;DR

This paper analyzes the timing and spectral properties of the X-ray pulsar 4U 1626-67 during its current spin-down phase, revealing distinct pulse profiles, a quasi-periodic oscillation, and spectral changes compared to previous states.

Contribution

It provides the first detailed timing and spectral analysis of 4U 1626-67 during its recent spin-down phase, highlighting new features and differences from prior torque states.

Findings

Detection of a ~46.8 mHz QPO during spin-down

Higher pulse fraction in spin-down phase

Significant flux drop by a factor of ~3 during spin-down

Abstract

We present a comprehensive analysis of X-ray pulsar 4U 1626-67 during its current spin-down (2SD) state, following a recent torque reversal. Since its discovery, this ultra-compact binary has experienced multiple torque states, transitioning from spin-up (1SU) during 1977-1990 to spin-down (1SD) during 1990-2008, and again spin-up (2SU) until 2023. From NuSTAR observation of May 2023, we have investigated the timing and spectral properties of this pulsar during its 2SD phase, while also comparing them with previous spin-up-down states. For energies upto 8 keV, a distinct bi-horned pulse profile was observed during the spin-up phase, while several sub-structures emerged during spin-down. Beyond 8 keV, a broad asymmetric peak was consistently observed across all torque states. The pulse fraction during the 2SD phase was higher than that during 2SU phase. A prominent ~46.8 mHz quasi-periodic oscillation has been exclusively detected during the spin-down phase. The broadband spectrum during the 2SD phase is described by empirical NPEX model, cyclotron absorption feature and its first harmonic. The spectrum during 2SU phase requires an additional blackbody component and asymmetry in the cyclotron absorption line. A significant flux drop by a factor of ~3 in the 2SD was observed.