Quasiperiodic Oscillations in a Highly Magnetized Ultracompact X-Ray Binary 4U 1626-67

We report the detection of mHz quasiperiodic oscillations (QPOs) in four Nuclear Spectroscopic Telescope Array observations of 4U 1626–67 during its recent spin-down episode. By using a novel method based on the Hilbert–Huang transform, we present the first QPO-phase-resolved timing and spectral analysis of accreting X-ray pulsars in low-mass X-ray binaries. Broadband QPO waveforms have been reconstructed and exhibit approximately sinusoidal shapes, with fractional amplitudes that vary with energy. In addition, we find that spin pulse profiles exhibit stable shapes between different QPO phases with different instantaneous fluxes, while the fractional rms is distinct for different observations. In this source, both QPO-phase-resolved and averaged spectra can be modeled with a negative and positive power-law exponential model, and their spectral evolutions show a similar trend, suggesting that the QPO modulation is caused by accretion rate variability instead of a geometric obscuration. These results provide new constraints on accretion physics in strongly magnetized neutron stars and the underlying mechanisms of QPOs.