QPOs in a highly magnetized ultra-compact X-ray binary 4U 1626-67
Abstract
We report the detection of mHz quasi-periodic oscillations (QPOs) in four NuSTAR observations of 4U 1626-67 during its recent spin-down episode. By using a novel method based on the Hilbert-Huang Transform (HHT), 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 root-mean-square (rms) is distinct for different observations. In this source, both QPO-phase-resolved and averaged spectra can be modeled with a negative and positive powerlaws exponential (NPEX) 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.
Cite
@article{arxiv.2510.09986,
title = {QPOs in a highly magnetized ultra-compact X-ray binary 4U 1626-67},
author = {Zi-Yi Zhou and Long Ji and Ling-Da Kong and Sergey S. Tsygankov and Qing-Cang Shui and Lian Tao},
journal= {arXiv preprint arXiv:2510.09986},
year = {2025}
}
Comments
Accepted for publication in APJ