Discovery of the Quasi-Periodic Oscillations from the X-Ray Pulsar X1627–673

Abstract Quasi-periodic oscillations (QPO) with a centroid frequency of 0.04 Hz were detected in the X-ray flux from a low-mass binary X-ray pulsar, X1627–673. The FWHM of the QPO peak in the power spectrum was 0.02 Hz. Though the QPO were observed over the entire energy range, 2 – 22 keV, they were most prominent in the energy band 14 – 18 keV. The QPO frequency is consistent with the beat frequency between the neutron star rotation and Keplerian motion at the Alfvén radius of the accretion disk. This is, however, not consistent with the theoretical model of the accretion torque which is widely accepted for interpreting the steady spin-up trend of this source.