Millihertz optical/ultraviolet oscillations in 4U 1626-67: Evidence for a warped accretion disk

We have detected large-amplitude 0.3-1.2 mHz quasi-periodic oscillations (QPOs) from the low-mass X-ray binary pulsar 4U 1626-67/KZ TrA, using UV photometry from the Hubble Space Telescope and ground-based optical photometry. These 1 mHz QPOs, which have coherence (nu/Delta nu) approximate to 8, are entirely distinct from the 130 mHz pulsar spin frequency, a previously known 48 mHz QPO, and the 42 minute binary period (independently confirmed here). Unlike the 48 and 130 mHz oscillations, which are present in both the optical/UV and the X-ray emission, the 1 mHz QPOs are not detected in simultaneous observations with the Rossi X-Ray Timing Explorer. The rms amplitude of the mHz QPO decreases from 15% in the far-UV to 3% in the optical, while the upper limit on a corresponding X-ray QPO is as low as 0.8%. We suggest that the mHz oscillations are due to warping of the inner accretion disk. We also report the detection of coherent upper and lower sidebands of the 130 mHz optical pulsations, with unequal amplitude and a spacing of 1.93 mHz around the main pulsation. The origin of these sidebands remains unclear.