High‐Speed Optical Photometry of the Ultracompact X‐Ray Binary 4U 1626−67

Rapid UBVRI photometry of the ultracompact low-mass X-ray binary (LMXB) pulsar 4U 1626-67/KZ TrA has detected 130.4 mHz (7.67 s) optical pulsations in all five bands. The optical pulsations, at the same frequency as the X-ray pulsations caused by rotation of the highly magnetized accreting neutron star primary, can be understood as a reprocessing of the pulsed X-ray emission in the accretion disk. The optical pulsed fraction is roughly 6%, independent of wavelength, indicating that the optical emission is dominated by X-ray reprocessing. A weaker (1.5%) sideband, downshifted 0.395(15) mHz from the main optical pulsation, is also present. This is consistent with a previously reported sideband downshifted 0.4011(21) mHz from the main pulsation, corroborating the 42 minute binary period proposed by Middleditch et al. A 0.048 Hz optical quasi-periodic oscillation (QPO), corresponding to a previously reported X-ray feature, was also detected in some of the observations, with a fractional rms amplitude of 3%-5%. This is the first measurement of an optical QPO in an X-ray binary pulsar. I discuss constraints on the nature of the mass donor and show that mass transfer via a radiatively driven wind is inconsistent with the optical data. I also review the basic theory of X-ray-heated accretion disks and show that such models provide a good fit to the optical photometry. If the effective X-ray albedo of LMXB accretion disks is as high as recently reported (η d ≳ 0.9), then the optical data imply a distance of ~8 kpc and an X-ray luminosity of ≈ 10 37 ergs s -1 .