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High-Speed Optical Photometry of the Ultracompact X-ray Binary 4U 1626-67

Rapid UBVRI photometry of this ultracompact LMXB pulsar has detected 7.67 s optical pulsations in all five bands. The optical pulsations, which are at the same frequency as the X-ray pulsations, are understood as reprocessing of pulsed X-rays in the accretion disk or on the companion surface. The optical pulsed fraction is 6%, independent of wavelength, indicating that the optical emission is dominated by X-ray reprocessing. A weaker (1.5%) sideband, shifted down 0.395(15) mHz from the main optical pulsation, is also present. This is consistent with a previously reported sideband and corroborating the 42-min binary period proposed earlier by Middleditch et al. (1981). A 0.048 Hz optical QPO, corresponding to a known X-ray feature, was also detected in some of the observations. 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 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 X-ray albedo of LMXB accretion disks is as high as recently reported, then the optical data imply a distance of 8 kpc and an X-ray luminosity of 10^37 erg/s.