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A number of black hole X-ray transients show quasi-periodic oscillations(QPOs) in the optical (ultraviolet) and X-ray bands at the same frequency,which challenge models for production of radiation at these wavelengths. Wepropose a model where the optical radiation is modulated by the oscillatingX-ray flux resulting in varying irradiation of the outer parts of the accretiondisc. The proposed QPO mechanism inevitably takes place in the systems withsufficiently small ratio of the outer disc radius to the QPO period. We showthat, unlike in the case of the aperiodic variability, it is not possible toobtain the optical QPO profiles from those observed in the X-rays through thetransfer function, because of different X-ray signals seen by the disc and bythe observer. We demonstrate that with the increasing QPO frequency, occurringat the rising phase of the X-ray outburst, the rms should be constant forsufficiently low frequencies, then to increase reaching the peak and finally todrop substantially when the QPO period becomes comparable to the light-crossingtime to the outer disc. We predict that the QPO rms in this model shouldincrease towards shorter wavelengths and this fact can be used to distinguishit from other QPO mechanisms.

Reprocessing model for the optical quasi-periodic oscillations in black hole binaries