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We present time-resolved optical photometry of the binary millisecond`redback' pulsar PSR J1023+0038 (=AY Sex) during its low-mass X-ray binaryphase. The light curves taken between 2014 January and April show an underlyingsinusoidal modulation due to the irradiated secondary star and accretion disc.We also observe superimposed rapid flaring on time-scales as short as ~20 swith amplitudes of ~0.1-0.5 mag and additional large flare events ontime-scales of ~5-60 min with amplitudes ~0.5-1.0 mag. The power densityspectrum of the optical flare light curves is dominated by a red-noisecomponent, typical of aperiodic activity in X-ray binaries. Simultaneous X-rayand UV observations by the Swift satellite reveal strong correlations that areconsistent with X-ray reprocessing of the UV light, most likely in the outerregions of the accretion disc. On some nights we also observe sharp-edged,rectangular, flat-bottomed dips randomly distributed in orbital phase, with amedian duration of ~250 s and a median ingress/egress time of ~20 s. Theserectangular dips are similar to the mode-switching behaviour between disc`active' and `passive' luminosity states, observed in the X-ray light curves ofother redback millisecond pulsars. This is the first time that the opticalanalogue of the X-ray mode-switching has been observed. The properties of thepassive and active state light curves can be explained in terms of clumpyaccretion from a trapped inner accretion disc near the corotation radius,resulting in rectangular, flat-bottomed optical and X-ray light curves.

The binary millisecond pulsar PSR J1023+0038 during its accretion state - I. Optical variability