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We present the results of a study of the low-mass X-ray binary 4U 1636-536.We have performed temporal analysis of all available RXTE/ASM, Swift/BAT andMAXI data. We have confirmed the previously discovered quasi-periodicity of ~45d present during ~2004, however we found it continued to 2006. At other epochs,the quasi-periodicity is only transient, and the quasi-period, if present,drifts. We have then applied a time-dependent accretion disc model to theinterval with the significant X-ray quasi-periodicity. For our best model, theperiod and the amplitude of the theoretical light curve agree well with thatobserved. The modelled quasi-periodicity is due to the hydrogenthermal-ionization instability occurring in outer regions of the accretiondisc. The model parameters are the average mass accretion rate (estimated fromthe light curves), and the accretion disc viscosity parameters, for the hot andcold phases. Our best model gives relatively low values of viscosity parameterfor cold phase 0.01 and for hot phase 0.03.

Long-term quasi-periodicity of 4U 1636-536 resulting from accretion disc instability