Long-term Quasiperiodicity in LMXB 4U 1636–536

We present the results of the low-mass X-ray binary 4U 1636–536 study. We performed a temporal analysis of all available RXTE/All-Sky Monitor, RXTE/Proportional Counter Array, Swift/Burst Alert Telescope, and MAXI data collected until the middle of 2022. Within our analysis we confirmed the previously discovered quasiperiodicity of ≃45 days present during ∼2004, and we found that it continued until 2007. Moreover, we showed that similar periodicity also appeared in 2010 and 2011. At other epochs, the quasiperiodicity is only transient. We applied a time-dependent accretion disk model to the interval with the most significant X-ray quasiperiodicity. Although 4U 1636–536 is persistent, the observed quasiperiodicity can be well modeled by the hydrogen thermal-ionization instability occurring in the outer regions of the accretion disk. For our best model, the theoretical light curve’s period and amplitude agree well with those observed.