Periodic Variability During the X-ray Decline of 4U 1636-53

We report the onset of a large amplitude, statistically significant periodicity (~46 d) in the RXTE/ASM data of the prototype X-ray burster 4U 1636-53, the X-ray flux of which has been gradually declining over the last four years. This behaviour is remarkably similar to that observed in the neutron star LMXB KS 1731-260, which is a long-term transient. We also report on an INTEGRAL/IBIS observation of 4U 1636-53 during its decline phase, and find that the hard X-ray flux (20-100 keV) indicates an apparent anti-correlation with soft X-rays (2-12 keV). We argue that 4U 1636-53 is transiting from activity to quiescence, as occurred in KS 1731-260. We also suggest that the variability during the X-ray decline is the result of an accretion rate variability related to the X-ray irradiation of the disc.