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 Rossi X-ray Timing Explorer/All Sky Monitor (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 4 yr. This behaviour is remarkably similar to that observed in the neutron star low mass X-ray binary (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 anticorrelation 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.