Effect of non-stationary accretion on spectral state transitions: An example of a persistent neutron star LMXB 4U 1636-536

Abstract

Observations of the black hole and neutron star X-ray binaries show that the luminosity of the hard-to-soft state transition is usually higher than that of the soft-to-hard state transition, indicating additional parameters other than the mass accretion rate is required to interpret spectral state transitions. It has been found in some individual black hole or neutron star soft X-ray transients that the luminosity corresponding to the hard-to-soft state transition is positively correlated with the peak luminosity of the following soft state. In this work, we report the discovery of the same correlation in a single persistent neutron star low mass X-ray binary (LMXB) 4U1636-536 based on the data from the All Sky Monitor(ASM) on board the {\it RXTE}, the Gas Slit Camera (GSC) on board the {\it MAXI} and the Burst Alert Telescope (BAT) on board the {\it Swift}. We also found such a positive correlation holds in the persistent neutron star LMXB in a luminosity range spanning by about a factor of four. Our results indicates that non-stationary accretion also plays an important role in driving X-ray spectral state transitions in persistent accreting systems with small accretion flares, which is much less dramatic compared with the bright outbursts seen in many Galactic LMXB transients.