Black hole-like hysteresis and accretion states in neutron star low-mass X-ray binaries

Abstract

We have systematically studied a large sample of the neutron star low-mass X-ray binaries (LMXBs) monitored by the Rossi X-ray Timing Explorer (50 sources; 10000+ observations). We find that the hysteresis patterns between Compton-dominated and thermal-dominated states, typically observed in black hole LMXBs, are also common in neutron star systems. These patterns, which also sample intermediate states, are found when looking at the evolution of both X-ray colour and fast variability of 10 systems accreting below ~30 per cent of the Eddington luminosity (LEdd). We show that hysteresis does not require large changes in luminosity and it is the natural form that state transitions take at these luminosities. At higher accretion rates, neutron stars do not show hysteresis, and they remain in a thermal-dominated, low-variability state, characterized by flaring behaviour and fast colour changes. Only at luminosities close to LEdd, are high variability levels seen again, in correspondence to an increase in the fractional contribution of the Comptonization component. We compare this behaviour with that observed in LMXBs harbouring black holes, showing that the spectral, timing and multiwavelength properties of a given source can be determined by its location in the fast variability-luminosity diagram, which, therefore, provides a common framework for neutron star and black hole accretion states. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.