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

TL;DR

This study reveals that neutron star low mass X-ray binaries exhibit hysteresis patterns similar to black hole systems at certain luminosities, with state transitions influenced by accretion rates and variability, suggesting a unified accretion state framework.

Contribution

It demonstrates that hysteresis in neutron star LMXBs occurs at sub-Eddington luminosities and provides a comparative analysis with black hole systems, proposing a unified variability-luminosity framework.

Findings

Hysteresis patterns are common in neutron star LMXBs at low accretion rates.

Neutron stars do not show hysteresis at higher accretion rates, remaining in a thermal state.

A variability-luminosity diagram can unify the understanding of neutron star and black hole accretion states.

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 ten systems accreting below ~ 30 % of the Eddington Luminosity. 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 the Eddington Luminosity, 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 multi-wavelength 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.