Simplified picture of low-mass X-ray binaries based on data from aquila X-1 and 4U 1608-52

Abstract

We propose a simplified picture of low-mass X-ray binaries containing a neutron star (NS-LMXBs) based on data obtained from Aql X-1 and 4U 1608-52, which often produce outbursts. In this picture we propose at least three states and three state transitions: i.e., the states: (1) soft state, (2) hard-high state, and (3) hard-low state, and the state transitions: (i) hard-high state to soft state, (ii) soft state to hard-high state, and (iii) hard-high state to hardlow state or vice versa. Gases from the accretion disc of an NS-LMXB penetrate almost the entire magnetic field and accrete onto the neutron star in cases (1) and (2), whereas in case (3) some gases accrete around the magnetic poles in a manner resembling the behavior of an X-ray pulsar, and considerable gas is dispersed or ejected by the propeller effect. Transition (iii) occurs when the Alfv́en radius is equal to the co-rotation radius. Therefore, in this case it is possible to estimate the strength of the neutron star's magnetic field by detecting transition (iii). We also discuss the no-accretion X-ray state or the recycled pulsar state, in which the Alfv́en radius is larger than the light cylinder radius. © 2013 Astronomical Society of Japan.