Accretion-rate threshold for thin-to-radiatively-inefficient transition in NS LMXBs

Determine the precise mass accretion rate, equivalently the Eddington fraction F_Edd, at which neutron star low-mass X-ray binaries transition from standard thin disk accretion to a radiatively inefficient accretion flow, by identifying the onset of the disappearance or weakening of disk reflection features across outburst decay trajectories.

Background

Standard thin disk accretion in neutron star low-mass X-ray binaries is expected to give way to a radiatively inefficient accretion flow at sufficiently low accretion rates. Reflection spectroscopy provides a way to track the inner disk and its emission features as sources evolve across accretion states.

Observations show mixed behavior at low Eddington fractions: for example, 4U 0614+091 has a disk extending to the innermost stable circular orbit at F_Edd < 0.01, while 4U 1608–52 exhibits a disappearance of reflection features at F_Edd ~ 0.002, suggesting a transition to a radiatively inefficient flow. These findings imply the threshold lies somewhere between F_Edd ≈ 0.002 and 0.006, but the exact value remains undetermined.