On the Origin of Quiescent Neutron Star Emission in Low-Mass X-ray Binaries

Many accreting neutron stars in low-mass X-ray binaries are transient X-ray sources, undergoing bright X-ray outbursts lasting a few weeks alternating with long quiescent intervals lasting months to years. The origin of their faint quiescent power-law X-ray emission has been a longstanding mystery, with theorists debating between Comptonization and synchrotron shock models. Recent observations of the nearby system Cen X-4 with the NuSTAR hard X-ray telescope have revealed an unexpected bremsstrahlung origin for this emission. I will discuss the implication of these results for low-luminosity accretion flows, and compare with the recently discovered "transitional" millisecond pulsars which exhibit markedly different behavior at comparable X-ray luminosity.