The Quiescent Emission Spectrum of Centaurus X‐4 and Other X‐Ray Transients Containing Neutron Stars

Abstract

We use the observed optical-UV and X-ray emission spectrum of Cen X-4 during quiescence to con-strain models for the accretion Ñow in this system. We argue that the optical-UV emission is not due to an optically thick quiescent accretion disk, nor due to synchrotron emission from an advection-dominated accretion Ñow (ADAF). Emission from the bright spot could account for the observed optical-UV component if the mass transfer rate in Cen X-4 is g s~1. Although the presence of Z2 ] 1016 an ADAF around the neutron star leads to Compton upscattering of the soft X-ray photons radiated from the stellar surface, we Ðnd that this process alone cannot account for the power-law component seen in the quiescent X-ray spectrum of Cen X-4 and other X-ray transients containing neutron stars ; this result is independent of whether the source of soft photons is incandescent thermal emission or accretion-powered emission. We conclude that, in models which invoke the presence of an ADAF and a propeller e ect for the quiescence of X-ray transients containing neutron stars, the intrinsic emission from the ADAF must contribute very little to the optical-UV and X-ray emission observed. If these ADAF ] propeller models are correct, the X-ray power-law component observed must arise from regions where the gas impacts the neutron star surface. Variability studies could greatly help clarify the role of the various emission mechanisms involved.