The Quiescent Emission Spectrum of Cen 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 constrain models for the accretion flow 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 Flow (ADAF). Emission from the bright spot could account for the observed optical-UV component if the mass transfer rate in Cen X-4 is >~ 2.10^16 g/s. Although the presence of an ADAF around the neutron star leads to Compton upscattering of the soft X-ray photons radiated from the stellar surface, we find 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 effect 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.