The Thermal X-Ray Spectra of Centaurus X-4, Aquila X-1 , and 4U 1608-522 in Quiescence

We reanalyze the available X-ray spectral data of the type I bursting neutron star transients Aql X-1, Cen X-4, and 4U 1608-522 using realistic hydrogen atmosphere models. Previous spectral fits assumed a blackbody spectrum; because the free-free-dominated photospheric opacity decreases with increasing frequency, blackbody spectral fits overestimate the effective temperature and underestimate, by as much as 2 orders of magnitude, the emitting area. Hydrogen atmosphere spectral models, when fit to the available observational data, imply systematically larger emission area radii, consistent with the canonical 10 km radius of a neutron star. This suggests that a substantial fraction of the quiescent luminosity is thermal emission from the surface of the neutron star. The magnitude of the equivalent hydrogen column density toward these systems, however, presents a considerable systematic uncertainty, which can be eliminated only by high signal-to-noise X-ray spectral measurements (e.g., with AXAF or XMM) that would permit simultaneous determination of the equivalent hydrogen column density, emission area, and thermal temperature.