  We re-analyze 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 two 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 only be eliminated by high signal-to-noise X-ray
spectral measurements (e.g., with AXAF or XMM) which would permit simultaneous
determination of the equivalent hydrogen column density, emission area, and
thermal temperature.
