The thermal X-ray spectra of Cen X-4, Aql X-1, and 4U 1608-522 in quiescence

We reanalyze the available X-ray spectral data of the type I bursting neutr on star transients Aql X-1, Cen X-4, and 4U 1608-522 using realistic hydrog en atmosphere models. Previous spectral fits assumed a blackbody spectrum; because the free-free-dominated photospheric opacity decreases with increas ing frequency, blackbody spectral fits overestimate the effective temperatu re and underestimate, by as much as 2 orders of magnitude, the emitting are a. Hydrogen atmosphere spectral models, when fit to the available observati onal data, imply systematically larger emission area radii, consistent with the canonical 10 km radius of a neutron star. This suggests that a substan tial fraction of the quiescent luminosity is thermal emission from the surf ace of the neutron star. The magnitude of the equivalent hydrogen column de nsity toward these systems, however, presents a considerable systematic unc ertainty, which can be eliminated only by high signal-to-noise X-ray spectr al measurements (e.g., with AXAF or XMM) that would permit simultaneous det ermination of the equivalent hydrogen column density, emission area, and th ermal temperature.