Searching for old neutron stars with ROSAT. II. Soft X-ray sources in galactic dark clouds

This is the second in a series of three papers constraining the number of detectable old neutron stars in the Galaxy. Here, I present the statistical analysis of a sample of X-ray sources coincident with areas of dark clouds in the Galactic plane. I compare this sample with all sources in the ROSAT All-Sky Survey bright source catalog within 20° of the Galactic plane. I present the results of an identification program of a subset of sources that are compatible with a soft, thermal X-ray spectrum and an effective source temperature of less than 70 eV. The three brightest sources in this sample form an intriguing subgroup. One of them is a previously identified candidate for an accreting neutron star. I identify the other two sources with hot white dwarf stars. I find no new accreting neutron star candidate in this sample. Based on this result, I derive an upper limit to the space density of accreting neutron stars in fields of Galactic dark clouds of ~ 2 sr^(-1) at a count rate > 0.05 s^(-1)