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A&A Supplement series, Vol. 128, March I 1998, 349-357

Received December 16, 1996; accepted August 1, 1997

Searching for old neutron stars with ROSAT

II. Soft X-ray sources in galactic dark clouds

R. Danner tex2html_wrap1099

tex2html_wrap1101  Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
tex2html_wrap1103  Palomar Observatory, California Institute of Technology, Pasadena, CA 91125, USA


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 tex2html_wrap_inline1081 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 70eV. 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 tex2html_wrap_inline1083 at a count rate tex2html_wrap_inline1085.

keywords: surveys -- X-rays: stars -- stars: neutron; white dwarfs

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