Searching for old neutron stars with ROSAT

Abstract

This is the first in a series of three papers aiming to put strong observational constraints on the number of old accreting neutron stars in the Galaxy. Old neutron stars have been predicted to be visible as soft X-ray sources. The brightest sources are expected where the density of the interstellar medium is high. I present an X-ray survey of a complete sample of molecular clouds at high galactic latitudes based on the ROSAT All-Sky Survey. I detect 89 sources inside the projected contours of the molecular clouds and report on a comprehensive identification program. For all sources I searched astronomical catalogs and created finding charts. I also observed all sources at radio frequencies, most of them at 1.4 GHz and at 8 GHz. For sources with bright counterparts, optical spectra are presented. For fainter sources, optical CCD images are shown. Of all sources, 54 are securely identified and for the majority of the remaining sources likely identifications are given. I find no candidate for an old accreting neutron star in this sample. From this survey I derive an upper limit to the neutron star density of 50 sr^(-1) at a ROSAT count rate of 0.012 s^(-1).