The discovery of surface emission from old isolated neutron stars has long been regarded as a key to the understanding of their equation of state and has caused a wealth of theoretical work over the last three decades, see e.g. (Ostriker et al. 1970; Helfand et al. 1980). Old isolated neutron stars were predicted to be visible as soft X-ray sources within the reach of current satellite missions, see Treves & Colpi (1991), Blaes & Madau (1993) and also (Madau & Blaes 1994; Colpi et al. 1993). Recently, three sources have been suggested to be actual detections of old neutron stars accreting from the interstellar medium (Stocke et al. 1995; Walter et al. 1996; Haberl et al. 1996). However, the number of candidates for accreting neutron stars fall surprisingly short of the most conservative estimates. Paper I (Danner 1998) described a comprehensive identification program of sources in molecular clouds at high galactic latitude aimed at the systematic search for such objects. Please see Paper I for a brief introduction to accreting neutron stars.
In the present paper, I describe a survey of a large area in the galactic plane coincident with galactic dark clouds. These areas are most promising for the detection of old neutron stars accreting from the interstellar medium because: (1) only the slow members of the total neutron star population are expected to accrete and will be therefore concentrated toward the Galactic plane; (2) the clouds with their high densities of gas and dust provide ample material for accretion; (3) these clouds occupy a much larger volume than the local molecular clouds we find at high Galactic latitudes; and (4) the high extinction through dark clouds obscures background sources and reduces so the number of chance coincidences.
These favorable conditions entail three limitations: (1) the stellar density in the Galactic plane is very high. In most situations several objects are found within a typical error circle of 30 arcsecond. As a consequence, optical spectroscopy is required to resolve ambiguities already on a bright optical level. Very deep searches (mV > 20) are feasible only with significantly smaller error circles. (2) Dark clouds fill a large fraction of the entire Galactic plane and are not easily surveyed by any telescope. A systematic search will quickly grow into a project too large to manage within a realistic time frame. (3) The softest sources, deep inside a cloud will suffer substantial absorption from the material in the immediate vicinity of the source.
Two groups have recently published studies of ROSAT sources in a subset of galactic dark clouds. See Motch et al. for a comprehensive list of identifications in Cygus (1997a,b) and Belloni et al. (1997) for an identification of further candidate objects in the Cygnus and Cygnus OB7 region.
I have analyzed the statistical properties of a sample of bright X-ray sources coincident with the clouds identified by Dame et al. (1987) and compared this sample with the average population in the galactic plane. From the dark cloud sample I selected a small group of sources that stand out on their X-ray properties. I studied these sources in detail through optical imaging and spectroscopy. The three brightest sources in this sample have exciting properties. One of them is an independently rediscovered neutron star candidate. The other two sources are identified with hot white dwarf stars.