The Einstein Galactic Plane Survey. II. Optical Observations of Northern Hemisphere X-Ray Sources

A statistically complete sample of x-ray sources observable from the northern hemisphere has been chosen from the Einstein Galactic Plane Survey, a flux- limited x-ray survey of the Galactic plane. The subset of the Galactic Plane Survey containing this northern sample covered 185 sq. deg of the Galactic plane with a median-limiting flux of 0.032 IPC counts s^-1^ (~7.4 X 10^-13^ erg cm^-2^ s^-1^). The northern sample contains 67 x-ray sources. A program of photometric and spectroscopic observations has been undertaken, and optical identifications are proposed for 45 (~67%) of the sources. Most of the unidentified sources must be noncoronal in nature, and statistical arguments imply that the majority of the unidentified sources are extragalactic. There is no evidence for a large class of low-luminosity Galactic accretion sources in the northern sample, and we place constraints on the spatial density of any such class. We present the results of our observing program, including finding charts of the unidentified x-ray sources. We note that the x-ray source 1E 1926.0 + 1927 is identified with the Wolf-Rayet star Roberts 93 and has an anomolously high x-ray-to-optical flux ratio for a Wolf-Rayet star. We suggest that this source may be a high-mass x-ray binary system containing a neutron star accreting stellar-wind-fed material from the Wolf-Rayet star.