The faint X-ray sources in and out of ω Centauri: X-ray observations and optical identifications

We present the results of an observation of the globular cluster ω Cen (NGC 5139) with the Einstein high-resolution imager (HRI). Of the five low-luminosity X-ray sources toward ω Cen which were first identified with the Einstein imaging proportional counter (IPC) (Hertz & Grindlay 1983a, b), two are detected in the Einstein HRI observation: IPC sources A and D. These detections provide source positions accurate to 3″-4″; the positions are confirmed in a ROSAT HRI observation reported here. Using CCD photometry and spectroscopy, we have identified both sources as foreground dwarf M stars with emission lines (dMe). The chance projection of two dMe stars within ∼ 13′ of the center of ω Cen is not extraordinary, given the space density of these stellar coronal X-ray sources. We discuss the possible nature of the three as yet unidentified IPC sources toward ω Cen, and consider the constraints that the Einstein observations place on the total population of X-ray sources in this cluster. The integrated luminosity from faint X-ray sources in ω Cen appears to be low relative to both the old open cluster M67 and the post-core-collapse globular, NGC 6397.