Spectroscopic Follow-up of X-Ray Sources in the ChaMPlane Survey: Identification of a New Cataclysmic Variable

We present a multi-object optical spectroscopy follow-up study of X-ray sources in a field along the Galactic plane (l = 327fdg42, b = 2fdg26) which is part of the Chandra Multi-wavelength Plane survey (ChaMPlane). We obtained spectra for 46 stars, including 15 likely counterparts to X-ray sources, and sources showing an Hα color excess. This has led to the identification of a new cataclysmic variable (CV), CXOPS J154305.5-522709, also named ChaMPlane Bright Source 7 (CBS 7), and we identified eight X-ray sources in the field as active late-type stars. CBS 7 was previously studied in X-rays and showed a hard spectrum and two periods: 1.22 ± 0.08 hr and 2.43 ± 0.26 hr. We present here clear evidence that the source is a CV through the detection of H, He I, and He II emission lines in its optical spectrum. The hard X-ray spectrum and the presence of the He II λ4686 in emission with a large equivalent width suggest a magnetic CV. The near-infrared counterpart is significantly variable, and we found a period consistent with the longest X-ray period at 2.39 ± 0.05 hr but not the shortest X-ray period. If this period is the orbital period, this would place the system in the CV period gap. The possible orbital period suggests a dM4 ± 1 companion star. The distance is then estimated to be ~1 kpc. The system could be a relatively hard and X-ray luminous polar or an intermediate polar, possibly nearly synchronous.