Abstract
We present a multi-object optical spectroscopy follow-up study of X-ray sources in a field along the Galactic Plane (l=327.42 deg, b=2.26 deg) 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 H-alpha 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 8 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 h and 2.43+/-0.26 h. We present here clear evidence that the source is a CV through the detection of H, HeI and HeII emission lines in its optical spectrum. The hard X-ray spectrum and the presence of the HeII 4686 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 h 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.
Highlights
Cataclysmic variables (CVs) are semi-detached binary stars with orbital periods typically of the order of hours, consisting of a white dwarf (WD) primary accreting via Roche lobe overflow from a companion star which is usually a late-type, mainsequence star
We clearly show here that ChaMPlane Bright Source 7 (CBS 7) is a CV through the detection of H, He i, and He ii emission lines which are consistent with the presence of accretion as observed for many CVs (e.g., Williams 1983)
We presented multi-wavelength follow-up observations of an archived Chandra observation from the ChaMPlane survey
Summary
Cataclysmic variables (CVs) are semi-detached binary stars with orbital periods typically of the order of hours, consisting of a white dwarf (WD) primary accreting via Roche lobe overflow from a companion star which is usually a late-type, mainsequence star. Subtypes of CVs include non-magnetic CVs where an accretion disk forms around the primary and magnetic systems (MCVs) where the WD magnetic field is strong enough to truncate (intermediate polars, IPs, with 105 < B < 107 G) or disrupt (polars, B > 107 G) the accretion disk. Polars are synchronous, locked systems, but the magnetic field in IPs is generally not strong enough to tidally lock the WD to the companion star and IPs are asynchronous rotators. IPs tend to have larger orbital periods than polars, and the latter could be the end-products of the evolution of part of the IPs (e.g., Norton et al 2008)
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