Abstract
The Chandra Multiwavelength Plane (ChaMPlane) Survey aims to constrain the Galactic population of mainly accretion-powered, but also coronal, low-luminosity X-ray sources (Lx <~ 1e33 erg/s). To investigate the X-ray source content in the plane at fluxes Fx >~ 3e-14 erg/s/cm^2, we study 21 of the brightest ChaMPlane sources, viz. those with >250 net counts (0.3-8 keV). By excluding the heavily obscured central part of the plane, our optical/near-infrared follow-up puts useful constraints on their nature. We have discovered two likely accreting white-dwarf binaries. CXOPS J154305.5-522709 (CBS 7) is a cataclysmic variable showing periodic X-ray flux modulations on 1.2 hr and 2.4 hr; given its hard spectrum the system is likely magnetic. We identify CXOPS J175900.8-334548 (CBS 17) with a late-type giant; if the X-rays are indeed accretion-powered, it belongs to the small but growing class of symbiotic binaries lacking strong optical nebular emission lines. CXOPS J171340.5-395213 (CBS 14) is an X-ray transient that brightened >~100 times. We tentatively classify it as a very late-type (>M7) dwarf, of which few have been detected in X-rays. The remaining sources are (candidate) active galaxies, normal stars and active binaries, and a plausible young T Tauri star. The derived cumulative number density versus flux (log N - log S) relation for the Galactic sources appears flatter than expected for an isotropic distribution, indicating that we are seeing a non-local sample of mostly coronal sources. Our findings define source templates that we can use, in part, to classify the >1e4 fainter sources in ChaMPlane.
Highlights
The Galactic population of low-luminosity X-ray sources (LX 1033 erg s−1) includes normal and active single stars and binaries, pre-main-sequence (PMS) stars, millisecond pulsars, and close binaries containing accreting compact objects (white dwarfs in cataclysmic variables (CVs); neutron stars and black holes in quiescent high- and low-mass X-ray binaries)
The ChaMPlane X-ray source catalog contains ∼15,000 sources from archival Advanced CCD Imaging Spectrometer (ACIS) imaging data at Galactic latitudes | b | < 12◦ that meet the primary criteria established in Grindlay et al (2005): preferably ACIS-I exposures with exposure times 20 ks and without bright or extended targets that limit the sensitivity to serendipitous detections
The quality flags indicate that the J and H magnitudes for CBS 19 are potentially contaminated by an image artifact. This star is included in the UKIDSS Galactic Plane Survey (GPS) catalog (Lucas et al 2008); a comparison of the J and H magnitudes shows them to be very similar to the 2MASS values with differences in the NIR colors of only van den Berg et al Figure 2
Summary
The Galactic population of low-luminosity X-ray sources (LX 1033 erg s−1) includes normal and active single stars and binaries, pre-main-sequence (PMS) stars, millisecond pulsars, and close binaries containing accreting compact objects (white dwarfs in cataclysmic variables (CVs); neutron stars and black holes in quiescent high- and low-mass X-ray binaries (qHMXBs; qLMXBs)). With XMM-Newton and especially Chandra, which combine sensitivity with excellent spatial resolution, existing imaging Galactic X-ray surveys (e.g., Hertz & Grindlay 1984) can be extended with two to three orders of magnitude higher sensitivity down to 10−15 erg s−1 cm−2 and with precise searches for counterparts at other wavelengths. This is what drives several ongoing campaigns, like the XMM-Newton Galactic Plane Survey (XGPS; Hands et al 2004) and our own Chandra Multi-wavelength Plane (ChaMPlane) survey (Grindlay et al 2005). Preliminary results were reported in Penner et al (2008)
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