The ChaMPlane bright X-ray sources - Galactic longitudes l = 2-358 deg

TL;DR

This study analyzes bright X-ray sources in the Galactic plane to identify their nature, discovering accreting white-dwarf binaries, active stars, and other objects, and providing a basis for classifying fainter sources.

Contribution

It presents the first detailed classification of the brightest ChaMPlane X-ray sources, revealing new accreting white-dwarf binaries and characterizing the source population.

Findings

Discovered two likely accreting white-dwarf binaries.

Identified a transient X-ray source possibly a very late-type dwarf.

Found the log N - log S relation to be flatter than isotropic expectations.

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.