Chandra Discovery of an Intermediate Polar in Baade's Window

Abstract

(Abridged) We have discovered an intermediate polar (IP) in the 100 ks Chandra observation of Baade's Window (BW), a low extinction region at ~ 4 deg south of the Galactic Center. The source exhibits large X-ray modulations at a period of 1028.4 s in the 0.3 - 8 keV band. The X-ray spectral fit with a power law model shows the integrated spectrum is intrinsically hard (photon index ~ 0.44) and moderately absorbed (NH22 ~ 0.15). Quantile analysis reveals that the modulations in the X-ray flux strongly correlate with spectral changes that are dominated by varying internal absorption. The X-ray spectrum of the source is heavily absorbed (NH22 > 1) during the faint phases, while the absorption is consistent with the field value during the bright phases. These X-ray properties are typical signatures of IPs. Images taken with the IMACS camera on the Magellan 6.5m telescope show a faint (V ~ 22), relatively blue object within the 2 sigma error circle of the Chandra source. If we assume a nominal range of absolute V magnitude for a cataclysmic variable (MV ~ 5.5-10.5) and the known reddening in the region, the source would likely be at a distance of 2-10 kpc. The corresponding average Lx would be $6\times 10^{31}-10^{33}$ erg s${}^{-1}$ in the 2-8 keV band. Assuming the space density of IPs follows the stellar distribution, which is highly concentrated in the Galactic Bulge, the source is probably a relatively bright IP belonging to the Galactic Bulge X-ray population, the majority of which is now believed to be magnetic cataclysmic variables.

6 pages, 6 figures, 1 table, accepted for publication in Astrophysical Journal

References in corpus (12)

• Modelling the Galactic Interstellar Extinction Distribution in Three Dimensions
• The XMM-Newton Serendipitous Survey. V. The Second XMM-Newton Serendipitous Source Catalogue
• The spin periods and magnetic moments of white dwarfs in magnetic cataclysmic variables
• The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center
• X-ray Processing of ChaMPlane Fields: Methods and Initial Results for Selected Anti-Galactic Center Fields
• New spectral classification technique for X-ray sources: quantile analysis
• Chandra Multiwavelength Plane (ChaMPlane) Survey: an Introduction
• Constraining the Nature of the Galactic Center X-ray Source Population
• ChaMPlane Optical Survey: Mosaic Photometry
• The Nature of the Faint Chandra X-ray Sources in the Galactic Centre
• ChaMPlane Discovery of Candidate Symbiotic Binaries in Baade's and Stanek's Windows
• Cataclysmic Variables in Globular Clusters, the Galactic Center, and Local Space

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• Chasing the Identification of ASCA Galactic Objects (ChIcAGO) - An X-ray Survey of Unidentified Sources in the Galactic Plane I: Source Sample and Initial Results
• Spectroscopic Follow-up of X-ray Sources in the ChaMPlane Survey: Identification of a New Cataclysmic Variable
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• Multi-Epoch X-ray Imaging of Globular Cluster M62 with Chandra
• Deep Chandra Survey of the Small Magellanic Cloud. II. Timing Analysis of X-ray Pulsars
• New transient Galactic bulge intermediate polar candidate XMMU J175035.2-293557