Abstract
(Abridged) We present the LogN-LogS and spatial distributions of X-ray point sources in seven Galactic Bulge (GB) fields within 4 deg from the Galactic Center (GC). We compare the properties of 1159 X-ray point sources discovered in our deep (100 ks) Chandra observations of three low extinction Window fields near the GC with the X-ray sources in the other GB fields centered around Sgr B2, Sgr C, the Arches Cluster and Sgr A* using Chandra archival data. To reduce the systematic errors induced by the uncertain X-ray spectra of the sources coupled with field-and-distance dependent extinction, we classify the X-ray sources using quantile analysis and estimate their fluxes accordingly. The result indicates the GB X-ray population is highly concentrated at the center, more heavily than the stellar distribution models. We also compare the total X-ray and infrared surface brightness using the Chandra and Spitzer observations of the regions. The radial distribution of the total infrared surface brightness from the 3.6 band $\mu$m images appears to resemble the radial distribution of the X-ray point sources better than predicted by the stellar distribution models. Assuming a simple power law model for the X-ray spectra, the closer to the GC the intrinsically harder the X-ray spectra appear, but adding an iron emission line at 6.7 keV in the model allows the spectra of the GB X-ray sources to be largely consistent across the region. This implies that the majority of these GB X-ray sources can be of the same or similar type. Their X-ray luminosity and spectral properties support the idea that the most likely candidate is magnetic cataclysmic variables (CVs), primarily intermediate polars (IPs). Their observed number density is also consistent with the majority being IPs.
Highlights
The Chandra X-ray Observatory has opened a new era in studies of the X-ray source population in the Galactic bulge (GB)
While the leading candidate that fits the properties of these X-ray sources is magnetic cataclysmic variables (CVs; Muno et al 2004, L05), the relatively hard X-ray spectra of some of the most recently discovered quiescent low-mass X-ray binaries (qLMXBs) imply qLMXBs could be misrecognized as CVs and be more common in the GB than thought in the past (Wijnands et al 2005; Bogdanov et al 2005)
The results indicate that the Eddington bias (EB) is noticeable in the sources with
Summary
The Chandra X-ray Observatory has opened a new era in studies of the X-ray source population in the Galactic bulge (GB). The X-ray luminosities and relatively hard spectra ruled out that the majority of the GC X-ray point sources are normal stars, active binaries, young stellar objects, or quiescent low-mass X-ray binaries (qLMXBs; M03). We have conducted a series of deep (100 ks) Chandra observations of three low extinction Window fields—Baade’s Window (BW), Stanek’s Window (SW; Stanek 1998), and the “Limiting Window” (LW)—near the GC (Section 2). These Window fields allow us to observe the GB X-ray population and their Galactic radial distribution with minimal obscuration by dust. This work is part of our Chandra Multi-wavelength Plane (ChaMPlane) Survey designed to measure the space density and probable nature of the low-luminosity accretionpowered sources in the Galaxy (Grindlay et al 2005)
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