Abstract
[Abridged] We have discovered 10 periodic X-ray sources from the 1 Ms Chandra ACIS observation of the Limiting Window (LW), a low extinction region (A_V~3.9) at 1.4 Deg south of the Galactic center. The observed periods (~1.3 to 3.4 hours) and the X-ray luminosities (10^{31.8-32.9} erg s^-1 at 8 kpc) of the 10 periodic sources, combined with the lack of bright optical counterparts and thus high X-ray-to-optical flux ratios, suggest that they are likely accreting binaries, in particular, magnetic cataclysmic variables (MCVs). All of the 10 sources exhibit a relatively hard X-ray spectrum (PLI<2 for a power law model) and X-ray spectra of at least five show an extinction larger than the field average expected from the interstellar medium in the region. The discovery of these periodic X-ray sources in the LW further supports the current view that MCVs constitute the majority of low luminosity hard X-ray sources (~10^{30-33} erg s^-1) in the Bulge. The period distribution of these sources resembles those of polars, whereas the relatively hard spectra suggest that they could be intermediate polars (IPs). These puzzling properties can be explained by unusual polars with buried magnetic fields or a rare sub-class of MCVs, nearly synchronous MCVs. These unusual MCVs may provide important clues in the evolutionary path of MCVs from IPs to polars. The completeness simulation indicates >~40% of the hard X-ray sources in the LW are periodic. Therefore, this discovery provides a first direct evidence of a large MCV population in the Bulge.
Highlights
The high sensitivity and superb spatial resolution of Chandra enabled a population study of low-luminosity X-ray sources (LX ∼ 1030–34 erg s−1) on Galactic scales beyond the local solar neighborhood
In ChaMPlane, we study the bulge through our dedicated surveys of low-extinction bulge regions (“Windows survey”) and a latitudinal strip around the Galactic center (“Bulge Latitude Survey”), where thousands of X-ray sources have been discovered
If most of these periodic sources are located near the Galactic center (8 kpc) as expected from the high absorption in their X-ray spectra and the high stellar density of the bulge, the X-ray luminosities of these sources are estimated at the high end of the magnetic cataclysmic variables (MCVs) range ( 1032 erg s−1, see Table 3), which is consistent with the above picture
Summary
The high sensitivity and superb spatial resolution of Chandra enabled a population study of low-luminosity X-ray sources (LX ∼ 1030–34 erg s−1) on Galactic scales beyond the local solar neighborhood. Several ongoing campaigns, including our own Chandra Multi-wavelength Plane (ChaMPlane) survey (Grindlay et al 2005), aim to improve the census of the Galactic low-luminosity X-ray sources. In ChaMPlane, we study the bulge through our dedicated surveys of low-extinction bulge regions (“Windows survey”) and a latitudinal strip around the Galactic center (“Bulge Latitude Survey”), where thousands of X-ray sources have been discovered We report the discovery of 10 periodic X-ray sources from the 1 Ms Chandra exposure of the Limiting Window (LW), a low-extinction region at 1◦.4 south of the Galactic center (see H09a). Revnivtsev et al (2009, hereafter R09) showed that the Galactic ridge X-ray emission (GRXE), the nature of which has been puzzling for decades, is mainly made up of discrete faint sources of known nature, primarily active binaries (ABs) or cataclysmic variables (CVs), based on the Chandra observations of the LW. We explore the X-ray and optical properties of the 10 periodic X-ray sources in the LW, their implication for evolutionary models of MCVs, and their connection to thousands of X-ray sources in the GCR
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