Discovery of a Significant Magnetic Cataclysmic Variable Population in the Limiting Window

Abstract

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) similar to 3.9) at 1 degrees.4 south of the Galactic center. The observed periods (similar to 1.3-3.4 hr) 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 them exhibit a relatively hard X-ray spectrum (Gamma < 2 for a power-law model), and relatively high extinction observed in the X-ray spectra of at least five sources indicates some intrinsic absorption in the system, which is also a typical sign of MCVs. On close inspection, 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 that greater than or similar to 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 and further supports the current view that MCVs constitute the majority of low-luminosity hard X-ray sources (similar to 10(30-33) erg s(-1)) in the bulge.