Chandra Discovery of an Intermediate Polar in Baade's Window

We have discovered an intermediate polar (IP) in the 100 ks Chandra observation of Baade's window (BW), a low extinction region at about 4° 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 that the integrated spectrum is intrinsically hard (photon index Γ = 0.44 ± 0.05) and moderately absorbed (N_H = 1.5 ± 1.0 × 10²¹ cm^-2). The relatively poor statistics only allow for a mild constraint on the presence of an iron emission line (equivalent width = 0.5 ± 0.3 keV at 6.7 keV). 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 (N_H > 10²² cm^-2) during the faint phases, while the absorption is consistent with the field value (~10²¹ cm^-2) during the bright phases. These X-ray properties are typical signatures of IPs. Images taken with the IMACS camera on the Magellan 6.5 m telescope show a faint (V ~ 22), relatively blue object (B ₀ - V ₀ gsim 0.05) within the 2σ error circle of the Chandra source, which is a good candidate for being the optical counterpart. If we assume a nominal range of absolute V magnitude for a cataclysmic variable (M_V ~ 5.5-10.5) and the known reddening in the region (A_V = 1.4 at >3 kpc), the source would likely be at a distance of 2-10 kpc and not in the local solar neighborhood. The corresponding average X-ray luminosity would be 6 × 10³¹ -10³³ 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 (~10³³ erg s^-1 if it is at 8 kpc) belonging to the Galactic bulge X-ray population, the majority of which is now believed to be magnetic cataclysmic variables.