CHANDRA DISCOVERY OF AN INTERMEDIATE POLAR IN BAADE'S WINDOW
- Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 (United States)
- Gemini Observatory, 670 N. A'ohoku Place, Hilo, HI 96720 (United States)
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 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 that the integrated spectrum is intrinsically hard (photon index {gamma} = 0.44 {+-} 0.05) and moderately absorbed (N{sub H} = 1.5 {+-} 1.0 x 10{sup 21} cm{sup -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{sub H} > 10{sup 22} cm{sup -2}) during the faint phases, while the absorption is consistent with the field value ({approx}10{sup 21} cm{sup -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 {approx} 22), relatively blue object (B {sub 0} - V {sub 0} {approx}> 0.05) within the 2{sigma} 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{sub V} {approx} 5.5-10.5) and the known reddening in the region (A{sub 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 x 10{sup 31} -10{sup 33} erg s{sup -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 ({approx}10{sup 33} erg s{sup -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.
- OSTI ID:
- 21333773
- Journal Information:
- Astrophysical Journal, Vol. 699, Issue 2; Other Information: DOI: 10.1088/0004-637X/699/2/1053; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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