Bulge Windows Survey
As a complement to our ChaMPlane survey, we have conducted a deep survey of low-extinction windows in the Galactic Bulge. The three windows — at 4°, 2° and 1.5° from the Galactic Center — were observed (nearly) simultaneously with Chandra and the Wide Field Camera on the Advanced Camera for Surveys (ACS) on HST. The combination of the superb spatial resolutions of both Chandra and HST provides accurate source identifications and classifications for X-ray sources down to a limiting unabsorbed flux Of 3.7-4.8 10-15 erg s-1 cm-2 (2-8 keV). The windows lie on a nearly radial alignment relative to the Galactic Center, and together with the results of our ChaMPlane and Bulge Latitude Surveys, will constrain the large-scale distribution of low-luminosity point sources in the Galactic-Center and Galactic-Bulge regions.
The Chandra and HST observations for the Bulge Window Survey were carried out from 2003 to 2005. This image shows the location of the three windows on an extinction map of the Galactic-Bulge region. The visual extinctions AV for Baade's Window (BW), Stanek's Window (SW) and the Limiting Window (LW) are approximately 1.6, 2.6 and 3.9, respectively. For each window, four HST/ACS pointings are arranged in a 2 x 2 mosaic centered close to the Chandra aimpoint, cover a region of 6.7′x 6.7′. The HST/ACS images were taken through B, R and Hα filters, to optimize the possibility to identify accretion-driven sources. (click image to enlarge)
The three windows have been imaged in the optical with the
CTIO-4m Mosaic imager (VRI and Hα); the moderate-extinction
Limiting Window has also been observed in the near-infrared (JHK and
Brγ) with ISPI on the CTIO-4m. These data, that cover the full
field observed by ACIS, will be used to look for bright candidate
counterparts outside the regions covered by HST/ACS.
Our search for accreting binaries has already resulted in several candidate cataclysmic variables in each of the windows, that distinguish themselves by their blue and Hα-bright colors compared to the overall star population in the field. A first paper on candidate accreting binaries in the Limiting Window describes identifications and constraints on the space density of cataclysmic variables. Aided by low- and high-resolution spectra obtained with the FAST spectrograph (FLWO-1.5m), the Hydra spectrograph (CTIO-4m) and NIRSPEC (Keck) we have identified a population of candidate symbiotic binaries, in which a white dwarf accretes from the wind of an evolved giant (van den Berg et al. 2006, ApJL 647, 135).