Paper detail

The Chandra Galactic Bulge Survey: optical catalogue and point-source counterparts to X-ray sources

As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of optical sources in the GBS footprint. This consists of two regions centered at Galactic latitude b = 1.5 degrees above and below the Galactic Centre, spanning (l x b) = (6x1) degrees. The catalogue consists of 2 or more epochs of observations for each line of sight in r&#39;, i&#39; and Hα filters. It is complete down to r&#39; = 20.2 and i&#39; = 19.2 mag; the mean 5σ depth is r&#39; = 22.5 and i&#39; = 21.1 mag. The mean root-mean-square residuals of the astrometric solutions is 0.04 arcsec. We cross-correlate this optical catalogue with the 1640 unique X-ray sources detected in Chandra observations of the GBS area, and find candidate optical counterparts to 1480 X-ray sources. We use a false alarm probability analysis to estimate the contamination by interlopers, and expect ~ 10 per cent of optical counterparts to be chance alignments. To determine the most likely counterpart for each X-ray source, we compute the likelihood ratio for all optical sources within the 4σ X-ray error circle. This analysis yields 1480 potential counterparts (~ 90 per cent of the sample). 584 counterparts have saturated photometry (r&#39;<17, i&#39;<16), indicating these objects are likely foreground sources and the real counterparts. 171 candidate counterparts are detected only in the i&#39;-band. These sources are good qLMXB and CV candidates as they are X-ray bright and likely located in the Bulge.