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

Abstract

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 • above and below the Galactic Centre, spanning (l × b) = (6 • ×1 • ). The catalogue consists of 2 or more epochs of observations for each line of sight in r , i and Hα filters. The catalogue is complete down to r = 20.2 and i = 19.2 mag; the mean 5σ depth is r = 22.5 and i = 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 17, i 16), indicating these objects are likely foreground sources and the real counterparts. 171 candidate counterparts are detected only in the i -band. These sources are good qLMXB and CV candidates as they are X-ray bright and likely located in the Bulge.

Candidate Hα emission and absorption line sources in the Galactic Bulge Survey

“…As the starting point of our work, we use the optical point source catalogue covering the GBS fields (Wevers et al 2016a). This catalogue consists of optical photometry obtained using the Mosaic-2 camera on the Victor M. Blanco telescope, located at the Cerro Tololo Inter-American Observatory (CTIO), in three filters: r , i and Hα.…”

Spectroscopic classification of X-ray sources in the Galactic Bulge Survey

“…Figure 15. The source was detected at r = 17.45 mag in the DECam images, 0.85 mag brighter than the earlier detection (in 2006) presented by Wevers et al (2016a). This indicates that the source is optically variable, although we caution that it is unresolved in the DECam data and we cannot completely rule out that another source is causing the apparent variability if it is blended.…”

“…The original X-ray error circle of CX266 (with radius 6. 49) contains two bright optical sources, one of which is saturated in the i -band photometry of Wevers et al (2016a). We designate this source A, while the other object, which was identified as a photometric H α outlier in Wevers et al (2017), is named source B.…”

Discovery of a long-lived, high-amplitude dusty infrared transient

“…In addition, the GBS collaboration took initial photometry of the bulge region prior to the X-ray observations in order to identify H α excess sources and provide a baseline for spectroscopic follow-up (Wevers et al 2016). In addition to these early observations, we have light curves and magnitudes in multiple colours from several other surveys from after the outburst began, and have both optical and near-infrared (NIR) spectroscopy of CX330 during outburst.…”

“…The catalogue of sources in the 12 deg 2 region is complete to r = 20.2 and i = 19.2, while the mean 5σ depth is r = 22.5 and i = 21.1 (Wevers et al 2016). Data reduction was carried out using a pipeline created by the Cambridge Astronomical Survey Unit (CASU; González-Solares et al 2008).…”