We present the first optical spectroscopy of five confirmed (or strong candidate) redback millisecond pulsar binaries, obtaining complete radial velocity curves for each companion star. The properties of these millisecond pulsar binaries with low-mass, hydrogen-rich companions are discussed in the context of the 14 confirmed and 10 candidate field redbacks. We find that the neutron stars in redbacks have a median mass of 1.78 ± 0.09M with a dispersion of σ = 0.21 ± 0.09. Neutron stars with masses in excess of 2M are consistent with, but not firmly demanded by, current observations. Redback companions have median masses of 0.36 ± 0.04M with a scatter of σ = 0.15 ± 0.04M , and a tail possibly extending up to 0.7-0.9M . Candidate redbacks tend to have higher companion masses than confirmed redbacks, suggesting a possible selection bias against the detection of radio pulsations in these more massive candidate systems. The distribution of companion masses between redbacks and the less massive black widows continues to be strongly bimodal, which is an important constraint on evolutionary models for these systems. Among redbacks, the median efficiency of converting the pulsar spindown energy to γ-ray luminosity is ∼ 10%.

We introduce the Galactic Bulge Survey (GBS) and we provide the Chandra source list for the region that has been observed to date. Among the goals of the GBS are constraining the neutron star equation of state and the black hole mass distribution via the identification of eclipsing neutron star and black hole low-mass X-ray binaries. The latter goal will, in addition, be obtained by significantly enlarging the number of black hole systems for which a black hole mass can be derived. Further goals include constraining X-ray binary formation scenarios, in particular the common envelope phase and the occurrence of kicks, via source-type number counts and an investigation of the spatial distribution of X-ray binaries, respectively. The GBS targets two strips of 6 • × 1 • (12 square degrees in total), one above (1 • < b < 2 • ) and one below (−2 • < b < −1 • ) the Galactic plane in the direction of the Galactic Center at both X-ray and optical wavelengths. By avoiding the Galactic plane (−1 • < b < 1 • ) we limit the influence of extinction on the X-ray and optical emission but still sample relatively large number densities of sources. The survey is designed such that a large fraction of the X-ray sources can be identified from their optical spectra. The X-ray survey, by design, covers a large area on the sky while the depth is shallow using 2 ks per Chandra pointing. In this way we maximize the predicted number ratio of (quiescent) low-mass X-ray binaries to Cataclysmic Variables. The survey is approximately homogeneous in depth to an 0.5-10 keV flux of 7.7×10 −14 erg cm −2 s −1 . So far, we have covered about two-thirds (8.3 square degrees) of the projected survey area with Chandra providing over 1200 unique X-ray sources. We discuss the characteristics and the variability of the brightest of these sources.

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We provide the Chandra source list for the last ≈quarter of the area covered by the Galactic Bulge Survey (GBS). The GBS targets two strips of 6 • × 1 • (12 square degrees in total), one above (1 • < b < 2 • ) and one below (−2 • < b < −1 • ) the Galactic plane in the direction of the Galactic Center at Xray, optical and near-infrared wavelengths. For the X-ray part of the survey we use 2 ks per Chandra pointing. We find 424 X-ray sources in the 63 Chandra observations we report on here. These sources are in addition to the 1216 X-ray sources discovered in the first part of the GBS survey described before. We discuss the characteristics and the X-ray variability of the brightest of the sources as well as the radio properties from existing radio surveys. We point out an interesting asymmetry in the number of X-ray sources as a function of their Galactic l and b coordinates which is probably caused by differences in average extinction towards the different parts of the GBS survey area.

We are undertaking a survey to characterize the X-ray sources found with the Chandra X-ray observatory in a strip of fields at −3 • < l < 3 • , b = +1.5 • and −3 • < l < 3 • , b = −1.5 • . This so-called Galactic Bulge Survey (GBS) targets X-ray emitting binaries in the bulge with the primary purpose of finding quiescent X-ray binaries. The aims of this survey are to quantify dynamically the mass of compact objects in these X-ray binaries in order to constrain the neutron star equation of state and to test black hole formation models. In addition, using the survey number counts of various sources we aim to test models for binary formation and evolution. Here, we present the identification of optical counterparts to twenty-three GBS X-ray sources. We report their accurate coordinates and medium resolution optical spectra acquired at the Very Large Telescope and Magellan. All sources are classified as accreting binaries according to their emission line characteristics. To distinguish accreting binaries from chromospherically active objects we develop and explain criteria based on Hα and Hei λλ5786, 6678 emission line properties available in the literature. The spectroscopic properties and photometric variability of all the objects are discussed and a classification of the source is given where possible. Among the twenty-three systems, at least nine of them show an accretion-dominated optical spectrum (CX28, CX63, CX70, CX128, CX142, CX207, CX522, CX794 CX1011) and another six show photospheric lines from a late-type donor star in addition to accretion disc emission (CX44, CX93, CX137, CX154, CX377 and CX1004) indicating that they are probably accreting binaries in quiescence or in a low accretion rate state. Two sources are confirmed to be eclipsing: CX207 and CX794. CX207 shows a broad asymmetric Hα profile blue-shifted by > 300 km s −1 . Such line profile characteristics are consistent with a magnetic (Polar) cataclysmic variable. CX794 is an eclipsing nova-like cataclysmic variable in the period gap. Time-resolved photometry and the large broadening of the Hα emission lines in CX446 (2100 km s −1 Full-Width at Half Maximum; FWHM) suggest that this is also an eclipsing or high-inclination accreting binary. Finally, the low-accretion rate source CX1004 shows a double-peaked Hα profile with a FWHM of 2100 km s −1 . This supports a high inclination or even eclipsing system. Whether the compact object is a white dwarf in an eclipsing cataclysmic variable or a black hole primary in a high-inclination low-mass X-ray binary remains to be established.

We present optical lightcurves of variable stars consistent with the positions of X-ray sources identified with the Chandra X-ray Observatory for the Chandra Galactic Bulge Survey. Using data from the Mosaic-II instrument on the Blanco 4m Telescope at CTIO, we gathered time-resolved photometric data on timescales from ∼ 2 hr to 8 days over the 3 4 of the X-ray survey containing sources from the initial GBS catalog. Among the lightcurve morphologies we identify are flickering in interacting binaries, eclipsing sources, dwarf nova outbursts, ellipsoidal variations, long period variables, spotted stars, and flare stars. 87% of X-ray sources have at least one potential optical counterpart. 24% of these candidate counterparts are detectably variable; a much greater fraction than expected for randomly selected field stars, which suggests that most of these variables are real counterparts. We discuss individual sources of interest, provide variability information on candidate counterparts, and discuss the characteristics of the variable population.

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.

We present optical light curves, spectroscopy, and classification of five X-ray sources in the Chandra Galactic Bulge Survey (CXOGBS J174009.1−284725 (CX5), CXOGBS J173935.7−272935 (CX18), CXOGBS J173946.9−271809 (CX28), CXOGBS J173729.1−292804 (CX37), CXOGBS J174607.6−261547 (CX561)). These objects were selected based on bright optical counterparts which were quickly found to have emission lines in their optical spectra. This paper presents an illustration of GBS optical follow-up, targeting emission line objects. Of these five objects, four exhibit photometric variability in the Sloan r band. CX5 shows a tentative period of 2.1 hr and is clearly an intermediate polar (IP). CX28 and CX37 both exhibit flickering with no clear period. Both are also suggested to be IPs. CX18 was observed to undergo two dwarf nova outbursts. Finally, CX561 shows no detectable variability, although its characteristics would be consistent with either a quiescent low-mass X-ray binary or cataclysmic variable.