The galactic bulge survey: Outline and X-ray observations

P. G. Jonker, SRON Netherlands Institute for Space Research
C. G. Bassa, SRON Netherlands Institute for Space Research
G. Nelemans, Radboud Universiteit
D. Steeghs, Harvard-Smithsonian Center for Astrophysics
M. A.P. Torres, SRON Netherlands Institute for Space Research
T. J. MacCarone, University of Southampton
R. I. Hynes, Louisiana State University
S. Greiss, University of Warwick
J. Clem, Louisiana State University
A. Dieball, University of Southampton
V. J. Mikles, Louisiana State University
C. T. Britt, Louisiana State University
L. Gossen, Louisiana State University
A. C. Collazzi, Louisiana State University
R. Wijnands, Astronomical Institute Anton Pannekoek
J. J.M. In'T Zand, SRON Netherlands Institute for Space Research
M. Méndez, University of Groningen, Kapteyn Astronomical Institute
N. Rea, Instituto de Estudios Espaciales de Cataluña
E. Kuulkers, European Space Astronomy Centre
E. M. Ratti, SRON Netherlands Institute for Space Research
L. M. Van Haaften, Radboud Universiteit
C. Heinke, University of Alberta
F. Özel, The University of Arizona
P. J. Groot, Radboud Universiteit
F. Verbunt, SRON Netherlands Institute for Space Research


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 (NS) equation of state and the black hole (BH) mass distribution via the identification of eclipsing NS and BH low-mass X-ray binaries (LMXBs). The latter goal will, in addition, be obtained by significantly enlarging the number of BH systems for which a BH 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 deg2 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) LMXBs to cataclysmic variables. The survey is approximately homogeneous in depth to a 0.5-10keV flux of 7.7 × 10 -14ergcm-2s-1. So far, we have covered about two-thirds (8.3 deg2) 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. © 2011. The American Astronomical Society. All rights reserved..