The Galactic Bulge Survey

This page provides details of a large research project that we are undertaking. The main idea is to image an area of 6x1 degrees centered on the Galactic Center at |b|=1.5 degrees: the Galactic Bulge Survey. These regions have been selected because of their lower extinction and crowding in the optical and near-infrared than in the Galactic Center area, while still having a high density of X-ray sources. We have detected a plethora of faint X-ray sources that will constrain binary evolution models by way of a number count and by identifying predicted X-ray binary types that so far have eluded identification. We expect to find (quiescent) eclipsing neutron star and black hole LMXBs. These are important for neutron star and black hole mass measurements.

Many high-energy phenomena are driven by accretion onto compact objects. The close binaries responsible for these sources went through one or two phases of common-envelope evolution. This includes white dwarf binaries such as CVs and AM CVns, and the various families of X-ray binaries. Also the rates of Type Ia SNe from different mechanisms and the rates of production of Gravitational wave radiation sources are strongly dependent on how common envelope evolution works. However, that phase is not yet understood (i.e. Ivanova et al. 2012). Compact binaries have much lower orbital energy and angular momentum than the progenitor binary that contained giants (Paczynski, 1976). The binary semi-major axis is thought to shrink mainly during a phase of unstable mass transfer and ejection, the spiral-in. If the outcome of this process is derived by assuming that the change in orbital energy is enough to eject the giant's mantle, the predicted properties do not match the observations of double white dwarf binaries. These properties can be matched with the assumption that the giant's mantle is ejected, carrying the specific orbital angular momentum (Nelemans et al. 2000), but this begs the question how the required energy is provided. It is clear that a more complete theoretical description is required that takes into account both energy and angular momentum. \indent To make progress on this issue we envisage a two-pronged approach by detailed studies of individual systems on one hand, and of the population on the other hand. Any viable evolutionary scheme must be able to reproduce the specific properties (such as component masses, orbital period, age, system velocity) of each observed individual system. Any viable evolution scheme must also reproduce the distributions of and correlations between these properties in the population of X-ray binaries. Observationally this implies the production of large homogeneous samples of X-ray binaries, and the detailed follow-up of a number of individual systems (i.e. Ratti, Jonker et al. 2012). In table 2 of Jonker et al. (2011) we provide the number of sources that we expect to discover in the full GBS area. We will compare our identifications with the predicted numbers of binaries in each category and that way place strong constraints on the common-envelope phase in binary evolution.

For dynamical mass measurements in LMXBs one needs to measure three parameters: the radial velocity amplitude of the companion star (K), the ratio between the mass of the companion star and the neutron star or black hole (q) and the inclination. A measurement of the rotational broadening of the stellar absorption lines (v sin i) combined with K gives this determination of q. The system inclination can be determined through modelling of the multi-colour optical lightcurves or, in systems with favorable viewing angles the X-ray eclipse duration can be used to accurately determine the inclination (Horne 1985). Since the inclination is constrained by the geometry, mass measurements in eclipsing systems are independent of the modelling that lies behind inclinations derived from ellipsoidal variations. Furthermore, the compact object mass is much more weakly sensitive to inclination when the inclination is high - one does not need to know the inclination very precisely to still have a small uncertainty in the mass measurement. Therefore, quiescent eclipsing systems are *prime* targets for mass measurements. Such mass measurements provide constraints on the neutron star equation of state (EoS). Constraining the neutron star EoS remains one of the final goals for neutron star studies. Note that with the GBS we are discovering the first-ever low-mass X-ray binaries selected in X-rays while in quiescence. These systems have not recently gone through an outburst cycle! Hence, we will be able to investigate if there are selection effects in, for instance, neutron star or black hole mass between systems that went through a recent outburst cycle or not. One can envisage models where high mass-ratio systems have outbursts more frequently (tidal effects are stronger in high mass-ratio systems). This would imply that the currently known black hole sample favors high mass-ratio systems hence relatively high mass black hole systems. This could potentially explain the apparent lack of low-mass (3-5 Msun) black holes found (Remillard & McClintock 2006) which is important input for supernova models (but see Kriedberg et al. 2012 for an alternative explaination for the lack of low-mass black holes). Eclipsing black hole LMXB systems should exist, but have not yet been found although black hole sources showing dips are known (e.g. Corral-Santana et al. 2013). It has been proposed that they are too weak to be detected by current X-ray all sky monitors because they are obscured behind the accretion disc rim (Narayan & McClintock 2005). If so, they should turn up in our GBS. The number of eclipsing sources depends on the distribution of the mass ratio between the accretor and the donor star (see e.g. Horne 1985). For mass ratios q~0.3 approximately 20-25 per cent of the 120 new quiescent LMXBs we expect to discover should be eclipsing of which several should be black holes.