[Surveys -- Oral ]

The Galactic Bulge Survey: finding eclipsing X-ray binaries and understanding the Galactic X-ray binary population

Peter Jonker, SRON \& CfA
C. Bassa (Machester Univ), G. Nelemans (Nijmegen Univ) et al.

The Galactic Bulge Survey is a multi-wavelength project aimed at understanding the population of faint X-ray sources towards the Galactic center, though avoiding the Galactic plane itself to avoid the huge extinction and problems caused by crowding. In order to sample the populations sufficiently, the total survey area will be 12square degrees. It consists of 2ks-long Chandra observations and optical Blanco images down to r' = 24. Thus far, the optical data is complete and the Chandra data covering 8.4 square degrees have revealed ~1350 X-ray sources. For ~1000 sources we have discovered counterparts on our optical Blanco images. A fraction of ~20 per cent of these are expected to be interloper stars. The plethora of faint X-ray sources will be compared to detailed population models in order to constrain binary evolution models, especially the common envelope phase, by way of number counts. We are using optical to X-ray flux ratios, optical and X-ray colours and follow-up optical spectroscopyof the detected optical counterparts to classify all sources. We expect to find several quiescent and eclipsing neutron star and black hole LMXBs. These will be used for detailed neutron star and black hole mass measurements. Mass measuremetns are vital for studies of the neutron star equation of state and supernova modelling. Lastly, wewill use the distribution of (quiescent) LMXBs to test if LMXB formation requires a kick imparted on the compact object or not as the apparent asymmetry in the Galactic distribution of LMXBs suggests. We will present the first results, and first surprises, of our optical and Chandra observations and the multi-wavelength follow-up of theseChandra X-ray discovered sources.