Population synthesis of classical low-mass X-ray binaries in the Galactic Bulge
1 Department of Astrophysics/ IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
2 Department of Physics, Texas Tech University, Box 41051, Lubbock TX 79409-1051, USA
3 Institute for Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
4 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
Received: 8 November 2014
Accepted: 13 May 2015
Aims. We model the present-day population of classical low-mass X-ray binaries (LMXBs) with neutron star accretors, which have hydrogen-rich donor stars. Their population is compared with that of hydrogen-deficient LMXBs, known as ultracompact X-ray binaries (UCXBs). We model the observable LMXB population and compare it to observations. We model the Galactic Bulge because it contains a well-observed population and it is the target of the Galactic Bulge Survey.
Methods. We combine the binary population synthesis code SeBa with detailed LMXB evolutionary tracks to model the size and properties of the present-day LMXB population in the Galactic Bulge. Whether sources are persistent or transient, and what their instantaneous X-ray luminosities are, is predicted using the thermal-viscous disk instability model.
Results. We find a population of ~2.1 × 103 LMXBs with neutron star accretors. Of these about 15−40 are expected to be persistent (depending on model assumptions), with luminosities higher than 1035 erg s-1. About 7−20 transient sources are expected to be in outburst at any given time. Within a factor of two these numbers are consistent with the observed population of bright LMXBs in the Bulge. This gives credence to our prediction of the existence of a population of ~1.6 × 103 LMXBs with low donor masses that have gone through the period minimum, and have present-day mass transfer rates below 10-11 M⊙ yr-1.
Conclusions. Even though the observed population of hydrogen-rich LMXBs in the Bulge is larger than the observed population of (hydrogen-deficient) UCXBs, the latter have a higher formation rate. While UCXBs may dominate the total LMXB population at the present time, the majority would be very faint or may have become detached and produced millisecond radio pulsars. In that case UCXBs would contribute significantly more to the formation of millisecond radio pulsars than hydrogen-rich LMXBs.
Key words: binaries: close / Galaxy: bulge / X-rays: binaries
© ESO, 2015