Program: GN-2005A-Q-75

In low-mass X-ray binaries (LMXBs), a neutron star or black hole primary accretes material from its companion star, producing intense X-ray emission. Since many of the brightest LMXBs are located in the obscured region of the Galactic Bulge, IR observations are crucial for their study. A long-standing puzzle is the origin of the dichotomy between the two classes of bright persistent neutron star LMXBs, known as "Z" and "atoll" systems, which exhibit different X-ray spectra and luminosities. We have hypothesized that the mass donors in the Z LMXBs may all be early-type (A-G) evolved stars, in contrast to the K-M companions expected in canonical (atoll) LMXBs. However, we require very high S/N spectra to definitively determine if the lack of absorption features in their spectra is intrinsic to the spectral type of the mass donor or is due to obscuration by an exceptionally luminous accretion disc. We propose to obtain spectra of three Z sources with identified IR counterparts, and of three of the brightest atoll LMXBs, to test our hypothesis. We will thus have a sufficient sample of spectra to determine whether differences in the nature of the system secondaries is one of the primary distinctions in the evolutionary history of the Z and atoll LMXBs. These observations will provide a direct link between X-ray behaviour and the luminosity class of the mass donor. We will then be able to directly compare the properties of these neutron star binaries to black hole binaries with similar mass donors, thus constraining theoretical models of XRB formation and evolution.