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Collective Properties of X-ray Binary Populations of Galaxies III. The Low-mass X-ray Binary Luminosity Function

Continuing our exploration of the collective properties of low-mass X-ray binaries (LMXBs) in the stellar fields of normal galaxies, we compute in this paper the expected X-ray luminosity function (XLF) of LMXBs, starting from the results of the previous paper in the series (Paper II). We treat separately two classes of LMXB evolution, the first being close systems whose initial orbital periods are below the bifurcation period, wherein the companion is on the main sequence when Roche-lobe contact occurs, the subsequent evolution is driven by angular-momentum loss from the system, and the second being wider systems whose initial orbital periods are above the bifurcation period, wherein the companion is on the giant branch when Roche-lobe contact occurs, and the evolution of these systems is driven by the nuclear evolution of the companion. We obtain model luminosity profiles L(t) for individual LMXBs of both classes. We then compute the LMXB XLF by folding in the inputs for the pre-LMXB collective properties and formation rates with the above luminosity profiles. Because of the long timescale on which LMXBs evolve, one needs to keep track of the evolution of the star-formation rate (SFR) on the same timescale, and we use star-formation histories given by canonical models. We compare the observed LMXB XLF with our computed one, keeping in mind that we have included only neutron-star systems in this work, so that there would be an unaccounted-for population of black-hole binaries at the high-luminosity end. We show that a qualitative similarity already exists between the two, and we discuss the role of the giant fraction, i.e., that fraction of all LMXBs which harbors a low-mass giant companion, on the shape of the XLF at the high-luminosity end.