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Pranab Ghosh, Nicholas E. White (NASA/GSFC)
We report on the implications of the peak in the cosmic star-formation rate (SFR) at redshift z\approx 1.5 for the resulting population of low-mass X-ray binaries(LMXB) and for that of their descendants, the millisecond radio pulsars (MRP). Since the evolutionary timescales of LMXBs, their progenitors, and their descendants are thought be significant fractions of the time-interval between the SFR peak and the present epoch, there is a lag in the turn-on of the LMXB population, with the peak activity occurring at z~0.5-1. The peak in the MRP population is delayed further, occurring at z\lesssim 0.5. We show that the much-debated apparent discrepancy between the birthrate of LMXBs and MRPs, found under the assumption of a steady-state SFR, can be resolved for the population as a whole when the effects of a time-variable SFR are included. A discrepancy may persist for LMXBs with short orbital periods, although a detailed population synthesis will be required to confirm this. Further, since the integrated X-ray luminosity distribution of normal galaxies is dominated by X-ray binaries, it should show a second enhancement at a redshift between ~0.5 and ~1 due to the delayed turn-on of the LMXB population, in addition to a first enhancement near the peak (z\approx 1.5) of the SFR due to the prompt turn-on of the massive X-ray binaries and young supernova remnants. Deep X-ray observations of galaxies by Chandra should be able to detect this enhancement, and so provide an important new method for constraining evolutionary models of X-ray binaries. We argue that the current uncertainties in the SFR at large redshifts (z~2.5-5) have little effect on the resolution of the birthrate problem, but may have a larger effect on theoretical expectations of the X-ray luminosity evolution of normal galaxies.
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