Birthrates of low-mass binary pulsars and low-mass X-ray binaries

Abstract

We examine the commonly accepted hypothesis that low-mass binary radio pulsars (LMBPs) are the end products of low-mass X-ray binaries (LMXBs). In steady state, the birthrates of these two classes of binaries should be equal to each other. We determine the birthrate of LMBPs from a careful analysis of nine pulsar surveys, including several recent ones that were sensitive to millisecond pulsars. We show that only the long-period LMXBs can evolve to form objects similar to the known LMBPs. Using published X-ray catalogs and accretion lifetimes from stellar evolution models we evaluate the birthrate of the Galactic LMXBs. The birth-rate of LMXBs is found to exceed that of LMBPs by a factor of ~ 10. Equality of the two birthrates can be obtained by appealing to small number statistics. However, the observed distribution of the orbital periods of the LMBPs cannot be satisfactorily explained. Specifically, we find that the birthrate of the small orbital period systems like PSR 1855 + 09 exceeds the birthrate of the progenitor LMXB systems by a factor of 100. Even with pessimistic estimates of the uncertainties, this discrepancy cannot be reduced to below a factor of ~10. This large discrepancy suggests that the mass transfer rates in the short orbital period systems are much higher than those obtained from stellar evolution models. Another possible solution is that mass transfer need not necessarily be accompanied by copious X-ray emission. If this speculation is correct then there should be a large number of hitherto undiscovered accretion-powered sources in our Galaxy, with most of the accretion power channeled not into X-ray emission but other channels such as jets, synchrotron nebulae, etc. We suggest that a systematic search for such sources be undertaken.