The low-mass X-ray binary-millisecond radio pulsar birthrate problem revisited

We investigate the birthrate problem for low-mass X-ray binaries (LMXBs) and millisecond radio pulsars (MRPs) in this paper. We consider intermediate-mass and low-mass X-ray binaries (I/LMXBs) to be the progenitors of MRPs, and calculate their evolutionary response to the cosmic star formation rate (SFR) both semi-analytically and numerically. With a typical value (1 Gyr) of the LMXB lifetime, one may expect comparable birthrates of LMXBs and MRPs, but the calculated number of LMXBs is an order of magnitude higher than that observed in the Galaxy. Instead, we suggest that the birthrate problem could be solved if most MRPs have evolved from faint to rather than bright LMXBs. The former may have a population of ∼104 in the Galaxy.