Eclipsing Millisecond Pulsars with He Star Companions

Eclipsing millisecond pulsars (MSPs) are a type of pulsar binaries with close orbits (≲1.0 day). They are important objects for studying the accretion history of neutron stars (NSs), pulsar winds, and the origin of isolated MSPs, etc. Recently, a new eclipsing MSP, PSR J1928+1815, was discovered by the Five-hundred-meter Aperture Spherical radio Telescope. It is the first known pulsar with a He star companion, as suggested in Yang et al. The system features a short orbital period of ∼0.15 days and a relatively massive companion ≳1.0 M _⊙ . However, the origin of PSR J1928+1815 remains highly uncertain. In this paper, we investigated the formation of the new subclass of eclipsing MSPs containing (evolved) He star companions through the NS + He star channel. We found that if an NS binary undergoes subsequent mass-transfer phases following Case BA or Case BB, it may appear as an eclipsing MSP during the detached phase. Additionally, we obtained the initial parameter space for producing eclipsing MSPs with He star companions. Using the binary population synthesis approach, we estimated their birth rate to be ∼2.1–4.7 × 10 ^−4 yr ^−1 , corresponding to a total number of ∼220 systems in the Galaxy. Moreover, we concluded that PSR J1928+1815 may originate from the evolution of an NS + He star system with an initial orbital period of ∼0.1 days, which can undergo the Case BB mass transfer.