Evolution of ultracompact neutron star–helium star binaries

Context. Ultracompact binaries (detached and X-ray-emitting) are regularly detected and studied; however, it is not known whether even more compact configurations featuring degenerate stars exist. The recent discovery of PSR J1928+1815, which has a helium companion in a 3.6 h orbit, suggests a hitherto largely unexplored evolution toward “hyper-compact” configurations with P orb of ≤1 min. Aims. This work aims to establish whether helium companions drive systems into hyper-compact configurations, and to clarify the evolutionary status of PSR J1928+1815. Methods. We modeled the evolution of binary systems formed by a neutron star and a helium star that possibly previously experienced a common envelope phase. After an initial mass transfer episode, the donor detaches and this leads to the formation of a system with a white dwarf companion. We followed the evolution after detachment, when the donor becomes a compact degenerate star, up to the onset of the final Roche lobe overflow in hyper-compact conditions of the binary. Results. We show that a sufficiently light helium secondary is compatible with the current state of PSR J1928+1815. After undergoing a Roche lobe overflow, the system first evolves into a detached configuration, with orbital periods in the range of the observed value of the PSR J1928+1815 system, and later into a hyper-compact configuration. We predict extremely short orbital periods for the latter state. Conclusions. Our results indicate that the PSR J1928+1815 system is young (≲10 7 yr), since the second epoch that could match the observed orbital period is unable to explain the length of pulsar eclipses. We find that the evolution of such systems toward extremely short orbital periods ( P orb ) of ≲1 min is unavoidable unless the secondary becomes massive enough to explode as an electron-capture supernova. The hyper-compact stage is not prevented by evaporation, and strong gravitational wave emission is expected during this phase, with the system eventually ending up as a bright optical and gamma-ray transient.