Accretion from a Shock-inflated Companion: Spinning Down Neutron Stars to Hour-long Periods

Abstract Recent observations have unveiled a population of pulsars with spin periods of a few minutes to hours that lie beyond the traditional “death line.” If they originate from neutron stars (NSs), the existence of such ultralong-period pulsars (ULPs) challenges our current understanding of NS evolution and emission. In this work, we propose a new channel for disk formation based on NSs born in close binaries with main-sequence companion stars. Using a hydrodynamic simulation of supernova–companion interactions, we show that a newborn NS may gravitationally capture gas as it moves through the complex density field shaped by the explosion. For a binary separation of 20 R ⊙ and a companion mass of 4 M ⊙ , we find the occurrence fraction for disk formation around unbound NSs to be ∼10%. By modeling the disk evolution and its interaction with the NS, we find a bimodal distribution in spin periods: canonical pulsars with P ≲ 10 s are the ones who lack disks or whose magnetospheres never interacted with the disk, and ULPs with 10 3 ≲ P < 10 5 s are produced when the system undergoes a short-lived “propeller” phase during which the NS undergoes rapid spin-down. Such ULPs are formed under strong initial dipolar magnetic field strengths B 0 ≳ 10 14 G, with a formation rate of 10 −4 yr −1 in the Milky Way. We also find that a small population of pulsars with moderate magnetic field strengths (10 13 ≲ B 0 ≲ 10 14 G) and relatively slow initial periods ( P 0 ≳ 0.1 s) evolve to P ∼ 10 2 s, filling the gap between the bimodal distribution. Thus, our model provides a unified explanation for pulsar spins beyond the death line.