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 ultra-long 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 $20{\mathrm{R}}_{\odot }$ and a companion mass of $4{\mathrm{M}}_{\odot }$, we find the occurrence fraction for disk formation around unbound NSs to be $\sim 10\%$. By modeling the disk evolution and its interaction with the NS, we find a bimodal distribution in spin periods: canonical pulsars with $P\lesssim 10\mathrm{s}$ are the ones who lack disks or whose magnetospheres never interacted with the disk, and ULPs with $10^3\lesssim P<10^5\mathrm{s}$ 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 $B_0\gtrsim 10^{14}\mathrm{G}$, with a formation rate of $10^{-4}\mathrm{y}{\mathrm{r}}^{-1}$ in the Milky Way. We also find that a small population of pulsars with moderate magnetic field strengths ($10^{13}\lesssim B_0\lesssim 10^{14}\mathrm{G}$) and relatively slow initial periods ($P_0\gtrsim 0.1\mathrm{s}$) evolve to $P\sim 10^2\mathrm{s}$, filling the gap between the bimodal distribution. Thus, our model provides a unified explanation for pulsars beyond the ``death line.''

24 pages, 16 Figures