Instability in Supernova Fallback Disks and Its Effect on the Formation of Ultralong Period Pulsars

Abstract Several pulsars with unusually long periods were discovered recently, comprising a potential population of ultralong period pulsars (ULPPs). The origin of their long periodicity is not well understood, but may be related to magnetars spun down by surrounding fallback disks. While there are few systematic investigations on the fallback-disk-assisted evolution of magnetars, instability in the disk has received little attention, which determines the lifetime of the disk. In this work we simulate the evolution of the magnetic field, spin period, and magnetic inclination angle of magnetars with a supernova fallback disk. We find that a thermal viscous instability in the disk could significantly affect the formation of ULPPs. Our simulation results also reveal that a large fraction of ULPPs seem to be nearly aligned and orthogonal rotators. This might help place ULPPs above the death line in the pulse period–period derivative plane. However, some extra mechanisms seem to be required to account for the radio emission of ULPPs.