On Young Neutron Stars as Propellers and Accretors with Conventional Magnetic Fields

The similarity of rotation periods of, the anomalous X-ray pulsars (AXPs), the soft gamma ray repeaters (SGRs) and the dim isolated thermal neutron stars (DTNs) suggests a common mechanism with an asymptotic spindown phase, extending through the propeller and early accretion stages. The DTNs are interpreted as sources in the propeller stage. Their luminosities arise from frictional heating in the neutron star. If the 8.4 s rotation period of the DTN RXJ 0720.43125 is close to its rotational equilibrium period, the estimated propeller torque indicates a magnetic field in the 10 Gauss range. The mass inflow rate onto the propeller is of the order of the accretion rates of the AXPs. The limited range of rotation periods, taken to be close to equilibrium periods, and conventional magnetic fields in the range 5 × 10 5 × 10 Gauss correspond to a range of mass inflow rates 3.2 × 10 gm s < Ṁ < 4.2 × 10 gm s. Observed spindown rates of the AXPs and SGRs also fit in with estimates for these magnetic fields and equilibrium periods. The source of the mass inflow is a remnant accretion disk formed as part of the fallback during the supernova explosion. These classes of sources thus represent the alternative pathways for those neutron stars that do not become radio pulsars. For the highest mass inflow rates the propeller action may support enough circumstellar material so that the optical thickness to electron scattering destroys the X-ray beaming, and the rotation period is not observable. These are the radio quiet neutron stars (RQNSs) at the centers of supernova remnants Cas A, Puppis A, RCW 103 and 296.5+10. The statistics and ages of DTNs suggest that sources in the propeller phase are quite common, maybe accounting for the majority of neutron stars formed in supernovae. AXPs are the rare cases whose Ṁ history allows them to reach the post-propeller accretion phase. This model obviates the need to postulate magnetars for AXPs