Isolated thermal neutron stars, SGRs and AXPs: propellers and early accretors with conventional magnetic fields?

Abstract

The similarity of rotation periods from three interesting classes of neutron stars, 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 spindownphase, extending through the propeller and early accretion stages. The DTNs are interpreted as sources in the propeller stage. Their low luminosities arise from frictional heating in the neutron star. SGRs and AXPs are accreting at $\dot{M} \sim 10^{15} gm/s $. The limited range of near equilibrium periods corresponds to a limited range of mass inflow rates $\dot{M}$. For lower rates the source of mass inflow may be depleted before the asymptotic stage is reached, while sources with higher $\dot{M}$ or later ages possess circumstellar material that is optically thick to electron scattering, destroying the X-ray beaming and the modulation at the rotation period. The model works with conventional magnetic fields of 10$^{11}-10^{12}$ G, obviating the need to postulate magnetars. Frequently sampled timing observations of AXPs, SGRs and DTNs can distinguish between this explanation and the magnetar model.