Isolated Magnetar Spin-Down, Soft X-Ray Emission, and RX J1856.5–3754

When an isolated magnetar with magnetic dipole field B ~ 1015 G moves at high velocity (v > 107 cm s-1) through the interstellar medium, its transition into propeller effect-driven spin-down may occur in less than 106 yr. We propose that the nearby neutron star RX J1856.5-3754 is such a magnetar and has spun down by the propeller effect to a period greater than 104 s within ~5 × 105 yr. This magnetar scenario is consistent with observed thermal X-ray emission properties and the absence of detectable spin modulations of them. Detection of other rapidly moving long-period (>100 s) magnetars with known ages would strongly constrain the very great variety of predicted propeller effect torque magnitudes.