Isolated Magnetar Spin-down, Soft X-ray Emission and Rxj1856.5-3754

When an isolated magnetar with magnetic dipole field B ∼ 10 G moves at high velocity (v > 10 cm s) through the ISM, its transition into propeller effect driven spin-down may occur in less than 10 years. We propose that the nearby neutron star RXJ1856.5-3754 is such a magnetar, and has spun down by the propeller effect to a period greater than 10 sec within ∼ 5 × 10 years. This magnetar scenario is consistent with observed thermal X-ray emission properties and the absence of detectable spinmodulations of them. Detection of other rapidly moving long period (> 100 sec) magnetars with known ages would strongly constrain the very great variety of predicted propeller-effect torque magnitudes. Subject headings: stars: neutron – X-rays: stars – stars: RX J1856.5-3754