0 The Cause of the Age Discrepancy in Pulsar B 1757 – 24 ?

Radio pulsars are thought to spin-down primarily due to torque from magnetic dipole radiation (MDR) emitted by the time-varying stellar magnetic field as the star rotates. This assumption produces a 'characteristic age' for a pulsar which has generally been assumed to be comparable to the actual age. Recent observational limits on the proper motion of pulsar B1757–24, however, revealed that the actual age (> 39 kyr) of this pulsar is much greater than its MDR characteristic age (16 kyr) – calling into question the assumption of pure MDR spin-down for this and other pulsars. To explore the possible cause of this discrepancy, we consider a scenario in which the pulsar acquired an accretion disk from supernova ejecta, and the subsequent spin-down occurred under the combined action of MDR and accretion torques. A simplified model of the accretion torque involving a constant mass accretion rate can explain the age and period derivative of the pulsar for reasonable values of the pulsar magnetic field and accretion rate. We discuss testable predictions of this model.