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Register a new userA large age for the pulsar B1757-24 from an upper limit on its proper motion
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Authors
Bryan Gaensler
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The characteristic age of a pulsar usually is considered to approximate its true age, but this assumption has led to some puzzling results, including the fact that many pulsars with small characteristic ages have no associated supernova remnants. The pulsar B1757-24 is located just beyond the edge of a supernova remnant; the properties of the system indicate that the pulsar was born at the centre of the remnant, but that it has subsequently overtaken the expanding blast-wave. With a characteristic age of 16,000 yr, this implies an expected proper motion by the pulsar of 63-80 milliarcsec per year. Here we report observations of the nebula surrounding the pulsar which limit its proper motion to less than 25 mas/yr, implying a minimum age of 39,000 yr. A more detailed analysis argues for a true age as great as 170,000 yr, significantly larger than the characteristic age. From this result and other discrepancies associated with pulsars, we conclude that characteristic ages seriously underestimate the true ages of pulsars.
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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 yields 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 inflow rate at the pulsar magnetosphere can explain the age and period derivative of the pulsar for reasonable values of the pulsar magnetic field and inflow rate. We discuss testable predictions of this model.
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