Pulsar motion effect and Geminga's high braking index

Physics

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Scientific paper

THE pulsar braking index, n, is a dimensionless quantity describing the rate at which a magnetized neutron star loses rotational energy1. It can be determined from pulsar timing measurements, and for distant pulsars is found to lie close to n = 3 (ref. 2), as predicted by theoretical models of pulsar emission mechanisms3-5. In contrast, the timing parameters-in particular the second derivative of the pulsation frequency-of the nearby pulsar Geminga6-11 indicate an extremely large braking index of about 10-30. To understand this property of Geminga, we consider here the effect on the measured timing parameters of a pulsar's motion through space. We find that the Doppler effect alone can give a high apparent braking index, but only if the pulsar is very close and has an abnormally high velocity (>1000kms-1). A more likely (but related) cause of the high braking index is the pulsar's proper motion: failure to correct for changes in the source coordinates with time can greatly influence the higher derivatives of the pulsar frequency, and lead to an erroneous value of n. A self-consistent analysis of the timing data, which account for both the proper motion and the Doppler effect, should permit a reliable estimate of the distance to Geminga.

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