Non-monotonous character of single radio pulsar spin-down |
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Authors: | A V Biryukov G M Beskin S V Karpov |
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Institution: | (1) Department of Astronomy, Nanjing University, Nanjing, 210093, China |
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Abstract: | Our paper is dedicated to the problem of anomalous values of braking indices n
obs and spin frequency second derivatives (n)\ddot]\ddot \nu of isolated radio pulsars. Observations of these objects for over 40 years have shown that in addition to the complex short-term
irregular component in the evolution of the pulsars’ frequency, secular values of its second derivative are orders of magnitude
greater than the predicted theoretical ones, and in a good half of cases—they are even negative. We earlier attributed this
behavior of secular values of the second derivative to the presence of a cyclic component in the secular evolution of ν(t), with a characteristic recurrence time of thousands to tens of thousand years. We continue to develop this hypothesis based
on a more detailed statistical analysis of the characteristics of 297 isolated radio pulsars: we analyze the model of these
objects spin-down, consisting of two components, monotonic and cyclic, and determine their parameters. We demonstrate that
the monotonic spin-down component is described by the classical magnetodipolar power law with an braking index of about 3,
while the large amplitude of the cyclic component causes a significant variation of the observed spin-down rate ((n)\dot] )(\dot \nu ) (with respect to magnetodipolar one), and fully determines the anomalous values of (n)\ddot]\ddot \nu and n
obs. An important consequence of the existence of a cyclic component of the pulsar rotational variations is the difference between
their characteristic ages and respective secular values (by about 0.5–5 times). This allows to explainthe observed discrepancy
of the characteristic and physical ages of some objects, as well as very large, up to 108 years, characteristic ages of some old pulsars. The paper argues that the cyclic component of the observed spin-down is due
to the long-term precession of neutron stars around their magnetic axes, which, in particular, may be driven by the anomalous
braking torque. In the model of purely magnetodipolar braking this torque is a consequence of emission in the near field zone. |
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