Radio Emission Properties of Pulsars
1 Pulsar parameters used in this paper have been obtained from the ATNF Pulsar Catalogue, Version 1.29, http://www.atnf.csiro.au/research/pulsar/psrcat [44]. Pulsars are fascinating objects with a wide range of applications in physics and astronomy. Chara
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Radio Emission Properties of Pulsars Richard N. Manchester
2.1 Introduction Pulsars are fascinating objects with a wide range of applications in physics and astronomy. Characterised observationally by a highly periodic pulse train with periodicities typically in the range a few milliseconds to several seconds, they are generally identified with highly magnetised and rapidly rotating neutron stars formed in supernova explosions. Rotation of the star causes beamed emission, probably emanating from open field lines associated with the magnetic poles, to sweep across the sky generating one observed pulse per rotation period. A total of 1,765 pulsars are now known and almost all of these lie within our Galaxy.1 As Fig. 2.1 illustrates, pulsars come in two main classes, those with periods in the millisecond range and the so-called “normal” pulsars with periods of order 1 s. Most millisecond pulsars (MSPs) are binary, that is, in an orbit with another star, whereas only a few percent of normal pulsars are binary. MSPs, which comprise about 10% of the known population, are believed to be relatively old pulsars which have been spun up or “recycled” by accretion from a binary companion [3]. Because of exchange interactions occurring in their dense cores, globular clusters are a fertile breeding ground for MSPs [10] and about three-quarters of the known MSPs are associated with these clusters. Pulsar periods are extremely stable but they are not constant. All pulsars are slowing down because of loss of rotational kinetic energy to some combination of magnetic-dipole radiation (electro-magnetic waves at the pulsar spin frequency) and relativistic particle outflow. The spin-down rate can be expressed as
ν˙ = −K ν n ,
(2.1)
R.N. Manchester Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia e-mail: [email protected] 1
Pulsar parameters used in this paper have been obtained from the ATNF Pulsar Catalogue, Version 1.29, http://www.atnf.csiro.au/research/pulsar/psrcat [44]. W. Becker (ed.), Neutron Stars and Pulsars, Astrophysics and Space Science Library 357,
c Springer-Verlag Berlin Heidelberg 2009
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R.N. Manchester
Fig. 2.1 Histogram of observed pulsar periods. Pulsars which are members of a binary system are identified
where K depends on the magnetic field strength at the stellar surface, Bs , and the neutron-star moment of inertia, I, and n is the braking index. For pure magneticdipole braking, n = 3 and 8π 2 B2s R6 sin2 α , (2.2) K= 3Ic3 where α is the inclination angle of the dipole magnetic axis relative to the spin axis. ˙ where P = 1/ν We can define a “characteristic age” for the pulsar τc = P/(2P), ˙ is the pulsar period and P is its first time derivative. If the pulsar was born with a period much less than the present value and its spin-down is characterized by a braking index close to 3.0, then the characteristic age is a good indicator of the true age. We can also estimate the strength of the dipole field at the neutron-star surface, ˙ 1/2 G, where n = 3, I =
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