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2.3 Pulse profiles

Pulsars are weak radio sources. Measured intensities, usually quoted in the literature for a radio frequency of 400 MHz, vary between 0.1 mJy and 5 Jy (1 Jy =_ 10-26 W m -2 Hz -1). As a result, even with a large radio telescope, the coherent addition of many hundreds or even thousands of pulses is usually required in order to produce a detectable “integrated profile”. Remarkably, although the individual pulses vary dramatically, the integrated profile at any particular observing frequency is very stable and can be thought of as a fingerprint of the neutron star’s emission beam. Profile stability is of key importance in pulsar timing measurements discussed in Section 4.

The selection of integrated profiles in Figure 4View Image shows a rich diversity in morphology including two examples of “interpulses” - a secondary pulse separated by about 180 degrees from the main pulse. The most natural interpretation for this phenomenon is that the two pulses originate from opposite magnetic poles of the neutron star (see however [209]). Since this is an unlikely viewing angle we would expect interpulses to be a rare phenomenon. Indeed, the fraction of known pulsars in which interpulses are observed in their pulse profiles is only a few percent [159].

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Figure 4: A variety of integrated pulse profiles taken from the available literature. References: Panels a, b, d, f [107], Panel c [23Jump To The Next Citation Point], Panels e, g, i [165Jump To The Next Citation Point], Panel h [29]. Each profile represents 360 degrees of rotational phase. These profiles are freely available from an on-line database [327].
Two contrasting phenomenological models to explain the observed pulse shapes are shown in Figure 5View Image. The “core and cone” model [261] depicts the beam as a core surrounded by a series of nested cones. Alternatively, the “patchy beam” model [202Jump To The Next Citation Point109] has the beam populated by a series of randomly-distributed emitting regions. Further work in this area is necessary to improve our understanding of the shape and evolution of pulsar beams and fraction of sky they cover. This is of key importance to the results of population studies reviewed in Section 3.2.
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Figure 5: Phenomenological models for pulse shape morphology produced by different line-of-sight cuts of the beam. Figure designed by Michael Kramer.

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