2.3 The pulsar distance scale2 An Introduction to Pulsar 2.1 The lighthouse model

2.2 Pulse profiles 

Pulsars are weak radio sources. Measured flux densities, usually quoted in the literature for a radio frequency of 400 MHz, vary between 0.1 and 5000 mJy (1 Jy tex2html_wrap_inline9101). This means that, even with a large radio telescope, the coherent addition of many thousands of pulses is required in order to produce an integrated profile. Remarkably, although the individual pulses vary dramatically from pulse to pulse, at any particular observing frequency the integrated profile is very stable. The pulse profile can thus be thought of as a fingerprint of the emission beam.


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Figure 3: Single pulses from PSR B0329+54. Click here to see the movie in action.

The animation in Fig.  3 shows a sequence of consecutive single pulses from PSR B0329+54 Popup Footnote, one of the brightest pulsars. This pulsar is seen in the animation to stabilise into its characteristic 3-component form after the summation of a number of seemingly erratic single pulses. Stabilization time-scales are typically several hundred pulses [94]. This property is of key importance in pulsar timing measurements discussed in detail in §  4 .

Fig.  4 shows the rich diversity in morphology from simple single-component profiles to examples in which emission is observed over the entire pulse. The astute reader will notice 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 [158]). Since this is an unlikely viewing angle we would expect interpulses to be a rare phenomenon. Indeed this is the case: the fraction of known pulsars in which interpulses are observed in their pulse profiles is only a few percent.


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Figure 4: A variety of integrated pulse profiles taken from the available literature. References: (a, b, d, f: [87]); (c: [20Jump To The Next Citation Point In The Article]); (e, g, i: [122Jump To The Next Citation Point In The Article]); (h: [26]). Each profile represents 360 degrees of rotational phase. These profiles are part of a database of over 2600 multi-frequency pulse profiles for over 600 pulsars that is available on-line [164].


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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 M. Kramer and A. von Hoensbroech).

Two contrasting phenomenological models to explain the observed pulse shapes are shown in Fig.  5 . The ``core and cone'' model, proposed by Rankin [198], depicts the beam as a core surrounded by a series of nested cones. Alternatively, the ``patchy beam'' model, championed by Lyne and Manchester [149Jump To The Next Citation Point In The Article, 89], has the beam populated by a series of randomly-distributed emitting regions. Further work in this area, particularly in trying to quantify the variety of pulse shapes (number of distinct components and the relative fraction that they occur) is necessary to improve our understanding of the fraction of sky covered by the radio pulsar emission beam. We return to this topic in the context of pulsar demography later on in §  3.2 .

2.3 The pulsar distance scale2 An Introduction to Pulsar 2.1 The lighthouse model

image Binary and Millisecond Pulsars at the New Millennium
Duncan R. Lorimer
© Max-Planck-Gesellschaft. ISSN 1433-8351
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