3 The Galactic Pulsar Population

Soon after the discovery of pulsars, it was realised that the observed sample is heavily biased towards the brighter objects that are the easiest to detect. What we observe therefore most likely represents only the tip of the iceberg of a much larger underlying population [88]. The extent to which the sample is incomplete is well demonstrated by the projection of pulsars onto the Galactic plane and their cumulative number distribution as a function of distance shown in Fig. 12 . Although the clustering of sources around the Sun seen in the left panel of Fig. 12 would be consistent with Ptolemy's geocentric picture of the heavens, it is clearly at variance with what we now know about the Galaxy, where the massive stars show a radial distribution about the Galactic center.

Figure 12: Left: The sample of radio pulsars from the Princeton catalog [196 ] projected onto the Galactic plane. The Galactic center is at (0,0) and the Sun is at (-8.5,0). Right: Cumulative number of observed pulsars (solid line) as a function of projected distance, d. The dashed line shows the expected distribution for a model population (see text).

The extent to which the pulsar sample is incomplete is shown in the right panel of Fig. 12 where the cumulative number of pulsars is plotted as a function of the projected distance from the Sun. The observed distribution is compared to the expected distribution for a simple model population in which there are errors in the distance scale, but no selection effects. We see that the observed sample becomes strongly deficient in terms of the number of sources for distances beyond a few kpc. We now discuss the main selection effects that hamper the detection of pulsars in some detail.

Binary and Millisecond Pulsars at the New Millennium
Duncan R. Lorimer
http://www.livingreviews.org/lrr-2001-5
© Max-Planck-Gesellschaft. ISSN 1433-8351
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