The exact mechanism by which a pulsar radiates the energy observed as radio pulses is still a subject of vigorous debate. The basic picture of a misaligned magnetic dipole, with coherent radiation from charged particles accelerated along the open field lines above the polar cap [55, 128], will serve adequately for the purposes of this article, in which pulsars are treated as a tool to probe other physics. While individual pulses fluctuate severely in both intensity and shape (see Figure 1), a profile ``integrated'' over several hundred or thousand pulses (i.e., a few minutes) yields a shape - a ``standard profile'' - that is reproducible for a given pulsar at a given frequency. (There is generally some evolution of pulse profiles with frequency, but this can usually be taken into account.) It is the reproducibility of time-averaged profiles that permits high-precision timing.
Of some importance later in this article will be models of the
pulse beam
shape, the envelope function that forms the standard profile. The
collection of pulse profile shapes and polarization properties
have been used to formulate phenomenological descriptions of the
pulse emission regions. At the simplest level (see,
e.g., [112] and other papers in that series), the classifications can be
broken down into Gaussian-shaped ``core'' regions with little
linear polarization and some circular polarization, and
double-peaked ``cone'' regions with stronger linear polarization
and S-shaped position angle swings in accordance with the
``Rotating Vector Model'' (RVM; see [111]). While these models prove helpful for evaluating observed
changes in the profiles of pulsars undergoing geodetic
precession, there are ongoing disputes in the literature as to
whether the core/cone split is physically meaningful, or whether
both types of emission are simply due to the patchy strength of a
single emission region (see,
e.g., [90]).
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Testing General Relativity with Pulsar Timing
Ingrid H. Stairs http://www.livingreviews.org/lrr-2003-5 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |