Pulsar properties

2.1 Pulsar properties

Radio pulsars were firmly established to be neutron stars by the discovery of the pulsar in the Crab nebula [119 ]; its 33-ms period was too fast for a pulsating or rotating white dwarf, leaving a rotating neutron star as the only surviving model [106

, 55

]. The 1982 discovery of a 1.5-ms pulsar, PSR B1937+21 [13 ], led to the realization that, in addition to the “young” Crab-like pulsars born in recent supernovae, there exists a separate class of older “millisecond” or “recycled” pulsars, which have been spun up to faster periods by accretion of matter and angular momentum from an evolving companion star. (See, for example, [22

] and [108

] for reviews of the evolution of such binary systems.) It is precisely these recycled pulsars that form the most valuable resource for tests of GR.

Figure 1: Top: 100 single pulses from the 253-ms pulsar B0950+08, demonstrating pulse-to-pulse variability in shape and intensity. Bottom: Cumulative profile for this pulsar over 5 minutes (about 1200 pulses); this approaches the reproducible standard profile. Observations taken with the Green Bank Telescope [58

]. (Stairs, unpublished.)

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 [57 , 127 ], 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., [110 ] 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 [109 ]). 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., [91 ]).