4.7 Going further4 Pulsar Timing4.5 Post-Keplerian parameters

4.6 Geodetic precession 

Shortly after the discovery of PSR B1913+16 it was realized that, if the spin axis of the visible pulsar was misaligned with the angular momentum axis of the binary system, the perturbing effect of the companion on the space-time around the radio pulsar would cause it to precess around the angular momentum axis [63, 76]. Within the framework of general relativity, the rate of precession tex2html_wrap_inline9629 was shown [22Jump To The Next Citation Point In The Article] to be

equation984

where we assume the same notation used for the discussion in §  4.4 and §  4.5 . Inserting the parameters of PSR B1913+16 yields tex2html_wrap_inline9631 . The period of the precession is 297.5 yr. The observational consequence of geodetic precession is a secular change in the pulse profile as the line-of-sight cut through the emission beam changes (recall Fig.  5).

Early qualitative evidence for profile evolution due to this effect [236] was substantiated with long-term Arecibo measurements of component changes by Weisberg et al. [264Jump To The Next Citation Point In The Article]. Further changes were seen by Kramer with new Effelsberg data acquired in the 1990s [119Jump To The Next Citation Point In The Article]. In addition to relative amplitude variations, the expected changes in component separation for a hollow-cone beam model were also seen in the Effelsberg data. These observations are summarized in Fig.  24 .

In addition to the above results, there is now evidence for geodetic precession in the other classic neutron star binary, PSR B1534+12 [9, 223]. Although geodetic precession in binary pulsars is another successful test of general relativity (albeit at a lower precision than e.g. orbital decay measurements), what is perhaps more interesting are the various consequences it has. Geodetic precession only occurs when the spin and orbital axes are misaligned [22]. This is most likely to occur if the neutron star received an impulsive ``kick'' velocity at birth (§  2.4.4). Wex et al. [265] have investigated the B1913+16 observations and find that the kick magnitude was at least 250  tex2html_wrap_inline9183 and was directed almost perpendicular to the spin axis of the neutron star progenitor. This places stringent constraints on any kick mechanism. Detailed monitoring of the pulse profile and polarization properties now underway [262, 120Jump To The Next Citation Point In The Article] will allow the first map of the emission beam of a neutron star to be made. This has important implications for the various beaming models described in §  3.2.3 . There are already indications that the beam is circular [120Jump To The Next Citation Point In The Article].

  

Click on thumbnail to view image

Figure 24: Geodetic precession in the binary pulsar B1913+16 system: (a) Changes in the observed pulse shapes between 1981-1988 seen as a decrease in amplitude between the left and right components in the pulse profile [240Jump To The Next Citation Point In The Article, 264]; (b) relative heights of the two amplitudes plotted between 1975-1998 [119Jump To The Next Citation Point In The Article]; (c) component separation change [119Jump To The Next Citation Point In The Article].

The current results predict that B1913+16 will completely precess out of the line of sight by around 2025 and re-appear some 240 years later [119]. Although we shall lose a most treasured pulsar, we can take comfort from the fact that other pulsars will precess into our field of view. Perhaps one example is the newly-discovered relativistic binary J1141-6545 [116Jump To The Next Citation Point In The Article] discussed in §  2.6.2 and §  3.4.2 . This relatively bright object was apparently missed by two previous searches during the early 1990s [109Jump To The Next Citation Point In The Article, 160, 152].



4.7 Going further4 Pulsar Timing4.5 Post-Keplerian parameters

image 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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