where we assume the same notation used for the discussion in
§
4.4
and §
4.5
. Inserting the parameters of PSR B1913+16 yields
. 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. [264]. Further changes were seen by Kramer with new Effelsberg data
acquired in the 1990s [119
]. 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
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,
120
] 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 [120
].
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 [116] discussed in §
2.6.2
and §
3.4.2
. This relatively bright object was apparently missed by two
previous searches during the early 1990s [109
,
160,
152].
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Binary and Millisecond Pulsars at the New Millennium
Duncan R. Lorimer http://www.livingreviews.org/lrr-2001-5 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |