The relative dearth of timing noise for the older pulsars is a very important finding. It implies that,
presently, the measurement precision depends primarily on the particular hardware constraints of
the observing system. Consequently, a large effort in hardware development is presently being
made to improve the precision of these observations using, in particular, coherent dedispersion
outlined in Section 4.1. Much progress in this area has been made by groups at Princeton [258],
Berkeley [325
], Jodrell Bank [326], UBC [323
], Swinburne [305] and ATNF [12]. From high quality
observations spanning over a decade [274, 275, 155
], these groups have demonstrated that the
timing stability of millisecond pulsars over such time-scales is comparable to terrestrial atomic
clocks.
This phenomenal stability is demonstrated in Figure 24 which shows
[214
], a parameter closely
resembling the Allan variance used by the clock community to estimate the stability of atomic
clocks [308
, 1]. Both PSRs B1937+21 and B1855+09 seem to be limited by a power law component
which produces a minimum in
after
and
respectively. This is most likely a
result of a small amount of intrinsic timing noise [155
]. Although the baseline for the bright
millisecond pulsar J0437
4715 is shorter, its
is already an order of magnitude smaller
than the other two pulsars or the atomic clocks. Timing observations of an array of millisecond
pulsars in the context of detecting gravitational waves from the Big Bang are discussed further in
Section 4.5.3.
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