A number of examples are shown in Fig.
20
. These are taken from the Jodrell Bank timing program [216]. Such ``timing noise'' is most prominent in the youngest of the
normal pulsars [162,
59] and virtually absent in the much older millisecond
pulsars [117]. While the physical processes of this phenomena are not well
understood, it seems likely that it may be connected to
superfluid processes and temperature changes in the interior of
the neutron star [3], or processes in the magnetosphere [55,
54].
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 §
4.1
. Much of the pioneering work in this area has been made by
Joseph Taylor and collaborators at Princeton University [196]. From high quality observations made using the Arecibo radio
telescope spanning almost a decade [206
,
207,
117
], the group has demonstrated that the timing stability of
millisecond pulsars over such time-scales is comparable to
terrestrial atomic clocks.
This phenomenal stability is demonstrated in Fig.
21
. This figure shows
, a parameter closely resembling the Allan variance used by the
clock community to estimate the stability of atomic clocks [233
,
1]. Atomic clocks are known to have
on time-scales of order 5 years. The timing stability of PSR
B1937+21 seems to be limited by a power law component which
produces a minimum in its
after
yr. This is most likely a result of a small amount of intrinsic
timing noise [117
]. No such noise component is observed for PSR B1855+09. This
demonstrates that the timing stability for PSR B1855+09 becomes
competitive with the atomic clocks after about 3 yr. The absence
of timing noise for B1855+09 is probably related to its
characteristic age
Gyr which is about a factor of 20 larger than B1937+21. Timing
observations of millisecond pulsars are discussed further in the
context of the pulsar timing array in §
5.2
.
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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 |