4.4 Binary pulsars and Kepler's 4 Pulsar Timing4.2 The timing model

4.3 Timing stability 

Ideally, after correctly applying a timing model, we would expect a set of uncorrelated timing residuals scattered in a Gaussian fashion about a zero mean with an rms consistent with the measurement uncertainties. This is not always the case; the residuals of many pulsars exhibit a quasi-periodic wandering with time.

  

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Figure 20: Examples of timing residuals for a number of normal pulsars. Note the varying scale on the ordinate axis, the pulsars being ranked in increasing order of timing ``activity''.

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 [117Jump To The Next Citation Point In The Article]. 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 [196Jump To The Next Citation Point In The Article]. From high quality observations made using the Arecibo radio telescope spanning almost a decade [206Jump To The Next Citation Point In The Article, 207, 117Jump To The Next Citation Point In The Article], the group has demonstrated that the timing stability of millisecond pulsars over such time-scales is comparable to terrestrial atomic clocks.

  

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Figure 21: Fractional timing instabilities for PSRs B1855+09 and B1937+21 as a function of time. (After Kaspi, Taylor & Ryba 1994 [117Jump To The Next Citation Point In The Article].)

This phenomenal stability is demonstrated in Fig.  21 . This figure shows tex2html_wrap_inline9543, a parameter closely resembling the Allan variance used by the clock community to estimate the stability of atomic clocks [233Jump To The Next Citation Point In The Article, 1]. Atomic clocks are known to have tex2html_wrap_inline9545 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 tex2html_wrap_inline9543 after tex2html_wrap_inline9549 yr. This is most likely a result of a small amount of intrinsic timing noise [117Jump To The Next Citation Point In The Article]. 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 tex2html_wrap_inline9551 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 .



4.4 Binary pulsars and Kepler's 4 Pulsar Timing4.2 The timing model

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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