Such “timing noise” is most prominent in the youngest of the normal pulsars [256, 87] and present at a lower level in the much older millisecond pulsars [188, 12]. While the physical processes of this phenomenon are not well understood, it seems likely that they may be connected to superfluid processes and temperature changes in the interior of the neutron star , or to processes in the magnetosphere [78, 77].
The relative dearth of timing noise for the older pulsars is a very important finding. It implies that the measurement precision presently depends primarily on the particular hardware constraints of the observing system. Consequently, a large effort in hardware development is now 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 , Berkeley , Jodrell Bank , UBC , Swinburne  and ATNF . From high quality observations spanning over a decade [320, 321, 188], these groups have demonstrated that the timing stability of millisecond pulsars over such timescales is comparable to terrestrial atomic clocks.
This phenomenal stability is demonstrated in Figure 24 which shows [257, 330], a parameter closely resembling the Allan variance used by the clock community to estimate the stability of atomic clocks [355, 3]. Both PSRs B1937+21 and B1855+09 seem to be limited by a power law component which produces a minimum in after 2 yr and 5 yr respectively. This is most likely a result of a small amount of intrinsic timing noise . The based on timing observations  of the bright millisecond pulsar J0437–4715 is now 1 – 2 orders of magnitude smaller than the other two pulsars or the atomic clock!
This work is licensed under a Creative Commons License.