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 , Berkeley , Jodrell Bank , UBC , Swinburne  and ATNF . 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 , 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 . Although the baseline for the bright millisecond pulsar J04374715 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.
© Max Planck Society and the author(s)