We have conducted radio timing observations of the eclipsing millisecond binary pulsar J2051-0827 with the European Pulsar Timing Array network of telescopes and the Parkes radio telescope, spanning over 13 years. The increased data span allows signi
ficant measurements of the orbital eccentricity, e = (6.2 {pm} 1.3) {times} 10^{-5} and composite proper motion, {mu}_t = 7.3 {pm} 0.4 mas/yr. Our timing observations have revealed secular variations of the projected semi-major axis of the pulsar orbit which are much more extreme than those previously published; and of the orbital period of the system. Investigation of the physical mechanisms producing such variations confirm that the variations of the semi-major axis are most probably caused by classical spin-orbit coupling in the binary system, while the variations in orbital period are most likely caused by tidal dissipation leading to changes in the gravitational quadrupole moment of the companion.
Analysis of high-precision timing observations of an array of approx. 20 millisecond pulsars (a so-called timing array) may ultimately result in the detection of a stochastic gravitational-wave background. The feasibility of such a detection and the
required duration of this type of experiment are determined by the achievable rms of the timing residuals and the timing stability of the pulsars involved. We present results of the first long-term, high-precision timing campaign on a large sample of millisecond pulsars used in gravitational-wave detection projects. We show that the timing residuals of most pulsars in our sample do not contain significant low-frequency noise that could limit the use of these pulsars for decade-long gravitational-wave detection efforts. For our most precisely timed pulsars, intrinsic instabilities of the pulsars or the observing system are shown to contribute to timing irregularities on a five-year timescale below the 100 ns level. Based on those results, realistic sensitivity curves for planned and ongoing timing array efforts are determined. We conclude that prospects for detection of a gravitational-wave background through pulsar timing array efforts within five years to a decade are good.
