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Most millisecond pulsars, like essentially all other radio pulsars, showtiming errors well in excess of what is expected from additive radiometer noisealone. We show that changes in amplitude, shape and pulse phase for themillisecond pulsar J1713+0747 cause this excess error. These changes appear tobe uncorrelated from one pulse period to the next. The resulting time ofarrival variations are correlated across a wide frequency range and areobserved with different backend processors on different days, confirming thatthey are intrinsic in origin and not an instrumental effect or caused bystrongly frequency dependent interstellar scattering. Centroids of singlepulses show an rms phase variation \approx 40 microsec, which dominates thetiming error and is the same phase jitter phenomenon long known in slowerspinning, canonical pulsars. We show that the amplitude modulations of singlepulses are modestly correlated with their arrival time fluctuations. We alsodemonstrate that single-pulse variations are completely consistent with arrivaltime variations of pulse profiles obtained by integrating N pulses such thatthe arrival time error decreases proportional to 1/\sqrt{N}. We investigatemethods for correcting times of arrival for these pulse shape changes,including multi-component TOA fitting and principal component analysis. Thesetechniques are not found to improve the timing precision of the observations.We conclude that when pulse shape changes dominate timing errors, the timingprecision of PSR J1713+0747 can be only improved by averaging over a largernumber of pulses.

Pulse intensity modulation and the timing stability of millisecond pulsars: A case study of PSR J1713+0747