Aqueye optical observations of the Crab Nebula pulsar

Context. We observed the Crab pulsar in October 2008 at the Copernico Telescope in Asiago - Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum Eye), which has the best temporal resolution and accuracy ever achieved in the optical domain (hundreds of picoseconds). Aims. Our goal was to perform a detailed analysis of the optical period and phase drift of the main peak of the Crab pulsar and compare it with the Jodrell Bank ephemerides. Methods. We determined the position of the main peak using the steepest zero of the cross-correlation function between the pulsar signal and an accurate optical template. Results. The pulsar rotational period and period derivative have been measured with great accuracy using observations covering only a two day time interval. The error on the period is 1.7 ps, limited only by the statistical uncertainty. Both the rotational frequency and its first derivative agree with those from the Jodrell Bank radio ephemerides archive. We also found evidence that the optical peak precedes the radio peak by similar to 230 mu s. The distribution of phase residuals of the whole dataset is slightly more scattered than that of a synthetic signal generated as a sequence of pulses distributed in time with the probability proportional to the pulse shape. Conclusions. The counting statistics and quality of the data allowed us to determine the pulsar period and period derivative with great accuracy in two days only. The time of arrival of the optical peak of the Crab pulsar precedes the radio peak in agreement with what was recently reported in the literature. The distribution of the phase residuals can be approximated with a Gaussian and is consistent with being completely caused by photon noise (for the best data sets).