Absolute timing of the Crab Pulsar at optical wavelengths with STJs

Context. We have observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with μs time resolution. Aims. Our aim was to study the delay between the radio and optical pulse. Methods. The Crab Pulsar was observed three times over a time span of almost 7 years, on two different locations, using three different versions of the instrument, and using two different GPS units. Results. We consistently find that the optical peak leads the radio peak by 49±90, 254±170, and 291±100 μs. On assumption of a constant optical lead, the weighted-average value is ∼170 μs, or when rejecting (based on a perhaps questionable radio ephemeris) the first measurement, 273±100 μs.