Absolute timing of the Crab Pulsar at optical wavelengths with STJs

Research and Scientific Support Department of ESA, ESTEC, Po stbus 299, 2200 AG Noordwijk, The NetherlandsReceived March 23, 2006; accepted May 24, 2006ABSTRACTContext.We have observed the Crab Pulsar in the optical with S-Cam, aninstrument 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 differentversions of the instrument, and using two different GPS units.Results.We consistently find that the optical peak leads the radio pea k by 49±90, 254±170, and 291±100 µs. On assumption of a constantoptical lead, the weighted-average value is∼170 µs, or when rejecting (based on a perhaps questionable radio ephemeris) the first measurement,273±100 µs.Key words. pulsars: individual (Crab Pulsar, PSR J0534+2200)