Precise optical timing of PSR J1023+0038, the first millisecond pulsar detected with Aqueye+ in Asiago

We report the first detection of an optical millisecond pulsar with the fast photon counter Aqueye+ in Asiago. This is an independent confirmation of the detection of millisecond pulsations from PSR J1023+0038 obtained with SiFAP at the Telescopio Nazionale Galileo. We observed the transitional millisecond pulsar PSR J1023+0038 with Aqueye+ mounted at the Copernicus telescope in 2018 January. Highly significant pulsations were detected. The rotational period is in agreement with the value extrapolated from the X-ray ephemeris, while the time of passage at the ascending node is shifted by 11.55 ± 0.08 s from the value predicted using the orbital period from the X-rays. An independent optical timing solution is derived over a baseline of a few days that has an accuracy of ∼0.007 in pulse phase (∼12 μs in time). This level of precision is needed to derive an accurate coherent timing solution for the pulsar and to search for possible phase shifts between the optical and X-ray pulses using future simultaneous X-ray and optical observations.