A semi-coherent search for optical pulsations from Scorpius X-1

Abstract: We present the first application of semi-coherent strategies in the search for optical pulsations from binary systems, which resulted in the tightest constraints on pulsations from Sco X-1. We analysed observations from the SiFAP2 fast photometer mounted at the TNG spread across four years, for a total of $\sim$56 ks divided in two datasets. The great efficiency of semi-coherent techniques when only limited knowledge on the orbital parameters is available allowed us to set an upper limit at $9.23 \cdot 10^{-5}$ in pulsed amplitude for this source, improving on previous results by a factor of 4. Reaching the same upper limit with fully coherent searches would have required a number of trials more than two orders of magnitude larger. We also applied the same algorithm to an optical observation of a system containing a known pulsar, PSR J1023+0038, ignoring the refined knowledge of the orbital parameters that was allowed by the identification of the pulsar itself in the radio band: through this analysis, we proved that detection of the pulsar would have followed even with data in the optical band alone.