2025A&A...702L..21P

Recent detections of optical pulsations from both a transitional and an accreting millisecond pulsar have revealed unexpectedly bright signals, suggesting that the presence of an accretion disk enhances the efficiency of optical emission, possibly via synchrotron radiation from accelerated particles. In this work we present optical observations of the redback millisecond pulsar PSR J2339-0533, obtained with the SiFAP2 photometer mounted on the Telescopio Nazionale Galileo. Data accumulated during the campaign with the longest exposure time (12 h) suggest that its ∼18 mag optical counterpart pulsates at the neutron star's spin frequency. This candidate signal was identified by folding the optical time series using the pulsar ephemeris derived from nearly simultaneous observations with the 64 m Murriyang (Parkes) radio telescope. The detection significance of the candidate optical signal identified in those data lies between 2.9 and 3.5σ, depending on the statistical test employed. The pulse duty cycle is ≲1/32, and the de-reddened pulsed magnitude in the V band is (26.0 ± 0.6) mag. At a distance of 1.7 kpc, this corresponds to a conversion efficiency of ∼3 × 10^–6 of the pulsar's spin-down power into pulsed optical luminosity - which is comparable to the values observed in young, isolated pulsars like the Crab but 50-100 times lower than in disk-accreting millisecond pulsars. If confirmed, these findings suggest that optical pulsations of MSPs arise independently of an accretion disk and support the notion that such disks boost the optical emission efficiency.