Letter to the Editor

Evidence of optical pulsations from a redback millisecond pulsar

¹ INAF Osservatorio Astronomico di Roma, via di Frascati 33, I-00078 Monteporzio Catone, Roma, Italy
² INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy
³ Max Planck Institute for Gravitational Physics (Albert Einstein Institute), D-30167 Hannover, Germany
⁴ Leibniz Universität Hannover, D-30167 Hannover, Germany
⁵ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185 Rome, Italy
⁶ INAF Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate (LC), Italy
⁷ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁸ Fundación Galileo Galilei – INAF, Rambla Josè Ana Fernández Pèrez, 7, E-38712 Breña Baja, TF, Spain
⁹ Università degli Studi di Catania, Via S. Sofia, 64, I-95123 Catania, Italy

^⋆ Corresponding author: alessandro.papitto@inaf.it

Received: 8 August 2025
Accepted: 24 September 2025

Abstract

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.