Evidence of optical pulsation from a redback millisecond pulsar

Recent research, published in the journal Astronomy and Astrophysics Letters and led by Alessandro Papitto (INAF-Osservatorio Astronomico di Roma) and other international collaborators, has unveiled evidence of optical pulsations from the redback millisecond pulsar PSR J2339-0533. This exceptional result was obtained thanks to an intensive observation campaign conducted with the fast single-photon photometer SiFAP2, installed at the Telescopio Nazionale Galileo (TNG). PSR J2339-0533 is the third optical millisecond pulsar ever discovered, all by SiFAP2, and the first not surrounded by an accretion disk.

Millisecond pulsars are rapidly rotating neutron stars, often found in compact binary systems, which can exhibit two states: powered by the accretion of matter from the companion star or powered by their rotation (as radio pulsars).

Data collected by SIFAP2 suggest that the optical counterpart, at magnitude V ~18, is pulsating at the neutron star's rotation frequency with a fractional amplitude corresponding to a pulsating magnitude V ~ 26. The results obtained from the dataset open an important window into the emission mechanisms of these cosmic objects.

The potential detection of these optical pulsations is particularly significant because it suggests that the pulsed optical emission in these systems may not depend on the presence of an accretion disk. The amount of rotational energy converted into pulsed optical luminosity was found to be comparable to that observed in young isolated pulsars, such as the famous Crab Pulsar, but 50 to 100 times lower than that observed in systems with an accretion disk. These results, if confirmed, support the hypothesis that the accretion disk serves to "boost" the efficiency of the optical emission, but optical pulsations can also be seen from millisecond pulsars that are not surrounded by a disk.

The research once again demonstrates the importance of the SiFAP2@TNG observational capabilities in studying the fastest and most violent phenomena in the Universe.

More information in the paper.

Graphics: Phase-aligned pulse profiles observed in W2 by SiFAP2 (black points), Parkes (green line), and Fermi-LAT (magenta line; taken from 3PC catalogue, S23, and shifted assuming that the lag with the radio profile is the same as reported there). The normalisation of the radio and gamma-ray profiles is arbitrary. The reference epoch was set to align with the radio peak at phase 0.5. The blue line is a fit to the SiFAP2 pulse profile with four Gaussians added to a constant level.

Image of a redback binary system consisting of a pulsar and a main-sequence companion star with a mass below one solar mass. Credits: John A. Paice https://www.johnapaice.com/?itemId=0trt0rpzwifakmtf3775qaqogudutc