The orbital phase coverage of radio observations of a new black-widow millisecond pulsar, PSR J1544−2555, obtained with MeerKAT, Murriyang/Parkes, Nançay, and Effelsberg in red (from Belmonte Diaz et al. 2025).

Millisecond pulsars (MSPs) differ from the regular pulsar population due to their fast spin periods and low spin-down rates MSPs are thought to be formed in binary systems through the so-called recycling scenario, where accretion from the companion star spins up the pulsar through mass transfer. Spider pulsars are an emerging class of MSP binaries. They contain an MSP and a tidally-locked low-mass stellar companion in a compact (less than one day) orbit. The companion mass divides the population into two sub-categories: black widows contain a lower mass companion (less than 0.1 solar masses) in comparison to redbacks (greater than 0.1M solar masses). These energetic systems can emit radiation across the entire electromagnetic spectrum. While the MSP dominates the gamma-ray and radio emission, the optical emission is dominated by the companion star.

Belmonte Díaz report the discovery of a new black-widow millisecond pulsar, PSR J1544−2555, associated with a Fermi gamma-ray source. Optical, radio, and gamma-ray observations confirmed its nature as a compact spider binary system. Optical photometry from ULTRACAM revealed a ∼2.7-hour orbital period, guiding MeerKAT observations that detected ∼2.4-ms radio pulsations. Subsequent timing campaigns using the Murriyang Parkes Telescope, the Effelsberg 100-m Radio Telescope, and the Nançay Radio Telescope yielded a preliminary timing solution, which enabled gamma-ray pulsations to be found. The figure above shows the orbital phase coverage of radio observations obtained with different telescopes: MeerKAT (MKT) in cyan, Murriyang/Parkes (PKS) in blue, Nançay (NRT) in green, and Effelsberg (EFF) in red. Phase 0 corresponds to the pulsar at the ascending node, and phase 0.25 corresponds to the pulsar’s superior conjunction. Strong detections are displayed by brighter solid lines, while weak detections and non-detections are displayed with a dimmer dashed and dotted line respectively. The grey-shaded region indicates when a clear eclipse was detected in the observation. This discovery exemplifies the proven capability of the Fermi-LAT catalogue in identifying millisecond pulsar candidates and highlights the role of optical surveys in detecting variable sources suitable for radio follow-up.