Published November 2025 | Version Published
Journal Article Open

Multiwavelength observations of a new black-widow millisecond pulsar PSR J1544−2555

  • 1. ROR icon University of Manchester
  • 2. ROR icon National Astronomical Research Institute of Thailand
  • 3. ROR icon Osservatorio Astronomico di Cagliari
  • 4. ROR icon Max Planck Institute for Gravitational Physics
  • 5. ROR icon Leibniz University Hannover
  • 6. ROR icon Max Planck Institute for Extraterrestrial Physics
  • 7. ROR icon University of Erlangen-Nuremberg
  • 8. ROR icon Max Planck Institute for Radio Astronomy
  • 9. ROR icon South African Radio Astronomy Observatory
  • 10. ROR icon California Institute of Technology
  • 11. ROR icon University of Sheffield
  • 12. ROR icon Instituto de Astrofísica de Canarias
  • 13. ROR icon University of Maryland, College Park
  • 14. ROR icon Goddard Space Flight Center
  • 15. ROR icon University of Orléans
  • 16. ROR icon Paris Observatory
  • 17. ROR icon University College Cork
  • 18. ROR icon Durham University

Abstract

We report the discovery of a new black-widow millisecond pulsar, PSR J1544–2555, associated with the Fermi-Large Area Telescope (LAT) source 4FGL J1544.2–2554. Optical, radio, and gamma-ray observations confirmed its nature as a compact spider binary system. Optical photometry from ULTRACAM revealed a ~2.7-h 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 allowed us to obtain a preliminary timing solution, which enabled us to find gamma-ray pulsations. The final timing solution, spanning 16 yr of Fermi-LAT gamma-ray data, also displays orbital period variations typical of spider pulsars. X-ray observations from eROSITA indicate non-thermal emission, but the relatively low count rate prohibits the search for X-ray pulsations. Optical light curve modelling using Icarus suggests the asymmetry is best explained by a spot model, where uneven heating creates localized temperature variations on the companion. While the optical spectra we obtained are compatible with the physical properties we infer for the companion star, they were not of sufficient signal-to-noise to allow for radial velocity measurements, thus limiting constraints on the neutron star’s mass. The observed bluer colour near the light curve minimum suggests possible non-thermal emission from intrabinary shocks, supported by the presence of an X-ray source. 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.

Copyright and License

© The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

Acknowledgement

The authors would like to thank Soheb Mandhai and Pengyue Sun for their help with conducting optical observations.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory (SARAO), which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. We thank staff at SARAO for their help with observations and commissioning. TRAPUM observations used the FBFUSE and APSUSE computing clusters for data acquisition, storage, and analysis. These clusters were funded and installed by the Max-Planck-Institut für Radioastronomie (MPIfR) and the Max-Planck-Gesellschaft. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Parkes radio telescope is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Wiradjuri people as the traditional owners of the Observatory site. The Nançay Radio Observatory is operated by the Paris Observatory, associated with the French Centre National de la Recherche Scientifique (CNRS) and Université d’Orléans. It is partially supported by the Region Centre Val de Loire in France.

The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l’Energie Atomique and the Centre National de la Recherche Scientifique / Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK), and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

TT is grateful to the National Astronomical Research Institute of Thailand (NARIT) for awarding a student scholarship. RPB acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programe (grant agreement No. 715051; Spiders). SBD acknowledges the support of a Science and Technology Facilities Council (STFC) stipend (grant number: ST/X001229/1) to permit work as a postgraduate researcher. ECF was supported by NASA under award number 80GSFC24M0006. ULTRACAM and VSD are supported by STFC. Lastly, we would like to thank Lucas Guillemot for reviewing the manuscript on behalf of the Fermi-LAT Collaboration.

Data Availability

TRAPUM and ULTRACAM observations are available upon reasonable request to the corresponding author. Reduced and calibrated optical light curves are available in accompanying online material. The Fermi-LAT data are available from the Fermi Science Support Center (FSSC; http://fermi.gsfc.nasa.gov/ssc). Gamma-ray timing solutions are also available from the FSSC (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/ephems/).

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2509.09605 (arXiv)

Funding

National Astronomical Research Institute of Thailand
European Research Council
715051
Science and Technology Facilities Council
ST/X001229/1
National Aeronautics and Space Administration
80GSFC24M0006

Dates

Accepted
2025-09-11
Available
2025-09-16
Published
Available
2025-10-15
Corrected and typeset

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Caltech groups
Division of Physics, Mathematics and Astronomy (PMA)
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Published