Is the Fermi source 4FGL J1824.2+1231 a transitional millisecond pulsar?

¹ Ioffe Institute, 26 Politekhnicheskaya, St. Petersburg 194021, Russia
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 106, Baja California, México 22860, Mexico
³ Department of Astronomy and Satellite Geodesy, Kazan Federal University, Kremlevskaya Str. 18, Kazan 420008, Russia
⁴ Academy of Sciences of Tatarstan, Baumana Str. 20, 420111 Kazan, Russia
⁵ Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany
⁷ Ulugh Beg Astronomical Institute, Uzbekistan Academy of Sciences, Tashkent 100052, Uzbekistan
⁸ Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaya st., 29, St. Petersburg 195251, Russia
⁹ SECIHTI, Instituto de Astronomía, Universidad Nacional Autónoma de México, 22860 Ensenada, BC, Mexico

^⋆ Corresponding author: da.zyuzin@gmail.com

Received: 2 May 2025
Accepted: 26 May 2025

Abstract

Context. Transitional millisecond pulsars (tMSPs) in tight binary systems represent an important evolutionary link between low-mass X-ray binaries and radio millisecond pulsars. To date, only three confirmed tMSPs and a few candidates have been discovered. Most of them are γ-ray sources. For this reason, searching for multi-wavelength counterparts to unassociated Fermiγ-ray sources can help us find new tMSPs.

Aims. We investigate whether the unassociated γ-ray source 4FGL J1824.2+1231 belongs to the tMSP family.

Methods. To find the counterpart to 4FGL J1824.2+1231, we used data from the SRG/eROSITA and Swift X-ray catalogues and from different optical catalogues. We also performed time-series photometric optical observations of the source with the 2.1 m telescope of the Observatorio Astronómico Nacional San Pedro Mártir, the 1.5 m telescope of the Maidanak Astronomical Observatory, and the 1.5 m Russian-Turkish telescope. In addition, we carried out optical spectroscopic observations with the Russian-Turkish telescope and used archival spectroscopic data obtained with the Gemini-North telescope.

Results. Within the position error ellipse of 4FGL J1824.2+1231, we find only one X-ray source that coincides with an optical object. We consider it a likely multi-wavelength counterpart to 4FGL J1824.2+1231. The source shows strong optical variability and significant proper motion. The latter strongly implies that it is a Galactic source. Double-peaked H and He emission lines are detected in its spectrum with a flat continuum, as often observed in accretion disks of compact binary systems. The X-ray spectrum is well fitted by a power law with a photon index of ∼1.7. The derived intrinsic X-ray-to-γ-ray flux ratio is about 0.2.

Conclusions. Assuming the X-ray/optical source is the true counterpart to 4FGL J1824.2+1231, all its properties suggest that it is a tMSP in the sub-luminous disk state.