The one and the only: The pulsar-white dwarf system in NGC 6749

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² School of Physics and Astronomy, Beijing Normal University, Beijing 100875, China
³ Department of Physics, Faculty of Arts and Sciences, Beijing Normal University, Zhuhai 519087, China
⁴ Dipartimento di Fisica e Astronomia “Augusto Righi,” Alma Mater Studiorum Università di Bologna, via Piero Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF—Osservatorio di Astro sica e Scienze dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁶ National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China
⁷ Key Laboratory of Time Reference and Applications,Chinese Academy of Sciences, Xi’an 710600, China
⁸ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, PR China
⁹ Guizhou Radio Astronomical Observatory, Guizhou University, Guiyang 550025, PR China
¹⁰ Key Laboratory of Radio Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
¹¹ College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China
¹² Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
¹³ Department of Physics, University of Alberta, CCIS 4-183, Edmonton, AB T6G 2E1, Canada

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Received: 31 March 2026
Accepted: 6 May 2026

Abstract

PSR J1905+0154A is a binary millisecond pulsar (MSP) located in the globular cluster (GC) NGC 6749. It was discovered in 2004 in a search for pulsars in GCs carried out with the Arecibo 305-m radio telescope. The pulsar has a spin period of 3.2 ms, an orbital period of 0.81 days, and is in a low-eccentricity orbit with a low-mass white dwarf (WD) companion. By combining timing data from the early Arecibo observations of NGC 6749 with timing data from observations of this GC made with the Five-hundred-meter Aperture Spherical Telescope (FAST), we derived a phase-coherent timing solution for this pulsar, which now spans 20 years. This solution includes a precise measurement of the astrometric, spin, and orbital parameters of the system. The small range of predicted accelerations expected from the gravitational field of this GC allows us to give an estimate of the intrinsic spin-down: the inferred magnetic field at the surface (2.2-2.4 ×10⁸ G) and characteristic age (2.8-3.5 Gyr) are typical of what one finds among MSPs in the Galactic field. The position of this pulsar, the only confirmed to date in this GC, coincides with the position of one of the very few candidate WDs in the whole HST dataset on this GC. The position of the companion in the colour-magnitude diagram is consistent with a helium WD with a mass of 0.17-0.19 M_⊙ (which implies an orbital inclination between 28 and 40 degrees), a cooling age of 0.4-0.7 Gyr, and a surface temperature of 11 600-14 800 K. A comparison with the characteristic age of the pulsar indicates that at the time of Roche lobe detachment, the spin period was between 1.98 and 2.62 ms. The relatively large proper motion difference relative to the motion of the GC, which is 4.5σ significant and an order of magnitude larger than the escape velocity, raises the possibility that, despite its location close to the centre of the GC, the pulsar might not be associated with it. Finally, our effort to confirm a second pulsar candidate in this GC did not yield a positive confirmation, nor the discovery of any additional pulsar in this GC.