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

TL;DR

This study presents a 20-year timing analysis of the pulsar PSR J1905+0154A in NGC 6749, revealing its binary nature with a white dwarf companion and discussing its properties and potential GC association.

Contribution

The paper provides the first precise timing solution for this pulsar, characterizes its companion as a Helium white dwarf, and explores its dynamics within the globular cluster.

Findings

The pulsar has a spin period of 3.2 ms and an orbital period of 0.81 days.

The white dwarf companion is a Helium WD with a mass of 0.17-0.19 M_sun.

The system's velocity suggests it may not be gravitationally bound to the GC.

Abstract

PSR J1905+0154A is a binary millisecond pulsar 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 WD companion. Combining early Arecibo and latter Five Hundred meter Aperture Spherical Telescope (FAST) data, we were able to derive a phase-coherent timing solution for this pulsar, which now spans 20 years. This 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 an estimate of the intrinsic spin-down: the inferred magnetic field at the surface (2.2 - 2.4 * 10^8 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 coincides with the position of one of the very few candidate white dwarfs (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_sun, 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 velocity of the system relative to the GC, which is 4.5-sigma 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.