NGC 1851A: Revealing an ongoing three-body encounter in a dense globular cluster

PSR J0514−4002A is a binary millisecond pulsar located in the globular cluster NGC 1851. The pulsar has a spin period of 4.99 ms, an orbital period of 18.8 days, and is in a very eccentric (e = 0.89) orbit around a massive companion. In this work, we present the updated timing analysis of this system, obtained with an additional 1 yr of monthly observations using the Giant Metrewave Radio Telescope and 2.5 yr of observations using the MeerKAT telescope. Combined with the earlier data, this allowed for the precise measurement of the proper motion of the system (μα = 2.61(13) mas yr−1 and μδ = −0.90(11) mas yr−1). This implies that the transverse velocity relative to the cluster is 30 ± 7 km s−1, which is smaller than the escape velocity of the cluster, and thus is consistent with the pulsar’s association to NGC 1851. In addition to the spin frequency and its derivative, we also confirmed the large second spin frequency derivative and large associated jerk (which has increased the spin frequency derivative by a factor of 27 since the mid-2000s). A measurement of the third spin frequency derivative for the pulsar showed that the strength of this jerk has increased by ∼65% in the same time period, and we analysed the detailed implications of these measurements. First, we point out that to get a consistent picture of the orbital evolution, we must take the effect of the changing acceleration into account: this allowed for much improved estimates of the orbital period derivative and solved one of the puzzles raised by previous timing. Second, we find that the large and fast-increasing jerk implies the presence of a third body in the vicinity of the pulsar. Based on our measured parameters, we constrained the mass, distance, and orbital parameters for this third body. No counterpart is detectable within the distance limit from NGC 1851A in existing Hubble Space Telescope images. In any such configuration, the tidal contributions induced by the third body to the post-Keplerian parameters are relatively small, and the precise measurement of these parameters allowed us to obtain precise measurements of the total and component masses for the system: Mtot = 2.4734(3) M⊙, Mp = 1.39(3) M⊙, Mc = 1.08(3) M⊙. This also indicates that the companion to the pulsar is a massive white dwarf and resolves the earlier ambiguity regarding its nature. Further observations will allow for the precise measurement of other higher-frequency derivatives and the determination of the nature of the third body, and reveal whether it is gravitationally bound to the inner binary system.