Upgraded Giant Metrewave Radio Telescope timing of NGC 1851A: a possible millisecond pulsar - neutron star system

In this work, we present the results of 1 yr of upgraded Giant Metrewave Radio Telescope timing measurements of PSR J0514-4002A, a 4.99-ms pulsar in a 18.8-d eccentric (e = 0.89) orbit with a massive companion located in the globular cluster NGC 1851. Combining these data with earlier Green Bank Telescope data, we greatly improve the precision of the rate of advance of periastron, \dot{ω} = 0.0129592(16) \deg yr^{-1} which, assuming the validity of general relativity, results in a much refined measurement of the total mass of the binary, M_tot = 2.4730(6) M_{\odot }. Additionally, we measure the Einstein delay parameter, γ, something that has never been done for any binary system with an orbital period larger than {∼}10 h. The measured value, γ = 0.0216(9) s, is by far the largest for any binary pulsar. Furthermore, we measure the proper motion of the system (μ _{α } = 5.19(22) and μ _{δ } = -0.56(25) mas yr^{-1}), which is not only important for analysing its motion in the cluster, but is also essential for a proper interpretation of γ, given the latter parameter's correlation with the variation of the projected semimajor axis. The measurements of γ and the proper motion enable a separation of the system component masses: we obtain a pulsar mass of M_p = 1.25^{+0.05}_{-0.06} M_{\odot } and a companion mass of M_c = 1.22^{+0.06}_{-0.05} M_{\odot }. This raises the possibility that the companion is also a neutron star. Searches for radio pulsations from the companion have thus far been unsuccessful; hence, we cannot confirm the latter hypothesis. The low mass of this millisecond pulsar - one of the lowest ever measured for such objects - clearly indicates that the recycling process can be achieved with a relatively small amount of mass transfer.