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 timingmeasurements 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, (omega) over dot = 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-circle dot. Additionally, we measure the Einstein delay parameter, gamma, something that has never been done for any binary system with an orbital period larger than similar to 10 h. The measured value, gamma = 0.0216(9) s, is by far the largest for any binary pulsar. Furthermore, we measure the proper motion of the system (mu(alpha) = 5.19(22) and mu(delta) = -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 gamma, given the latter parameter's correlation with the variation of the projected semimajor axis. The measurements of gamma and the proper motion enable a separation of the system component masses: we obtain a pulsar mass of M-p = 1.25(-0.06)(+0.05)M(circle dot) and a companion mass of M-c = 1.22(-0.05)(+0.06)M(circle dot). 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.