A Highly Eccentric 3.9-Millisecond Binary Pulsar in the Globular Cluster NGC 6652

We present the Robert C. Byrd Green Bank Telescope discovery of the highly eccentric binary millisecond pulsar PSR J1835$-$3259A in the Fermi Large Area Telescope-detected globular cluster NGC 6652. Timing over one orbit yields the pulse period 3.89 ms, orbital period 9.25 d, eccentricity $\sim 0.95$, and an unusually high companion mass of $0.74\,M_{\odot}$ assuming a $1.4\,M_{\odot}$ pulsar. We caution that the lack of data near periastron prevents a precise measurement of the eccentricity, and that further timing is necessary to constrain this and the other orbital parameters. From tidal considerations, we find that the companion must be a compact object. This system likely formed through an exchange encounter in the dense cluster environment. Our initial timing results predict the measurements of at least two post-Keplerian parameters with long-term phase-connected timing: the rate of periastron advance $\dot{\omega} \sim 0.1^{\circ}\,$yr$^{-1}$, requiring 1 yr of phase connection; and the Einstein delay $\gamma_{\mathrm{GR}} \sim 10\,$ms, requiring 2-3 yr of timing. For an orbital inclination $i > 50^{\circ}$, a measurement of $\sin{i}$ is also likely. PSR J1835$-$3259A thus provides an opportunity to measure the neutron star mass with high precision; to probe the cluster environment; and, depending on the nature of the companion, to investigate the limits of general relativity.