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

Abstract

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 $0.74M_{\odot }$ assuming a $1.4M_{\odot }$ 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 $\dot{\ddot{I}}‰\sim 0.1^{\circ }$yr${}^{-1}$, requiring 1 yr of phase connection; and the Einstein delay ${\hat{I}}_{\mathrm{GR}}^3\sim 10$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.

Accepted by ApJ Letters; 7 pages, 3 tables, 2 figures