A Massive and Eccentric Binary Millisecond Pulsar in the Globular Cluster M10

We present the timing solution of the millisecond pulsar PSR J1657−0406A in the globular cluster M10, based on observations performed with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The timing results show that PSR J1657−0406A is in a long-period ( P b ∼ 37 days) and eccentric ( e ∼ 0.12) orbit. We also measured the periastron advance ω ̇ = 0.00090 ( 13 ) deg yr −1 , which implies a total system mass of 2.5(6) M ⊙ by assuming that general relativity accurately models the orbital motion. No optical counterpart was detected within the positional uncertainty, indicating that the companion is unlikely to be a main-sequence star. Considering possible formation channels for the orbital eccentricity, we favor a helium white dwarf companion, although a massive white dwarf or neutron star cannot be completely excluded.