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A Massive Millisecond Pulsar in an Eccentric Binary

The recent discovery of a population of eccentric (e ~ 0.1) millisecond pulsar (MSP) binaries with low-mass white dwarf companions in the Galactic field represents a challenge to evolutionary models that explain MSP formation as recycling: all such models predict that the orbits become highly circularised during a long period of accretion. The members of this new population exhibit remarkably similar properties (orbital periods, eccentricities, companion masses, spin periods) and several models have been put forward that suggest a common formation channel. In this work we present the results of an extensive timing campaign focusing on one member of this new population, PSR J1946+3417. Through measurement of the both the advance of periastron and Shapiro delay for this system, we determine the mass of the pulsar, companion and the inclination of the orbit to be 1.828(22) Msun, 0.2656(19) Msun and 76.4(6) , under the assumption that general relativity is the true description of gravity. Notably, this is the third highest mass measured for any pulsar. Using these masses and the astrometric properties of PSR J1946+3417 we examine three proposed formation channels for eccentric MSP binaries. While our results are consistent with eccentricity growth driven by a circumbinary disk or neutron star to strange star phase transition, we rule out rotationally delayed accretion-induced collapse as the mechanism responsible for the configuration of the PSR J1946+3417 system.