An eccentric binary millisecond pulsar in the Galactic plane

Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+ 0327, a radio pulsar with a rotational period of 2.15 milliseconds in a highly eccentric ( e = 0.44) 95- day orbit around a solar mass ( M.) companion. Infrared observations identify a possible main- sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main- sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster, then ejecting it into the Galactic disk, or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74 +/- 0.04 M. an unusually high value.