PROPERTIES and EVOLUTION of the REDBACK MILLISECOND PULSAR BINARY PSR J2129-0429

PSR J2129-0429 is a "redback" eclipsing millisecond pulsar binary with an unusually long 15.2 hr orbit. It was discovered by the Green Bank Telescope in a targeted search of unidentified Fermi gamma-ray sources. The pulsar companion is optically bright (mean m_R = 16.6 mag), allowing us to construct the longest baseline photometric data set available for such a system. We present 10 years of archival and new photometry of the companion from the Lincoln Near-Earth Asteroid Research Survey, the Catalina Real-time Transient Survey, the Palomar Transient Factory, the Palomar 60 inch, and the Las Cumbres Observatory Global Telescope. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200 inch indicates that the pulsar is massive: 1.74 ±0.18 M_O. The G-type pulsar companion has mass 0.44 ±0.04 M_O, one of the heaviest known red back companions. It is currently 95 ±1% Roche-lobe filling and only mildly irradiated by the pulsar. We identify a clear 13.1 mmag yr^-1 secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical light curve shape. This behavior may indicate that the companion is cooling. Binary evolution calculations indicate that PSR J2129-0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar-white dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation.