Eric C. Bellm (Caltech), David L. Kaplan (Wisconsin-Milwaukee), Rene P. Breton (Jodrell Bank, Southampton), E. Sterl Phinney (Caltech), Varun B. Bhalerao (IUCAA, India), Fernando Camilo (Columbia), Sumit Dahal (New York University Abu Dhabi), S. G. Djorgovski, Andrew J. Drake (Caltech), J. W. T. Hessels (ASTRON, Amsterdam), Russ R. Laher (IPAC, Caltech), David B.
Levitan (Caltech), Fraser Lewis (FT, LJMU), Ashish A. Mahabal (Caltech), Eran O. Ofek (Weizmann Institute of Science, Israel), Thomas A. Prince (Caltech), Scott M. Ransom (NRAO, Virginia), Mallory S. E. Roberts, David M. Russell (New York University Abu Dhabi), Branimir Sesar (MPIA, Germany), Jason A. Surace (IPAC, Caltech), Sumin Tang (Caltech)

PSR J2129−0429 is a “redback” eclipsing millisecond pulsar binary with an unusually long 15.2 hour 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 dataset available for such a system. We present ten years of archival and new photometry of the companion from LINEAR, CRTS, PTF, the Palomar 60-inch, and LCOGT. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200- inch indicates that the pulsar is massive: 1.74 ± 0.18 solar masses. The G-type pulsar companion has mass 0.44 ± 0.04 solar masses, one of the heaviest known redback 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 lightcurve 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.

Read the preprint here