Properties and Evolution of the Redback Millisecond Pulsar Binary PSR J2129-0429

TL;DR

This paper studies the properties and long-term evolution of the redback millisecond pulsar binary PSR J2129-0429, revealing its massive pulsar, heavy companion, and potential future evolutionary paths based on a decade of photometric and spectroscopic data.

Contribution

It provides the longest baseline photometry for such a system, measures the pulsar's mass accurately, and analyzes the binary's evolutionary status and future prospects.

Findings

Pulsar mass estimated at 1.74 solar masses.

Companion is one of the heaviest known redback stars.

Detected secular decline in optical brightness suggesting cooling.

Abstract

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: $1.74\pm0.18 M_\odot$. The G-type pulsar companion has mass $0.44\pm0.04 M_\odot$, one of the heaviest known redback companions. It is currently 95\% 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.