A 1.05 $M_\odot$ Companion to PSR J2222-0137: The Coolest Known White Dwarf?

TL;DR

This paper investigates the nature of the companion to PSR J2222-0137, providing evidence that it is likely a very cool white dwarf with a temperature below 3000 K, making it one of the coolest known white dwarfs.

Contribution

The study combines pulsar timing and deep optical/infrared searches to identify the companion as a cool white dwarf, ruling out the neutron star hypothesis based on eccentricity and non-detection.

Findings

Companion mass is approximately 1.05 solar masses.

Eccentricity is extremely low at 3e-4, inconsistent with a neutron star origin.

Companion's temperature is constrained to be below 3000 K.

Abstract

The recycled pulsar PSR J2222-0137 is one of the closest known neutron stars, with a parallax distance of $267_{-0.9}^{+1.2}\,$pc and an edge-on orbit. We measure the Shapiro delay in the system through pulsar timing with the Green Bank Telescope, deriving a low pulsar mass ($1.20\pm0.14$ $M_\odot$) and a high companion mass ($1.05\pm0.06$ $M_\odot$) consistent with either a low-mass neutron star or a high-mass white dwarf. We can largely reject the neutron star hypothesis on the basis of the system's extremely low eccentricity (3e-4) - too low to have been the product of two supernovae under normal circumstances. However, despite deep optical and near-infrared searches with SOAR and the Keck telescopes we have not discovered the optical counterpart of the system. This is consistent with the white dwarf hypothesis only if the effective temperature is <3000 K, a limit that is robust to distance, mass, and atmosphere uncertainties. This would make the companion to PSR J2222-0137 one of the coolest white dwarfs ever observed. For the implied age to be consistent with the age of the Milky Way requires the white dwarf to have already crystallized and entered the faster Debye-cooling regime.