A Dense Companion to the Short-Period Millisecond Pulsar Binary PSR J0636+5128

TL;DR

This paper reports the discovery and detailed characterization of the optical companion to the ultra-compact millisecond pulsar binary PSR J0636+5128, revealing a dense, likely degenerate remnant with implications for binary evolution.

Contribution

The study provides the first optical detection and modeling of the companion to PSR J0636+5128, including mass, radius, and density estimates, highlighting its unusual density among black widow systems.

Findings

Companion mass of approximately 0.0171 solar masses.

Companion radius around 0.076 solar radii.

Mean density of about 54 g/cm³, indicating a dense, possibly degenerate remnant.

Abstract

PSR J0636+5128 is a millisecond pulsar in one of the most compact pulsar binaries known, with a 96\,min orbital period. The pulsar mass function suggests a very low-mass companion, similar to that seen in so-called "black widow" binaries. Unlike in most of those, however, no radio eclipses by material driven off from the companion were seen leading to the possibility that the companion was a degenerate remnant of a carbon-oxygen white dwarf. We report the discovery of the optical counterpart of its companion in images taken with the Gemini North and Keck~I telescopes. The companion varies between $r=25$ and $r=23$ on the 96\,min orbital period of the binary, caused by irradiation from the pulsar's energetic wind. We modeled the multi-color lightcurve using parallax constraints from pulsar timing and determine a companion mass of $(1.71\pm0.23)\times 10^{-2}\,M_\odot$, a radius of $(7.6\pm1.4)\times 10^{-2}\,R_\odot$, and a mean density of $54\pm26\,{\rm g\,cm}^{-3}$, all for an assumed neutron star mass of $1.4\,M_\odot$. This makes the companion to PSR J0636+5128 one of the densest of the "black widow" systems. Modeling suggests that the composition is not predominantly hydrogen, perhaps due to an origin in an ultra-compact X-ray binary.