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

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 $(1.71\pm 0.23)\times 10^{-2}{M}_{\odot }$, a radius of $(7.6\pm 1.4)\times 10^{-2}{R}_{\odot }$, and a mean density of $54\pm 26{\mathrm{g}\mathrm{cm}}^{-3}$, all for an assumed neutron star mass of $1.4M_{\odot }$. 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.

4 figures, 1 table. Submitted to ApJ on June 29, 2018. Accepted on July 20, 2018