Evidence for gravitational quadrupole moment variations in the companion of PSR J2051-0827

TL;DR

This study presents 13 years of radio timing data of PSR J2051-0827, revealing orbital parameter variations likely caused by the companion's changing gravitational quadrupole moment due to tidal effects.

Contribution

It provides the first evidence linking orbital period variations in this pulsar to changes in the companion's gravitational quadrupole moment.

Findings

Measured orbital eccentricity and proper motion.

Detected secular variations in semi-major axis and orbital period.

Attributed period variations to quadrupole moment changes.

Abstract

We have conducted radio timing observations of the eclipsing millisecond binary pulsar J2051-0827 with the European Pulsar Timing Array network of telescopes and the Parkes radio telescope, spanning over 13 years. The increased data span allows significant measurements of the orbital eccentricity, e = (6.2 {\pm} 1.3) {\times} 10^{-5} and composite proper motion, {\mu}_t = 7.3 {\pm} 0.4 mas/yr. Our timing observations have revealed secular variations of the projected semi-major axis of the pulsar orbit which are much more extreme than those previously published; and of the orbital period of the system. Investigation of the physical mechanisms producing such variations confirm that the variations of the semi-major axis are most probably caused by classical spin-orbit coupling in the binary system, while the variations in orbital period are most likely caused by tidal dissipation leading to changes in the gravitational quadrupole moment of the companion.