We present the results of precision gamma-ray timing measurements of the binary millisecond pulsar PSR J2339−0533, an irradiating system of "redback" type, using data from the Fermi Large Area Telescope. We describe an optimized analysis method to determine a long-term phase-coherent timing solution spanning more than six years, including a measured eccentricity of the binary orbit and constraints on the proper motion of the system. A major result of this timing analysis is the discovery of an extreme variation of the nominal 4.6-hour orbital period Porb over time, showing alternating epochs of decrease and increase. We inferred a cyclic modulation of Porb with an approximate cycle duration of 4.2 years and a modulation amplitude of ΔPorb/Porb=2.3×10−7. Considering different possible physical causes, the observed orbital-period modulation most likely results from a variable gravitational quadrupole moment of the companion star due to cyclic magnetic activity in its convective zone.
@article{arxiv.1504.07466,
title = {Gamma-ray Timing of Redback PSR J2339-0533: Hints for Gravitational Quadrupole Moment Changes},
author = {Holger J. Pletsch and Colin J. Clark},
journal= {arXiv preprint arXiv:1504.07466},
year = {2015}
}