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Abstract

The Fermi Large Area Telescope gamma-ray source 3FGL J2039.6$-$5618 contains
a periodic optical and X-ray source that was predicted to be a "redback"
millisecond pulsar (MSP) binary system. However, the conclusive identification
required the detection of pulsations from the putative MSP. To better constrain
the orbital parameters for a directed search for gamma-ray pulsations, we
obtained new optical light curves in 2017 and 2018, which revealed long-term
variability from the companion star. The resulting orbital parameter
constraints were used to perform a targeted gamma-ray pulsation search using
the Einstein@Home distributed volunteer computing system. This search
discovered pulsations with a period of 2.65 ms, confirming the source as a
binary MSP now known as PSR J2039$-$5617. The pulsar's orbital Doppler shift
indicates a companion mass of 0.15 to 0.22 $M_{\odot}$, confirming the redback
classification. Optical light curve modelling is complicated by variability,
but we find an inclination $i > 70{\deg}$, for a low pulsar mass between $1.1
M_{\odot} < M_{\rm psr} < 1.35 M_{\odot}$. Timing the gamma-ray pulsations also
revealed significant variability in the orbital period, which we find to be
consistent with quadrupole moment variations in the companion star, suggestive
of convective activity. We also find that the pulsed flux is modulated at the
orbital period, potentially due to inverse Compton scattering between
high-energy leptons in the pulsar wind and the companion star's optical photon
field.