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 – 0.22 M _ { \odot } , confirming the redback classification .
Optical light curve modelling is complicated by variability , but we find an inclination i \gtrsim 70 \degr , for a low pulsar mass between 1.1 M _ { \odot } < M _ { 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 .