Evidence on the Orbital Modulated Gamma-Ray Emissions from the Redback Candidate 3FGL J2039.6-5618

Using 9.5 yr of Fermi Large Area Telescope data, we report the evidence on the orbital modulated gamma-ray emissions from the redback candidate 3FGL J2039.6-5618. We produced the folded light curve with the orbital period of ∼5.47 hr at a ∼4σ level. We also computed the gamma-ray spectra in two orbital phases corresponding to the inferior conjunction and the superior conjunction. We found that the <3 GeV excess in the spectrum of inferior conjunction can be modeled by the inverse Compton scattering between a relativistic pulsar wind and background soft photons of the companion star. The orbital modulation can also be explained by the evolving collision angle between the particles and photons in the same model. Through period searches by the Rayleigh test and the flux variability, we speculate that the orbital modulation is not detected after MJD ∼57,000. We propose a possible explanation in which the intrabinary shock is located closer to the pulsar so that the pulsar wind carries a smaller Lorentz factor. We estimated that the resultant inverse Compton component will be too soft and too weak to be observed.