Revisiting <i>γ</i> -ray Orbital Modulation in the Redback Millisecond Pulsar PSR J2039−5617

Abstract PSR J2039−5617 is a redback millisecond pulsar binary system consisting of a compact star with a mass of 1.1–1.6 M ⊙ and a low-mass companion of 0.15–0.22 M ⊙ . For this binary, we performed a timing analysis using 16 yr of data from the Fermi Large Area Telescope, covering the period from 2008 August to 2024 October. Our analysis detected an orbital modulation with a period of 0.2279781 days at a significance level of ∼4 σ , which is in good agreement with previous findings. However, unlike previous reports, we identified a transition in the orbital modulation around 2021 August, after which the orbital signal disappeared. We speculate that the system may be undergoing a transition from a rotation-powered to an accretion-powered state at this epoch. Additionally, we conducted the phase-resolved and spectral analyses, and in the phase-resolved results, we observed an anticorrelation between its γ -ray and X-ray emissions, which is consistent with the predictions of high-energy radiation models for such systems. We provide some predictive discussions based on the results of γ -ray data analysis, and future Fermi-LAT observations will determine whether these predictions hold true.