Revisiting $\gamma$-Ray Orbital Modulation in the Redback Millisecond Pulsar PSR J2039-5617

TL;DR

This study analyzes 16 years of Fermi-LAT data on PSR J2039-5617, revealing a transition in orbital modulation around 2021 and an anti-correlation between gamma-ray and X-ray emissions, suggesting a possible state change in the system.

Contribution

It provides the first evidence of a transition in orbital modulation and phase-resolved emission analysis in PSR J2039-5617, indicating a potential shift from rotation-powered to accretion-powered state.

Findings

Detected orbital modulation with ~4σ significance.

Observed disappearance of orbital signal after 2021 August.

Found anti-correlation between gamma-ray and X-ray emissions.

Abstract

PSR J2039-5617 is a redback millisecond pulsar binary system consisting of a compact star with a mass of 1.1-1.6 $M_\odot$ and a low-mass companion of 0.15-0.22 $M_\odot$. For this binary, we performed a timing analysis using 16 years 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 $\sim4\sigma$, 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 anti-correlation between its $\gamma$-ray and X-ray emissions, which consistent with the predictions of high-energy radiation models for such systems. We provide some predictive discussions based on the results of $\gamma$-ray data analysis, and future Fermi-LAT observations will determine whether these predictions hold true.