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

TL;DR

Revisits gamma-ray orbital modulation in the redback MSP PSR J2039-5617 using ~16 years of Fermi-LAT data to test the persistence of modulation and its possible link to an MSP-to-LMXB transition. The analysis combines aperture photometry timing with likelihood-based spectral and phase-resolved studies, updating the source model with added nearby gamma-ray sources. It finds a persistent modulation at $P_{ m orb}=0.2279781$ days that vanishes around $T_{ m transition}$ in 2021, and reports an anti-correlation between gamma-ray and X-ray emission with largely unchanged spectral shapes across orbital phase. These results support an IC-scattering–driven, extended emission region and highlight a potential evolutionary pathway for redback systems, underscoring the need for continued multiwavelength monitoring.

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 $\sim4σ$, 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 $γ$-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 $γ$-ray data analysis, and future Fermi-LAT observations will determine whether these predictions hold true.

Figures (6)

• Figure 1: 0.1--500.0 GeV TS maps centered on 4FGL J2039.5$-$5617 with a region of $10^{\circ}\times10^{\circ}$. The target is marked by a red crosse, while other $\gamma$-ray sources from the 4FGL catalog are denoted by orange crosses, the six NGSs are shown as green circles, with radii proportional to their positional uncertainties. Left panel: TS map showing the $\gamma$-ray emissions from 4FGL J2039.5$-$5617, derived from the best-fit model with the target excluded. Right panel: Residual TS map generated using the same model, but including 4FGL J2039.5$-$5617.
• Figure 2: SEDs of 4FGL J2039.5$-$5617 in 0.1--500.0 GeV. Left: SEDs for three time intervals, 2008-08-04 to 2024-10-23 (black, Whole), 2008-08-04 to 2021-08-01 (red, P$_1$), and 2021-08-01 to 2024-10-23 (orange, P$_2$), with their best-fit PLE4 models shown as a black solid, red dashed-dotted, and orange dotted line, respectively. For the Whole results, TS values for each energy bin are displayed as a gray histogram (on right y-axis). Right: SEDs during the epoch where the AP light curve shows periodicity (P1). The red points match those in the left panel, while purple and green points correspond to phase intervals around inferior and superior conjunction, respectively, as that indicated by the pink and cyan shaded regions in Figure \ref{['fig:phlc']}. Their best-fit models are shown as purple dashed and green dotted lines, respectively.
• Figure 3: LSP power spectra (black histogram) of 4FGL J2039.5$-$5617 derived from the AP light curve in 0.18--9.39 GeV. Left: LSP power spectrum for the Whole AP light curve, with the 3$\sigma$ and 4$\sigma$ confidence levels indicated by green dashed and blue dash-dotted lines, respectively. Right: LSP power spectra for the AP light curve in P$_1$ and P$_2$, shown in black and orange, respectively.
• Figure 4: LSP power values around the periodic signal obtained from the accumulated data. The time axis corresponds to the end dates of the AP light curves, all starting from the same epoch (MJD 54682.687). As an example, the AP light curve spanning from 2008 August 4 to 2021 August 1 yields the highest LSP power of $\sim$19.95 (blue dashed line). The gray dashed-dotted line indicates the end date of the dataset analyzed by Ng2018, while the gray dotted line marks the epoch when orbital modulation was reported to disappear in their work. In comparison, the blue dashed line denotes the epoch at which we find the modulation to vanish. The evolution of the signal period with accumulated data is plotted as orange circles on the right y-axis.
• Figure 5: Cumulative H-test results obtained using the orbital periods $P_{\rm orb}$ (derived in this work, black) and $P_{\rm Strader}$ (adopted by Ng2018, orange). All others are the same as in Figure \ref{['fig:pvst']}.