Kes 75 with IXPE: Detection of Nebular X-ray Polarization and Change in Pulsar Lightcurve

Technical Summary: Kes 75 with IXPE: Detection of Nebular X-ray Polarization and Change in Pulsar Lightcurve

Problem Statement
The paper addresses the lack of X-ray polarization measurements for the PSR/PWN complex within the supernova remnant Kes 75 (G29.7-0.3). This system hosts PSR J1846-0258, a young, high-field pulsar ($B_{surf} \sim 5 \times 10^{13}$ G) known for exhibiting magnetar-like outbursts and timing irregularities. While the nebula's morphology and radio polarization are well-studied, the X-ray polarization properties of the pulsar and its wind nebula (PWN) remained unmeasured. Furthermore, the stability of the pulsar's lightcurve and the nature of its emission mechanisms (synchrotron vs. thermal/curvature) in the context of its magnetar-like behavior required investigation.

Methodology
The authors utilized data from two $\sim$500 ks observations conducted by the Imaging X-ray Polarimetry Explorer (IXPE) in October/November 2024 (Epoch 1) and April 2025 (Epoch 2).

• Data Reduction: IXPE data were reprocessed using a neural network (NN)-based reconstruction algorithm to improve position and azimuthal angle accuracy, alongside standard moment-weighting methods for cross-validation. Solar background was handled via two methods: time cuts during flares and characterizing/subtracting flare contributions.
• Multi-wavelength Context: Contemporaneous NICER monitoring (Nov 2024) and archival Chandra (2023) observations were used to model the nebula flux and pulsar lightcurve. Radio polarization data from the eVLA (2–4 GHz) provided a comparison baseline.
• Analysis Techniques:
  • Phase-Integrated Polarization: Aperture photometry (PCUBE) and spectro-polarimetric fitting were performed on the PWN and SNR regions.
  • Timing Analysis: Pulse profiles were constructed using $Z_2$ statistics. The authors employed Kuiper's test (a variation of the Kolmogorov-Smirnov test) to compare pulse profiles between epochs and against historical NuSTAR data.
  • Phase-Resolved Polarization: An unbinned Maximum Likelihood fit using a Rotating Vector Model (RVM) was applied to Epoch 1 data. The model included a constant polarization component for the nebula and a pulsed component for the pulsar, allowing the polarization angle to sweep according to the RVM geometry.

Key Contributions and Results

1. First X-ray Polarization Detection: The study presents the first X-ray polarization measurements for the Kes 75 PWN.

   • Epoch 2 (April 2025): A significant phase-averaged polarization was detected in the 2–8 keV band with a polarization degree (PD) of $9.9\% \pm 2.5\%$ and a polarization angle (PA) of $36.8^\circ \pm 7.3^\circ$. This PA aligns with the PWN's toroidal symmetry axis, suggesting a toroidal magnetic field structure.
   • Epoch 1 (Oct/Nov 2024): No significant phase-averaged polarization was detected, yielding only a $3\sigma$ upper limit ($PD \sim 12\%$).

2. Anomalous Pulsar Lightcurve:

   • Timing analysis revealed a significant difference between the Epoch 1 and Epoch 2 pulse profiles. Epoch 1 exhibited an additional pulsed component at a phase offset of $\Delta\phi \approx 0.5$ (detected at $\sim 3.7\sigma$ in the lightcurve and confirmed via Kuiper's test with $p=0.0022$).
   • This anomalous component contributed approximately 30% of the total pulse counts but did not significantly alter the phase-averaged spectrum or total flux.
   • By Epoch 2, the lightcurve returned to its historical profile, consistent with previous NuSTAR observations.

3. Polarization Sweep and RVM Modeling:

   • An unbinned analysis of Epoch 1 revealed a rotating vector model (RVM) sweep in the polarization angle at the $\sim 99.5\%$ confidence level.
   • The best-fit model suggests a highly polarized, anomalous pulse component with a large impact parameter ($\beta \sim 60^\circ - 85^\circ$). This geometry is consistent with the non-detection of radio pulsations from PSR J1846-0258.
   • The model indicates that the superposition of this highly polarized, sweeping component with the unpolarized (or differently polarized) main pulse and the nebular emission resulted in the cancellation of the net phase-averaged polarization observed in Epoch 1.

4. Radio vs. X-ray Polarization: The study notes that the 2–4 GHz radio polarization ($PD \approx 14\%$) is higher than the X-ray polarization in Epoch 2 ($PD \approx 10\%$), a result that challenges simple synchrotron cooling models if electrons occupy complex spatial zones, though statistical fluctuations cannot be ruled out.

Significance and Claims
The paper claims that the primary significance of this work lies in the first detection of X-ray polarization from the Kes 75 PWN and the discovery of a transient, anomalous pulse component in PSR J1846-0258.

• The detection of a toroidal magnetic field in the PWN via X-ray polarization confirms the synchrotron origin of the emission and aligns with the morphology seen in other PWNe.
• The variability in the pulse profile and the associated change in polarization properties demonstrate that PSR J1846-0258 can undergo significant magnetospheric changes without the dramatic flux increases or glitches typically associated with its "magnetar-like" outbursts.
• The authors modestly conclude that while the RVM model successfully explains the Epoch 1 polarization data and the anomalous pulse, the specific emission mechanism (synchrotron vs. curvature) for the anomalous component remains undetermined. They posit that the anomalous state may be rare and that further high-sensitivity monitoring is required to capture the pulsar in this state again to fully elucidate the nature of the emission geometry and mechanism.