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

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Abstract We present the first X-ray polarization measurements of the PSR/PWN complex within SNR Kes 75. Two ∼500 ks IXPE observations were conducted in 2024 October/November and 2025 April. The second observation yields a significant phase-average 2–8 keV polarization degree of PD = 9.9% ± 2.5% at PA = 36 . ° 8 ± 7 . ° 3, implying a toroidal field aligned with the pulsar wind nebula (PWN) symmetry axis. The first epoch, however, has only a polarization upper limit. During this epoch, an additional pulsed component is visible at Δ ϕ ≈ 0.5, detected at ∼3.7 σ . An unbinned phase-resolved analysis reveals a high-polarization degree rotating vector model position angle sweep at the ∼99.5% confidence level, with angles fixed at those inferred from the PWN morphology; this can explain the loss of phase-average polarization. Additional observations are needed to pin down the nature of the anomalous pulse.

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