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

Abstract

We present the first X-ray polarization measurements of the PSR/PWN complex within SNR Kes 75. Two $\sim 500\mathrm{ks}$ IXPE observations were conducted in October/November 2024 and April 2025. The second observation yields a significant phase-average 2-8 keV polarization degree $\mathrm{PD}=9.9\%\pm 2.5\%$ at $\mathrm{PA}=36.8^{\circ }\pm 7.3^{\circ }$, implying a toroidal field aligned with the PWN symmetry axis. The first epoch, however, has only a polarization upper limit. During this epoch, an additional pulsed component is visible at $\hat{I}”\ddot{I}•\approx 0.5$, detected at $\sim 3.7\ddot{I}ƒ$. An unbinned phase-resolved analysis reveals a high-PD rotating vector model PA sweep at the $\sim 99.5\%$ 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.

13+3 pages, 9 figures, 4 tables. Accepted for publication in ApJ