Two correlated Gaussian noise models support a 65-day X-ray cycle in the April 2023–January 2026 Swift data for NGC 7793 P13

We test the long X-ray brightness cycle of NGC 7793 P13 with public Swift catalog data. The primary later sample has 133 good detections from April 2023 to January 2026. We fit annual means, a sine, measurement errors, and correlated Gaussian source noise. The search covers 40–100 d. The Ornstein–Uhlenbeck model gives 64.72 d and a semi-amplitude of 0.0226 counts/s. Its approximate local 95% profile range is 64.03–65.50 d. A Matérn 3/2 model gives 64.55 d and 63.81–65.41 d. Each later noise test uses 1,500 full-search null simulations. The OU test has zero exceedances. The Matérn test has one. The result agrees with the published 65.06 d X-ray period. It does not establish a period change, an orbital period, or phase continuity through the low state. The 2025–2026 subset alone gives a weak constraint. We also find 39 observation IDs in both blind and non-detection tables. These rows must not enter a fit as independent measurements. The contribution is a later-interval test, an extraction audit, and a frozen reproduction package. The cycle and the source recovery are prior results. The noise probabilities depend on the fitted stationary Gaussian models.

NGC 7793 P13 · Swift XRT · ultraluminous X-ray pulsar · period search · correlated noise · LSXPS · reproducible analysis