HZ Her kept a large r-band orbital flux contrast in May to October 2025
We test whether HZ Her kept a large optical orbital flux contrast during the early 2025 low X-ray state of Her X-1. Public ZTF DR24 data supply 45 r and 43 g measurements after the reported start at MJD 60805.3, through October 21, 2025. The seasonal reference contains 2,325 measurements from 2018 to 2024. We fit a shared orbital curve with a separate mean in each of five 35-day intervals. The r contrast ratio is 0.996. Its nominal 95% interval is 0.874 to 1.118, with baseline uncertainty included. This interval assumes independent 35-day error blocks and a shared curve shape. With 70-day error blocks it widens to 0.773 to 1.219. Free fits of the 2025 curve, removal of bright orbital phases, phase shifts, color checks and two nearby comparison stars support a large retained contrast. The g ratio is 0.936, with a nominal interval of 0.756 to 1.117. Bright-source saturation is a greater concern in g. The available daily Swift/BAT mean is 29% lower than the seasonal reference mean. This is a constraint on the early event, not a test of the deeper 2026 state or a measure of intrinsic X-ray power. The study follows data inspection. We preserve the numerical public responses, input hashes, source code, full results, three figures and an offline reproduction audit.
HZ Her · Her X-1 · ZTF DR24 · Swift BAT · optical orbital contrast · extended low state · X-ray binary · public astronomical data · reproducible research
References
8- usesdoi:10.26131/IRSA598
The analysis uses public ZTF PSF light curves. The archived query explicitly selects release DR24.
- doi:10.1088/1538-3873/aae8ac
This paper describes the ZTF data processing and archive.
- usesdoi:10.1088/0067-0049/209/1/14
The public Swift/BAT transient monitor supplies the daily 15-50 keV rates used for context.
- usesdoi:10.1051/0004-6361/200911690
The published quadratic orbital ephemeris supplies the phase reference.
- usesdoi:10.48550/arXiv.2610.04341
Version 1 reports the 2025 extended low state. We use its start time, MJD 60805.3, to define the optical test.
- doi:10.1111/j.1365-2966.2011.19494.x
This earlier study provides the precedent for optical modulation during a low X-ray state. The present study measures the early 2025 event.
- doi:10.48550/arXiv.2507.13515
Section II.15 describes real optical shape changes and retains the published X-ray period derivative. This supports use of the clock and checks with free curve shapes.
- The evidence for a 1.7-day signal in March–October 2026 Her X-1 MAXI data depends on the error model sci:sha256:8df02b074d3c2ae6c297ceb2c621334350915bbc013b2ed14c8bbf92500ea662
This related study tests an orbital signal in later 2026 MAXI data. Its error model checks motivate care with correlation assumptions. Its time interval and waveband differ from this optical study.
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