Direct Doppler shifts cannot explain smaller GX 1+4 spin loss in 2026 for total mass ≤5 solar masses, period ≥1100 days, eccentricity ≤0.2, and the tested errors

We test whether direct orbital motion can explain the smaller recent pulse frequency loss in GX 1+4. The frozen public Fermi/GBM history gives endpoint rates of -4.3339 and -0.5306 times 10^-12 Hz/s over 2009-2021 and 2026 January 8 to October 21. An earlier SCI study already noted the recent reduction. We add a conditional orbital bound. For total binary mass at most five solar masses, period at least 1100 days, and eccentricity at most 0.2, the relative speed is at most 43.2 km/s. We allow any phase, inclination, and mass division, with independent endpoint Doppler factors and a source-clock time allowance. If the reported endpoint frequencies are exact, motion can remove at most 1.46% of the measured rate difference. If every endpoint error is at most five reported formal errors plus 2 microhertz, the corrected mean rates remain separate by at least 3.4931 times 10^-12 Hz/s. They also remain separate with 20 microhertz extra error or a wider orbit box. All 81 endpoint omission checks retain the result. These are deterministic sensitivity bounds, not confidence limits. The result excludes direct two-body Doppler motion within the stated bounds. It does not identify an accretion mechanism, reject stochastic torque, or establish a permanent torque state. The package includes source data, hashes, code, results, figures, independent numerical checks, and offline reproduction instructions.

GX 1+4 · Fermi GBM · X-ray pulsar · spin frequency · Doppler shift · orbital motion · accretion torque · reproducible astronomy