A Spectral-model-independent Diagnostic of Phase-resolved Spectral Convergence in Swift J0243.6+6124

Abstract We introduce a spectral-model-independent spectral-shape metric, δ rms , to probe phase-resolved spectral evolution in the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124. The δ rms –luminosity relation reveals a reproducible convergence–redivergence behavior: spectra from two geometrically opposite pulse phases converge to near identity around L t * ∼ 8.8 × 1 0 37 erg s − 1 , with a mean shape difference of only ∼2%, and rediverge at higher luminosities, most prominently at higher energies. This strong phase-resolved spectral convergence coincides with a pulse-profile reversal and traces the same transition luminosities previously identified from broadband continuum evolution, including L t  ∼ 4.5 × 10 37 erg s −1 . These results suggest a possible luminosity-driven reweighting between a broadly distributed surface-reprocessed/reflected component and an increasingly important direct accretion-column contribution, while establishing δ rms as a robust, spectral-model-independent diagnostic of radiation-geometry restructuring in bright accreting X-ray pulsars.