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, ${\delta }_{\mathrm{rms}}$, to probe phase-resolved spectral evolution in the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124. The ${\delta }_{\mathrm{rms}}$--luminosity relation reveals a reproducible convergence--redivergence behavior: spectra from two geometrically opposite pulse phases converge to near identity around $L_{\mathrm{t}}^{\ast }\sim 8.8\times 10^{37}\mathrm{erg}{\mathrm{s}}^{-1}$, with a mean shape difference of only $\sim 2\%$, 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 $L_{\mathrm{t}}\sim 4.5\times 10^{37}\mathrm{erg}{\mathrm{s}}^{-1}$. 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 ${\delta }_{\mathrm{rms}}$ as a robust, spectral-model-independent diagnostic of radiation-geometry restructuring in bright accreting X-ray pulsars.

arXiv comment9 pages, 7 figures (4 main-text figures and 3 appendix figures)

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