Discovery of a New Spectral Transition in Swift J0243.6+6124 in the Sub-Eddington Regime

We conduct a detailed spectral analysis of the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124 in its sub-Eddington regime, using Insight-HXMT and NICER observations during multiple outbursts including the 2018 giant outburst. We discover a new transition at $L_{\mathrm{t}}\approx 4.5\times {10}^{37}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$, accompanied by systematic evolution of spectral parameters, in particular a significant turnover in the blackbody normalization. This transition luminosity in the sub-Eddington regime represents the fifth transition identified so far in Swift J0243.6+6124, further highlighting the complexity of its accretion-powered emission. We interpret the transition in terms of a multipolar magnetic-field configuration, where weak ($\sim 2.8\times {10}^{12}\mathrm{G}$) and strong ($\sim 1.6\times {10}^{13}\mathrm{G}$) magnetic poles dominate the emission at different accretion rates. On the magnetospheric scale, this configuration is equivalent to an effective dipole field of $\sim 6.6\times {10}^{12}\mathrm{G}$, while allowing the local surface field to exceed ${10}^{13}\mathrm{G}$.