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

Abstract We have conducted a detailed spectral analysis of Swift J0243.6+6124 in its sub-Eddington regime, as observed by Insight-HXMT and NICER during a series of outbursts including the giant one in 2018, and discovered a new transition at L t ∼ 4.5 × 10 37 erg s −1 accompanied by the evolution of the spectral parameters, in particular a significant turnover of the blackbody normalization. L t in the sub-Eddington regime represents the fifth transition luminosity identified so far, further increasing the complexity of Swift J0243.6+6124, and it may be explained by introducing a multipolar magnetic field configuration, where weak (∼2.8 × 10 12 G) and strong (∼1.6 × 10 13 G) magnetic poles govern the emission at different accretion rates. Such a magnetic field configuration is equivalent to a relatively weak pure dipole magnetic field of ∼6.6 × 10 12 G on the scale of the magnetospheric radius, and allows the local magnetic field of the neutron star to exceed 10 13 G.