Two Complete Spectral Transitions of Swift J0243.6+6124 Observed by Insight-HXMT

We performed the broadband (1-100 keV) spectral analysis of the first Galactic Be ultraluminous X-ray pulsar (BeULX) Swift J0243.6+6124 observed by Insight-HXMT during the 2017-2018 outburst. The results show spectral transitions at two typical luminosities, roughly consistently with those reported previously via pure timing analysis. We find that the spectrum evolves and becomes softer and has higher cutoff energies until the luminosity reachesL(1)(similar to 1.5 x 10(38)erg s(-1)). Afterwards the spectrum becomes harder with lower cutoff energies until the luminosity increases toL(2)(similar to 4.4 x 10(38)erg s(-1)), around which the second spectral transition occurs. BeyondL(2), the spectrum softens again and has larger cutoff energies. Similar behaviors were observed previously in other high-mass X-ray binary systems (HMXBs), especially for the second transition at higher luminosities, which is believed to have a correlation with the magnetic field of the harbored neutron star. Accordingly, we speculate that Swift J0243.6+6124 owns a neutron star with magnetic field strength >10(13)G. The spectral transition at aroundL(1)of Swift J0243.6+6124 is first observed thoroughly for any HMXB outburst characterized by strong evolution of the thermal component: the temperature of the blackbody drops sharply accompanied by a sudden increase of the blackbody radius. These spectral transitions can in principle be understood in a general scenario of balancing the emission patterns between the pencil and the fan beams at the magnetic pole, for which the extreme brightness of Swift J0243.6+6124 may provide an almost unique lab to probe the details.