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The detection of cyclotron resonance scattering features (CRSFs) is the onlyway to directly and reliably measure the magnetic field near the surface of aneutron star (NS). The broad energy coverage and large collection area of\emph{Insight}-HXMT in the hard X-ray band allowed us to detect the CRSF withthe highest energy known to date, reaching about 146 keV during the 2017outburst of the first galactic pulsing ultraluminous X-ray source (pULX) SwiftJ0243.6+6124. During this outburst, the CRSF was only prominent close to thepeak luminosity $\sim 2\times10^{39}$ erg s$^{-1}$, the highest to date in anyof the Galactic pulsars. The CRSF is most significant in the spin phase regioncorresponding to the main pulse of the pulse profile, and its centroid energyevolves with phase from 120 to 146 keV. We identify this feature as thefundamental CRSF, since no spectral feature exists at $60-70$ keV. This is thefirst unambiguous detection of an electron CRSF from an ULX. We also estimate asurface magnetic field $\sim1.6\times10^{13}$ G for Swift J0243.6+6124.Considering that the dipole magnetic field strengths, inferred from severalindependent estimates of magnetosphere radius, are at least an order ofmagnitude lower than our measurement, we argue that the detection of thehighest energy CRSF reported here unambiguously proves the presence ofmultipole field components close to the surface of the neutron star. Such ascenario has previously been suggested for several pulsating ULXs, includingSwift J0243.6+6124, and our result represents the first direct confirmation ofthis scenario.

Insight-HXMT discovery of the highest energy CRSF from the first Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124