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Description
This paper reports on the X-ray emission evolution of the ultraluminous Galactic X-ray pulsar Swift J0243.6+6124 during the giant outburst from 2017 October to 2018 January as observed by the MAXI GSC all-sky survey. The 2-30keV light curve and the energy spectra confirm the source luminosity LX assuming an isotropic emission reached 2.5x10^39^erg/s, 10 times higher than the Eddington limit for a 1.4M_{sun}_ neutron star. When the source was luminous with L_X_>~0.9x10^38^erg/s, it generally exhibited a negative correlation on a hardness-intensity diagram. However, two hardness ratios, a soft color (=4-10keV/2-4keV) and a hard color (=10-20keV/4-10keV), showed somewhat different behavior across a characteristic luminosity of L_c_~5x10^38^erg/s. The soft color changed more than the hard color when L_X_<L_c_, whereas the opposite was observed above Lc. The spectral change above Lc was represented by a broad enhanced feature at ~6keV on top of the canonical cutoff power-law continuum. The pulse profiles, derived daily, made the transition from a single-peak to a double-peak as the source brightened across Lc. These spectral and pulse-shape properties can be interpreted by a scenario in which the accretion columns on the neutron-star surface, producing the Comptonized X-ray emission, gradually became taller as LX increases. The broad 6keV enhancement could be a result of cyclotron-resonance absorption at ~10keV, corresponding to a surface magnetic field B_s_~1.1x10^12^G. The spin-frequency derivatives calculated with the Fermi GBM data showed a smooth positive correlation with LX up to the outburst peak, and its linear coefficient is comparable to those of typical Be binary pulsars whose B_s_ are (1-8)x10^12^G. These results suggest that the B_s_ of Swift J0243.6+6124 is a few times 10^12^G.
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