X-Ray Emission Evolution of the Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124 during the 2017-2018 Outburst Observed by the MAXI GSC

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 2-30 keV light curve and the energy spectra confirm the source luminosity L_X assuming an isotropic emission reached 2.5 × 10³⁹ erg s^-1, 10 times higher than the Eddington limit for a 1.4 M_⊙ neutron star. When the source was luminous with L_X ≳ 0. 9 × 10³⁸ erg s^-1, it generally exhibited a negative correlation on a hardness-intensity diagram. However, two hardness ratios, a soft color (=4-10 keV/2-4 keV) and a hard color (=10-20 keV/4-10 keV), showed somewhat different behavior across a characteristic luminosity of L_c ≃ 5 × 10³⁸ erg s^-1. The soft color changed more than the hard color when L_X < L_c, whereas the opposite was observed above L_c. The spectral change above L_c was represented by a broad enhanced feature at ∼6 keV 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 L_c. 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 L_X increases. The broad 6 keV enhancement could be a result of cyclotron-resonance absorption at ∼10 keV, corresponding to a surface magnetic field B_s ≃ 1.1 × 10¹² G. The spin-frequency derivatives calculated with the Fermi GBM data showed a smooth positive correlation with L_X up to the outburst peak, and its linear coefficient is comparable to those of typical Be binary pulsars whose B_s are (1-8) × 10¹² G. These results suggest that the B_s of Swift J0243.6+6124 is a few times 10¹² G.