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 LX 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 LX 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 Lc 5 × 1038 erg s−1. The soft color changed more than the hard color when LX < Lc, whereas the opposite was observed above Lc. The spectral change above Lc 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 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 6 keV enhancement could be a result of cyclotron-resonance absorption at ∼10 keV, corresponding to a surface magnetic field Bs 1.1 × 1012 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 Bs are (1-8) × 1012 G. These results suggest that the Bs of Swift J0243.6+6124 is a few times 1012 G.