X-ray emission evolution of the Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124 during the 2017-2018 outburst observed by the MAXI GSC
Abstract
This paper reports on the X-ray emission evolution of the ultra-luminous Galactic X-ray pulsar, Swift J0243.6+6124, during the 2017-2018 giant outburst observed by the MAXI GSC. The 2-30 keV light curve and the energy spectra confirm that the luminosity LX reached 2.5× 1039 erg s-1, 10 times higher than the Eddington limit. When the source was luminous with LX 0.9× 1038 erg s-1, it 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. The pulse profiles made a transition from a single-peak to a double-peak one as the source brightened across Lc. These spectral and pulse-shape properties can be interpreted by a scenario that the accretion columns on the neutron star surface, producing the Comptonized X-ray emission, gradually became taller as LX increased. 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 correlation with LX up to the outburst peak, and its linear coefficient is comparable to those of X-ray binary pulsars whose Bs are (1-8)× 1012 G. These results suggest that Bs of Swift J0243.6+6124 is a few times 1012 G.