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

Abstract

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 × 10 39 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 × 10 38 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 × 10 38 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 × 10 12 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) × 10 12 G. These results suggest that the B s of Swift J0243.6+6124 is a few times 10 12 G.