MAXI GSC X-ray LCs of the ULX Swift J0243.6+6124

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.