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 $L_{\mathrm{X}}$ reached $2.5\times 10^{39}$ erg s${}^{-1}$, 10 times higher than the Eddington limit. When the source was luminous with $L_{\mathrm{X}}\gtrsim 0.9\times 10^{38}$ 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 $L_{\mathrm{c}}\simeq 5\times 10^{38}$ erg s${}^{-1}$. The soft color changed more than the hard color when $L_{\mathrm{X}}<L_{\mathrm{c}}$, whereas the opposite was observed above $L_{\mathrm{c}}$. The spectral change above $L_{\mathrm{c}}$ was represented by a broad enhanced feature at $\sim 6$ keV. The pulse profiles made a transition from a single-peak to a double-peak one as the source brightened across $L_{\mathrm{c}}$. 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 $L_{\mathrm{X}}$ increased. The broad 6 keV enhancement could be a result of cyclotron-resonance absorption at $\sim 10$ keV, corresponding to a surface magnetic field $B_{\mathrm{s}}\simeq 1.1\times 10^{12}$ G. The spin-frequency derivatives calculated with the Fermi GBM data showed a smooth correlation with $L_{\mathrm{X}}$ up to the outburst peak, and its linear coefficient is comparable to those of X-ray binary pulsars whose $B_{\mathrm{s}}$ are $(1-8)\times 10^{12}$ G. These results suggest that $B_{\mathrm{s}}$ of Swift J0243.6$+$6124 is a few times $10^{12}$ G.

19 pages, 13 figures, accepted for publication in ApJ