Fermi Gamma-ray Space Telescope

MAXI results of the Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124

Mutsumi Sugizaki
(M. Oeda, N. Kawai, T. Mihara, K. Makishima, and M. Nakajima)


Swift J0243.6+6124 is a new Be/X-ray binary pulsar discovered on October 3, 2017. It was first identified as a new X-ray object by the Swift BAT survey. The coherent pulsation with the 9.86 s period was then recognized by the Swift XRT observation and the Fermi-GBM pulsar analysis. The MAXI all-sky survey had also detected the X-ray activity, but could not resolve the source from the known nearby X-ray object, LS I +61 303. After the discovery, Swift J0243.6+6124 has been observed by many X-ray satellites. X-ray monitoring by MAXI, Swift, Fermi/GBM revealed that the outburst continued for more than 4 months, and the flux reached over 5 Crab at the peak. On the other hand, the recent GAIA result suggests the source distance to be ~7 kpc, meaning that the X-ray luminosity reached the 10 times of the Eddington luminosity at the peak. We performed detailed MAX-GSC data analysis taken at around the flux peak, using the Fermi/GBM pulsar ephemeris. X-ray spectra in the 2-30 keV band were largely approximated by power-law functions with a higher-energy cutoff and an iron-K emission line, which is typical in X-ray binary pulsars. The results clarified the spectral softening due to the flux increase, as observed by NICER and Fermi/GBM. The folded pulse profiles show the energy-dependent change due to the intensity. We also investigated the relation between the luminosity and the spin-up rate and found that the positive correlation close to the proportionality smoothly extends up to the 10 Eddington luminosity. Assuming the accretion-torque model proposed by Ghosh & Lamb, the surface magnetic filed is estimated to be in a order of 10^12 G.