ULX pulsar Swift J0243.6+6124 observations with NuSTAR -- dominance of reflected emission in the super-Eddington state

Abstract

We report the discovery of the bright reflected emission component in the super-Eddington state of the ULX pulsar Swift J0243.6+6124, based on the NuSTAR observations of the source during its 2017 outburst. The flux of the reflected emission is weakly variable over the pulsar phase while the direct emission shows significantly larger pulsation amplitude. We propose that in this system the neutron star finds itself in the centre of the well formed by the inner edge of the geometrically thick super-Eddington accretion disc truncated by the magnetic field of the pulsar. The aspect ratio of the well is H/R \sim 1. The inner edge of the truncated disc is continuously illuminated by the emission of the accretion column giving rise to the weakly variable reflected emission. As the neutron star rotates, its emission sweeps through the line of sight, giving rise to the pulsating direct emission. From Doppler broadening of the iron line, we measure the truncation radius of the accretion disc \sim 50 R_g. The inferred dipole component of the magnetic field is consistent with previous estimates favouring a not very strong field. The uniqueness of this system is determined by its moderately super-Eddington accretion rate and the moderate magnetic field so that the inner edge of the truncated geometrically thick accretion disc is seen from the neutron star at a large solid angle.

12 pages, 9 figures, 2 tables. Monthly Notices of the Royal Astronomical Society Main Journal Accepted. Code available via this url: https://github.com/SergeiDBykov/nustar_sj0243

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• Ultraluminous X-ray sources
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• X-ray Binaries in External Galaxies
• Timing analysis of Swift J0243.6+6124 with NICER and Fermi/GBM during the decay phase of the 2017-2018 outburst
• Spectral analysis of ultraluminous X-ray pulsars with models of X-ray pulsars
• Unveiling the origin of the optical/UV emission from the Galactic ULX Swift J0243.6+6124 during its 2017-2018 giant outburst
• Discovery of a 0.8-mHz quasi-periodic oscillation in the transient X-ray pulsar SXP31.0 and associated timing transitions