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

TL;DR

This study reveals that in the super-Eddington state of the ULX pulsar Swift J0243.6+6124, reflected emission dominates, with the neutron star embedded in a truncated, geometrically thick accretion disc, providing new insights into accretion physics.

Contribution

The paper presents the first detection of reflected emission dominance in a super-Eddington ULX pulsar, linking it to the disc geometry and magnetic field characteristics.

Findings

Reflected emission is weakly variable over pulsar phase.

Inner disc radius is approximately 50 gravitational radii.

Magnetic field strength is moderate, consistent with previous estimates.

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.