An evolving broad iron line from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124

Understand

We present a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017--2018 giant outburst.

• The 1.2--10~keV energy spectrum of the source can be approximated with an absorbed cut-off power law model.
• We detect strong, luminosity-dependent emission lines in the 6--7 keV energy range.
• A narrow 6.42 keV line, observed in the sub-Eddington regime, is seen to evolve into a broad Fe-line profile in the super-Eddington regime.