(C. Malacaria (NPP/MSFC/USRA), P. A. Jenke (UAH), G. K. Jaisawal (DTU Space), M. Kerr (NRL), M. T. Wolff (NRL), Z. Arzoumanian (NASA/GSFC), D. Chakrabarty (MIT), J. P. Doty (Noqsi Aerospace Ltd.), K. C. Gendreau (NASA/GSFC), S. Guillot (CNRS/CNES), W. C. G. Ho (Haverford College/Univ. of Southampton), B. LaMarr (MIT), C. B. Markwardt (NASA/GSFC), F. Ozel (Univ. of Arizona), G. Y. Prigozhin (MIT), P. S. Ray (NRL), M. Ramos-Lerate (ESA/ESAC), R. A. Remillard (MIT), T. E. Strohmayer (NASA/GSFC), M. L. Vezie (MIT), K. S. Wood (Praxis Inc.) on behalf of the NICER and GBM teams)
Swift J0243.6+6124 is a ~9.8 second pulsar orbiting a Be-star companion that reached a peak luminosity of 2x10^39 erg/s (0.1-10 keV, d=7 kpc) in November 2017, making it the first known ultra luminous X-ray pulsar in our Galaxy. I will describe NICER and Fermi GBM observations of Swift J0243.6+6124, including our observations of a transition from sub-critical to super-critical accretion observed at L~10^38 ergs/s. As the accretion regime transitioned, the pulse profile evolved from single to doubled peaked, the pulse fraction reached a minimum, the energy spectrum softened, and the power spectrum changed. This behavior is suggestive of a transition from a Coulomb collisional stopping mechanism to a radiation dominated stopping mechanism in the accretion structure at the neutron star surface. The high peak luminosity and the high critical luminosity suggest a magnetic field of B~10^13 G. The initial giant outburst lasted about ~150 days and Swift J0243.6+6124 continues to undergo normal outbursts every ~27 days, giving us increasing insight into the dynamical behavior of this exciting system. The synergy between NICER and Fermi GBM observations resulted in an exquisite dataset of detailed measurements of this ULX pulsar from 0.2-100 keV.