  Swift J0243.6+6124 is a newly discovered Galactic Be/X-ray binary, revealed
in late September 2017 in a giant outburst with a peak luminosity of 2E+39 (d/7
kpc)^2 erg/s (0.1-10 keV), with no formerly reported activity. At this
luminosity, Swift J0243.6+6124 is the first known galactic ultraluminous X-ray
pulsar. We describe Neutron star Interior Composition Explorer (NICER)} and
Fermi Gamma-ray Burst Monitor (GBM) timing and spectral analyses for this
source. A new orbital ephemeris is obtained for the binary system using
spin-frequencies measured with GBM and 15-50 keV fluxes measured with the Neil
Gehrels Swift Observatory Burst Alert Telescope to model the system's intrinsic
spin-up. Power spectra measured with NICER show considerable evolution with
luminosity, including a quasi-periodic oscillation (QPO) near 50 mHz that is
omnipresent at low luminosity and has an evolving central frequency. Pulse
profiles measured over the combined 0.2-100 keV range show complex evolution
that is both luminosity and energy dependent. Near the critical luminosity of
L~1E+38 erg/s, the pulse profiles transition from single-peaked to double
peaked, the pulsed fraction reaches a minimum in all energy bands, and the
hardness ratios in both NICER and GBM show a turn-over to softening as the
intensity increases. This behavior repeats as the outburst rises and fades,
indicating two distinct accretion regimes. These two regimes are suggestive of
the accretion structure on the neutron star surface transitioning from a
Coulomb collisional stopping mechanism at lower luminosities to a
radiation-dominated stopping mechanism at higher luminosities. This is the
highest observed (to date) value of the critical luminosity, suggesting a
magnetic field of B ~1E+13 G.
