  We present broad-band X-ray spectroscopy of the energetic components that
make up the supernova remnant (SNR) Kesteven 75 using concurrent 2017 Aug 17-20
XMM-Newton and NuSTAR observations, during which the pulsar PSR J1846-0258 is
found to be in the quiescent state. The young remnant hosts a bright pulsar
wind nebula powered by the highly-energetic (Edot = 8.1E36 erg/s) isolated,
rotation-powered pulsar, with a spin-down age of only P/2Pdot ~ 728 yr. Its
inferred magnetic field (Bs = 4.9E13 G) is the largest known for these objects,
and is likely responsible for intervals of flare and burst activity, suggesting
a transition between/to a magnetar state. The pulsed emission from PSR
J1846-0258 is well-characterized in the 2-50 keV range by a power-law model
with photon index Gamma_PSR = 1.24+/-0.09 and a 2-10 keV unabsorbed flux of
(2.3+/-0.4)E-12 erg/s/cm^2). We find no evidence for an additional non-thermal
component above 10 keV in the current state, as would be typical for a
magnetar. Compared to the Chandra pulsar spectrum, the intrinsic pulsed
fraction is 71+/-16% in 2-10 keV band. A power-law spectrum for the PWN yields
Gamma_PWN = 2.03+/-0.03 in the 1-55 keV band, with no evidence of curvature in
this range, and a 2-10 keV unabsorbed flux (2.13+/-0.02)E-11 erg/s/cm^2. The
NuSTAR data reveal evidence for a hard X-ray component dominating the SNR
spectrum above 10 keV which we attribute to a dust-scattered PWN component. We
model the dynamical and radiative evolution of the Kes 75 system to estimate
the birth properties of the neutron star, the energetics of its progenitor, and
properties of the PWN. This suggests that the progenitor of Kes 75 was
originally in a binary system which transferred most its mass to a companion
before exploding.
