  We clarify the nature of the small-diameter supernova remnant (SNR) Kes 73
and its central compact source, 1E 1841-045, using X-ray data acquired with the
ASCA Observatory. We introduce a spatio-spectral decomposition technique
necessary to disentangle the ASCA spectrum of the compact source from the
barely resolved shell-type remnant. The source spectrum (1 - 8 keV) is
characterized by an absorbed power-law with a photon index ~ 3.4 and N_H ~
3.0E22 cm^-2, possibly non-thermal in nature. This bright X-ray source is
likely a slowly spinning pulsar, whose detection is reported in our companion
paper (Vasisht & Gotthelf 1997). The SNR spectrum is characteristic of a
thermal plasma, with kT ~ 0.7 keV, and emission lines typical of a young
remnants. The element Mg and possibly O and Ne are found to be over-abundant,
qualitatively suggesting an origin from a massive progenitor. We find that Kes
73 is a young (~ 2000 yr) type II/Ib SNR containing a neutron star pulsar
spinning anomalously slow for its age. Kes 73 is yet another member of a
growing class of SNRs containing radio-quiet compact sources with a hard
spectral signature.
