  In this paper we use high quality X-ray observations from XMM-Newton and
Chandra to gain new insights into the explosion that originated Tycho's
supernova 433 years ago. We perform a detailed comparison between the ejecta
emission from the spatially integrated X-ray spectrum of the supernova remnant
and current models for Type Ia supernova explosions. We use a grid of synthetic
X-ray spectra based on hydrodynamic models of the evolution of the supernova
remnant and self-consistent nonequilibrium ionization calculations for the
state of the shocked plasma. We find that the fundamental properties of the
X-ray emission in Tycho are well reproduced by a one-dimensional delayed
detonation model with a kinetic energy of 1.2e51 erg. All the other paradigms
for Type Ia explosions that we have tested fail to provide a good approximation
to the observed ejecta emission, including one-dimensional deflagrations,
pulsating delayed detonations and sub-Chandrasekhar explosions, as well as
deflagration models calculated in three dimensions. Our results require that
the supernova ejecta retain some degree of chemical stratification, with
Fe-peak elements interior to intermediate mass elements. This strongly suggests
that a supersonic burning front (i.e., a detonation) must be involved at some
stage in the physics of Type Ia supernova explosions.
