  X-ray observations of supernova remnants (SNRs) allow us to investigate the
chemical inhomogeneity of ejecta, offering unique insight into the
nucleosynthesis in supernova explosions. Here we present detailed imaging and
spectroscopic studies of the "Fe knot" located along the eastern rim of the
Type Ia SNR Tycho (SN 1572) using Suzaku and Chandra long-exposure data.
Surprisingly, the Suzaku spectrum of this knot shows no emission from Cr, Mn,
or Ni, which is unusual for the Fe-rich regions in this SNR. Within the
framework of the canonical delayed-detonation models for SN Ia, the observed
mass ratios M_Cr/M_Fe < 0.023, M_Mn/M_Fe < 0.012, and M_Ni/M_Fe < 0.029 (at 90%
confidence) can only be achieved for a peak temperature of (5.3-5.7) x 10^9 K
and a neutron excess of < 2.0 x 10^-3. These constraints rule out the deep,
dense core of a Chandrasekhar-mass white dwarf as the origin of the Fe knot,
and favors either incomplete Si burning or the alpha-rich freeze-out regime,
probably close to their boundary. An explosive He burning regime is a possible
alternative, although this hypothesis is in conflict with the main properties
of this SNR.
