  Asymmetry is required by most numerical simulations of stellar core-collapse
explosions, but the form it takes differs significantly among models. The
spatial distribution of radioactive 44Ti, synthesized in an exploding star near
the boundary between material falling back onto the collapsing core and that
ejected into the surrounding medium, directly probes the explosion asymmetries.
CassiopeiaA is a young, nearby, core-collapse remnant from which 44Ti emission
has previously been detected but not imaged. Asymmetries in the explosion have
been indirectly inferred from a high ratio of observed 44Ti emission to
estimated 56Ni emission, from optical light echoes, and from jet-like features
seen in the X-ray and optical ejecta. Here we report spatial maps and spectral
properties of the 44Ti in Cassiopeia A. This may explain the unexpected lack of
correlation between the 44Ti and iron X-ray emission, the latter being visible
only in shock-heated material. The observed spatial distribution rules out
symmetric explosions even with a high level of convective mixing, as well as
highly asymmetric bipolar explosions resulting from a fast-rotating progenitor.
Instead, these observations provide strong evidence for the development of
low-mode convective instabilities in core-collapse supernovae.
