The Distribution of Radioactive ⁴⁴Ti in Cassiopeia A

The distribution of elements produced in the inner-most layers of a supernova explosion is a key diagnostic for studying the collapse of massive stars. Here we present the results of a 2.4 Ms NuSTAR observing campaign aimed at studying the supernova remnant Cassiopeia A (Cas A). We perform spatially-resolved spectroscopic analyses of the ⁴⁴Ti ejecta which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the ⁴⁴Ti ejecta. We nd an initial 44Ti mass of 1.54±0.21x10^-4 M which has a present day average momentum direction of 340°±15° projected on to the plane of the sky (measured clockwise from Celestial North) and tilted by 58°±20° into the plane of the sky away from the observer, roughly opposite to the inferred direction of motion of the central compact object. We find some ⁴⁴Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to the reverse shock. Where we observe ⁴⁴Ti ejecta exterior to the reverse shock we also see shock-heated iron; however, there are regions where we see iron but do not observe ⁴⁴Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of ⁴⁴Ti in some regions by at least a factor of two, even in regions that are assumed to be the result of processes like α-rich freezeout that should produce both iron and titanium.