The distribution of radioactive $^{44}$Ti in Cassiopeia A

International audienceThe distribution of elements produced in the innermost 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 (44)Ti ejecta, which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the (44)Ti ejecta. We find an initial (44)Ti mass of (1.54 ± 0.21) × 10(−)(4) M (⊙), which has a present-day average momentum direction of 340° ± 15° projected onto the plane of the sky (measured clockwise from celestial north) and is 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 (44)Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to it. Where we observe (44)Ti ejecta exterior to the reverse shock we also see shock-heated iron, however, there are regions where we see iron but do not observe (44)Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of (44)Ti by at least a factor of two in some regions, even in regions that are assumed to be the result of processes like α-rich freezeout that should produce both iron and titanium