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

Brian W. Grefenstette, Chris L. Fryer, Fiona A. Harrison, Steven E. Boggs, Tracey DeLaney, J. Martin Laming, Stephen P. Reynolds, David M. Alexander, Didier Barret, Finn E. Christensen, William W. Craig, Karl Forster, Paolo Giommi, Charles J. Hailey, Alan Hornstrup, Takao Kitaguchi, J. E. Koglin, Laura Lopez, Peter H. Mao, Kristin K. Madsen, Hiromasa Miyasaka, Kaya Mori, Matteo Perri, Michael J. Pivovaroff, Simonetta Puccetti, Vikram Rana, Daniel Stern, Niels J. Westergaard, Daniel R. Wik, William W. Zhang, Andreas Zoglauer

Published 2016-12-08Version 1

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 \textit{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 $\pm$ 0.21 $\times 10^{-4}$ M$_{\odot}$ which has a present day average momentum direction of 340$^{\circ}$ $\pm$ 15$^{\circ}$ projected on to the plane of the sky (measured clockwise from Celestial North) and tilted by 58$^{\circ}$ $\pm$ 20$^{\circ}$ 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 the reverse shock. 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 in some regions by at least a factor of two, even in regions that are assumed to be the result of processes like $\alpha$-rich freezeout that should produce both iron and titanium.