Acknowledgements

This work was supported by NASA through the CXO grant GO5-16108X. We thank the referee, Tim Kallman, for his careful reading and valuable suggestions. The computations in this paper were run on the Odyssey cluster supported by the FAS Division of Science, Research Computing Group at Harvard University, the Smithsonian Institution High Performance Cluster (Hydra), and the Stampede cluster at the Texas Advanced Computing Center supported by NSF grant ACI-1134872.