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tokens: ACKNOWLEDGEMENTS We thank M. Begelman, A. Beloborodov, E. Quataert, K. Parfrey, L. Sironi, A. Spitkovsky, and V. Zhdankin for fruitful discussions. This work was supported by DOE grant DE-SC0008409, NASA grants NNX17AK57G and NNX16AB28G, NSF grant AST-1411879, and NASA through Einstein Postdoctoral Fellowship grant PF7-180165 awarded to AP by the Chandra X-ray Center, operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS803060. The simulations presented in this paper used computational resources of the NASA/Ames HEC Program and XSEDE/Stampede2 (allocation PHY140041) (Towns et al. 2014 ). DAU gratefully acknowledges the hospitality of the Inst. for Advanced Study and support from the Ambrose Monell Foundation.