tokens: We thank Andy Fabian, Rich Plotkin, Amy Reines, Claudia Scarlata, and Anil Seth for helpful discussions, and an anonymous referee for constructive comments. This work was supported in part by Chandra Award Number 11620915, by the National Science Foundation under grant no. NSF PHY11-25915 and by NASA through grant HST-GO-12591.01 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. J.H.W. acknowledges support by the National Research Foundation of Korea grant funded by the Korea government (No. 2012-006087 and No. 2010-0027910).
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