tokens: We acknowledge the use of data from the Fermi Science Support Center (FSSC) at NASA's Goddard Space Flight Center. JNZ thanks Dr Yizhong Fan for help in improving the presentation. This work is supported in part by 973 Programme of China under grant 2013CB837000, the National Natural Science Foundation of China under grants 11273064 and 11443004, the Strategic Priority Research Programme ‘The Emergence of Cosmological Structures’ of the Chinese Academy of Sciences (Grant No. XDB09000000) and by the China Postdoctoral Science Foundation under grant 2014M551680.
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