tokens: We thank the referee for his/her very helpful comments. We appreciate Jian Li, Feifei Kou, Xian Hou, and Shanshan Weng for their useful suggestions and discussions. We thank the High Energy Astrophysics Science Archive Research Center and Fermi Science Support Center for maintaining its online archive service that provided the data used in this research. This work is supported by the National Key RD Program of China (2016YFA0400803, 2016YFA0400800), National Science Foundation of China (11233001, 11503027 and 61471003), the Strategic Pionner Program on Space Science, Chinese Academy of Sciences, grant No. XDA15051400, and Anhui Province Key Laboratory of Intelligent Building Building Energy Saving.
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