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tokens: Acknowledgements We appreciate Paul Wiita, James Guillochon, Tinggui Wang, Wenfei Yu, Bin Liu, Morgan MacLeod, and Dacheng Lin for helpful discussions, the referee for suggestions, and Dimitris Emmanoulopoulos for providing us with the RXTE data of NGC 7213. This work was supported in part by the National Key Research and Development Program of China (2016YFA0400804), the Youth Innovation Promotion Association of CAS (id. 2016243), the Natural Science Foundation of Shanghai (No. 17ZR1435800), and the National Natural Science Foundation of China (grant Nos. 11403074, 11773055, 11333005 and 11350110498). This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory.