tokens: M.Y.S. thanks K. D. Denney for a beneficial discussion. We thank the anonymous referee for helpful comments that improved the paper. M.Y.S. and Y.Q.X. acknowledge the support from NSFC-11603022, NSFC-11473026, NSFC-11421303, the 973 Program (2015CB857004), the China Postdoctoral Science Foundation (2016M600485), the CAS Frontier Science Key Research Program (QYZDJ-SSW-SLH006), and the Fundamental Research Funds for the Central Universities. W.N.B. acknowledges support from NSF Grant AST-1516784 and Chandra X-ray Center grant GO6-17083X. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/ . SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University. Software: Astropy (Astropy Collaboration et al. 2013 ), Kapteyn (Terlouw Vogelaar 2015 ), Matplotlib (Hunter 2007 ), Numpy Scipy (Van Der Walt et al. 2011 ).
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