tokens: We wish to thank Dick Manchester and George Hobbs for helpful discussion and suggestions. Furthermore, we thank Hao Tong and Mingyu Ge for communication on the latest progress on the time delay measurements of the Crab Pulsar. This research is supported by the National Natural Science Foundation of China, under the grant NSFC11373018 and NSFC11503006. We are also thankful for the support of the grant Beyond the Horizons 2012, and the support of the Strategic Priority Research Program of the Chinese Academy of Sciences Grant No. XDB23010200.
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