tokens: We thank the anonymous referees for carefully reading the manuscript and for valuable comments that helped improve this paper substantially. We also thank Professor Yang Chen for discussions, Dr Rai Yuen for smoothing out the language and Dr Cristobal Espinoza for useful suggestions on the braking indices of several radio pulsars. This work was supported by Xinjiang Natural Science Foundation No. 2013211A053, by Chinese National Science Foundation through grants Nos 11173041, 11173042, 11273051, 11373006 and 11133001, by the National Basic Research Programme of China grants 973 Programmes 2012CB821801, by the Strategic Priority Research Programme ‘The Emergence of Cosmological Structures’ of the Chinese Academy of Sciences through No. XDB09000000 and by the West Light Foundation of Chinese Academy of Sciences No. 2172201302.
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