tokens: This work was supported by the National Basic Research Program of China (973 program, grant No. 2012CB821800), the talent team of science and technology innovation in Guizhou Province (Grant No. (2015) 4015) and the National Natural Science Foundation of China (grant Nos 11373011, 11303069, 11225314, 11103021, 11565010, and 11165006). We would like to thank Dr. Dipanjan Mitra for sharing the 317 MHz pulse profile data observed with the GMRT and Dr. Ramesh Karuppusamy for sharing the 116.75–173.75 MHz pulse profile data observed with the Westerbork Synthesis Radio Telescope (WSRT). We would also like to thank the Kunming 40 m Radio Telescope and EPN teams for providing their pulse profiles. In particular, we would like to greatly thank Prof. Richard de Grijs for his kind help in carefully improving the language. We sincerely thank the anonymous referee for insightful suggestions and valuable comments.
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