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We investigate the position of the radio core in a blazar by multi-epochastrometric observations at 43 GHz. Using the VLBI Exploration of RadioAstrometry (VERA), we have conducted four adjacent observations in February2011 and another four in October 2011, and succeeded in measuring the positionof the radio core in the TeV blazar Mrk 501 relative to a distant compactquasar NRAO 512. During our observations, we find that (1) there is nopositional change within ~0.2 mas or ~2.0 pc de-projected with $\pm1\sigma$error for the weighted-mean phase-referenced positions of Mrk 501 core relativeto NRAO 512 over four adjacent days, and (2) there is an indication of positionchange for 3C 345 core relative to NRAO 512. By applying our results to thestandard internal shock model for blazars, we constrain the bulk Lorenz factorsof the ejecta.

Probing Precise Location of Radio Core in TeV Blazar Mrk 501 with VERA at 43 GHz