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

TL;DR

This study uses multi-epoch VLBI astrometry at 43 GHz to precisely locate the radio core of the TeV blazar Mrk 501, finding no significant positional change over days and constraining jet dynamics.

Contribution

First multi-epoch astrometric measurements of Mrk 501's radio core at 43 GHz, providing constraints on jet physics and core stability.

Findings

No positional change within ~0.2 mas over four days.

Indication of position change in 3C 345.

Constraints on bulk Lorentz factors of jet ejecta.

Abstract

We investigate the position of the radio core in a blazar by multi-epoch astrometric observations at 43 GHz. Using the VLBI Exploration of Radio Astrometry (VERA), we have conducted four adjacent observations in February 2011 and another four in October 2011, and succeeded in measuring the position of the radio core in the TeV blazar Mrk 501 relative to a distant compact quasar NRAO 512. During our observations, we find that (1) there is no positional 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 relative to NRAO 512 over four adjacent days, and (2) there is an indication of position change for 3C 345 core relative to NRAO 512. By applying our results to the standard internal shock model for blazars, we constrain the bulk Lorenz factors of the ejecta.