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We investigate the location of the radio jet bases ("radio cores") of blazarsin radio images, and their stationarity by means of dense very long baselineinterferometry (VLBI) observations. In order to measure the position of a radiocore, we conducted 12 epoch astrometric observation of the blazar Markarian 421with the VLBI Exploration of Radio Astrometry at 22 GHz immediately after alarge X-ray flare, which occurred in the middle of 2011 September. For thefirst time, we find that the radio core is not stationary but rather changesits location toward 0.5 mas downstream. This angular scale corresponds to thede-projected length of a scale of $10^5$ Schwarzschild radii (Rs) at thedistance of Markarian~421. This radio-core wandering may be a new type ofmanifestation associated with the phenomena of large X-ray flares.

Discovery of a wandering radio jet base after a large X-ray flare in the blazar Markarian 421