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We report on the detailed radio status of the M87 jet during theVery-High-Energy (VHE) gamma-ray flaring event in April 2010, obtained fromhigh-resolution, multi-frequency, phase-referencing VLBA observations. Weespecially focus on the properties for the jet base (the radio core) and thepeculiar knot HST-1, which are currently favored as the gamma-ray emittingsites. During the VHE flaring event, the HST-1 region remains stable in termsof its structure and flux density in the optically thin regime above 2GHz,being consistent with no signs of enhanced activities reported at X-ray forthis feature. The radio core shows an inverted spectrum at least up to 43GHzduring this event. Astrometry of the core position, which is specified as ~20Rsfrom the central engine in our previous study, shows that the core position isstable on a level of 4Rs. The core at 43 and 22GHz tends to show slightly(~10%) higher flux level near the date of the VHE flux peak compared with theepochs before/after the event. The size of the 43-GHz core is estimated to be~17Rs, which is close to the size of the emitting region suggested from theobserved time scale of rapid variability at VHE. These results tend to favorthe scenario that the VHE gamma-ray flare in 2010 April is associated with theradio core.

VLBI Observations of the Jet in M87 During the Very-High-Energy Gamma-ray Flare in April 2010