VLBI Observations of the Jet in M 87 during the Very High Energy gamma-Ray Flare in 2010 April

We report on the detailed radio status of the M 87 jet during the very high energy (VHE) γ-ray flaring event in 2010 April, obtained from high-resolution, multi-frequency, phase-referencing Very Long Baseline Array observations. We especially focus on the properties of the jet base (the radio core) and the peculiar knot HST-1, which are currently favored as the γ-ray emitting sites. During the VHE flaring event, the HST-1 region remains stable in terms of its structure and flux density in the optically thin regime above 2 GHz, being consistent with no signs of enhanced activities reported at X-ray for this feature. The radio core shows an inverted spectrum at least up to 43 GHz during this event. Astrometry of the core position, which is specified as ~20 R s from the central engine in our previous study, shows that the core position is stable on a level of 4 R s. The core at 43 and 22 GHz tends to show slightly (~10%) higher flux level near the date of the VHE flux peak compared with the epochs before/after the event. The size of the 43 GHz core is estimated to be ~17 R s, which is close to the size of the emitting region suggested from the observed timescale of rapid variability at VHE. These results tend to favor the scenario that the VHE γ-ray flare in 2010 April is associated with the radio core.