A strong radio brightening at the jet base of M87 during the elevated
  very-high-energy gamma-ray state in 2012
  We report our intensive radio monitoring observations of the jet in M87 with
the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network
(EVN) from February 2011 to October 2012, together with contemporaneous
high-energy gamma-ray light curves obtained by the Fermi-LAT. During this
period, an elevated level of the M87 flux is reported at VHE gamma rays. We
detected a remarkable increase of the radio flux density from the unresolved
jet base (radio core) with VERA at 22 and 43GHz coincident with the VHE
activity. Meanwhile, we confirmed with EVN at 5GHz that HST-1 (an alternative
gamma-ray production candidate site) remained quiescent in terms of its flux
density and structure. These results in the radio bands strongly suggest that
the VHE gamma-ray activity in 2012 originates in the jet base within 0.03pc or
56 Schwarzschild radii from the central supermassive black hole. We further
conducted VERA astrometry for the M87 core during the flaring period, and
detected core shifts between 22 and 43GHz. We also discovered a clear
frequency-dependent evolution of the radio core flare at 43, 22 and 5GHz; the
radio flux density increased more rapidly at higher frequencies with a larger
amplitude, and the light curves clearly showed a time-lag between the peaks at
22 and 43GHz. This indicates that a new radio-emitting component was created
near the black hole in the period of the VHE event, and then propagated outward
with progressively decreasing synchrotron opacity. By combining these results,
we estimated an apparent speed of the newborn component, and derived a
sub-luminal speed of less than ~0.2c. This value is significantly slower than
the super-luminal (~1.1c) features that appeared from the core during the
prominent VHE flaring event in 2008, suggesting that the stronger VHE activity
can be associated with the production of the higher Lorentz factor jet.
