Pinpointing the TeV gamma-ray emission region in M87 using TeV and 43
  GHz radio monitoring
  The TeV radio galaxy M87 is the first radio galaxy detected in the TeV
regime. The structure of its jet, which is not pointing towards our line of
sight, is spatially resolved in X-ray (by Chandra), optical and radio
observations. In 2008, the three main Atmospheric Cherenkov Telescope
observatories VERITAS, MAGIC and H.E.S.S. coordinated their observations in a
joint campaign from January to May with a total observation time of approx. 120
hours. In February, strong and rapid day-scale TeV flares were detected. VLBA
monitoring observations during the same period showed that the 43 GHz radio
flux density of the unresolved core began to rise at the time of the TeV flares
and eventually reached levels above any previously seen with VLBI. New jet
components appeared during the flare. The localization accuracy of the TeV
instruments of many arcseconds, even for strong sources, is inadequate to
constrain the origin of the emission in the inner jets of AGNs. For M87, with a
6 billion solar mass black hole and a distance of 16.7 Mpc, the VLBA resolution
instead corresponds to 30 by 60 Schwarzschild radii. This is starting to
resolve the jet collimation region. The temporal coincidence of the TeV and
radio flares indicates that they are related and provides the first direct
evidence that the TeV radiation from this source is produced within a few tens
of RS of the radio core, thought to be coincident to within the VLBA resolution
with the black hole.
