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The TeV radio galaxy M87 is the first radio galaxy detected in the TeVregime. The structure of its jet, which is not pointing towards our line ofsight, is spatially resolved in X-ray (by Chandra), optical and radioobservations. In 2008, the three main Atmospheric Cherenkov Telescopeobservatories VERITAS, MAGIC and H.E.S.S. coordinated their observations in ajoint campaign from January to May with a total observation time of approx. 120hours. In February, strong and rapid day-scale TeV flares were detected. VLBAmonitoring observations during the same period showed that the 43 GHz radioflux density of the unresolved core began to rise at the time of the TeV flaresand eventually reached levels above any previously seen with VLBI. New jetcomponents appeared during the flare. The localization accuracy of the TeVinstruments of many arcseconds, even for strong sources, is inadequate toconstrain the origin of the emission in the inner jets of AGNs. For M87, with a6 billion solar mass black hole and a distance of 16.7 Mpc, the VLBA resolutioninstead corresponds to 30 by 60 Schwarzschild radii. This is starting toresolve the jet collimation region. The temporal coincidence of the TeV andradio flares indicates that they are related and provides the first directevidence that the TeV radiation from this source is produced within a few tensof RS of the radio core, thought to be coincident to within the VLBA resolutionwith the black hole.

Pinpointing the TeV gamma-ray emission region in M87 using TeV and 43 GHz radio monitoring