  To obtain a better understanding of the location and mechanisms for the
production of the gamma-ray emission in jets of AGN we present a detailed study
of the HST-1 structure, 0.8 arcsec downstream the jet of M87, previously
identified as a possible candidate for TeV emission. HST-1 shows a very
peculiar structure with superluminal as well as possible stationary
sub-components, and appears to be located in the transition from a parabolic to
a conical jet shape, presumably leading to the formation of a recollimation
shock. This scenario is supported by our new RHD simulations in which the
interaction of a moving component with a recollimation shock leads to the
appearance of a new superluminal component. To discern whether HST-1 is
produced by a recollimation shock or some other MHD instability, we present new
polarimetric 2.2 and 5 GHz VLBA, as well as 15, 22 and 43 GHz JVLA observations
obtained between November 2012 and March 2013.

High Energy Astrophysical Phenomena;Cosmology and Nongalactic Astrophysics