Abstract
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 ofM87, 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 are collimation 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.
Highlights
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
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
In the era of the Fermi satellite and of the new generation Cherenkov telescopes, there is an active debate over the location and the mechanisms for the production of MeV to very high energy (VHE) gamma rays in AGN jets
Summary
In the era of the Fermi satellite and of the new generation Cherenkov telescopes, there is an active debate over the location and the mechanisms for the production of MeV to very high energy (VHE) gamma rays in AGN jets. The inner jet region is favored by the short TeV variability timescales and the simultaneous increase in radio flux density for the VHE event in 2008. The presence of compact substructures with superluminal motion and the simultaneous radio and X-ray flare during the 2005 event favor the location at the HST-1 complex about 0.8” downstream the jet. There are, no clear observational indications for the existence of a stationary feature in HST-1 associated with the recollimation shock — as expected from nu-. Merical simulations [13, 14] and observed in other sources [15] —, other than the earlier 1.7 GHz VLBA observations by Cheung, Harris, & Stawarz [3], which measured an upper-limit for the proper motion of the upstream region of HST-1 (labeled by these authors as HST-1d) of 0.25c
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