Spine-sheath layer radiative interplay in subparsec-scale jets and the TeV emission from M87

Abstract

Simple one-zone homogeneous synchrotron self-Compton models have severe difficulties in explaining the TeV emission observed in the radio galaxy M87. Also, the site the TeV emission region is uncertain: it could be the unresolved jet close to the nucleus, analogously to what is proposed for blazars, or it could be the active knot, called HST-1, tens of parsec away. We explore the possibility that the TeV emission of M87 is produced in the misaligned subparsec-scale jet. We base our modelling on a structured jet, with a fast spine surrounded by a slower layer. In this context the main site responsible for the emission of the TeV radiation is the layer, while the (debeamed) spine accounts for the emission from the radio to the GeV band: therefore we expect a more complex correlation with the TeV component than that expected in one-zone scenarios, in which both components are produced by the same region. Observed from small angles, the spine would dominate the emission, with an overall spectral energy distribution close to those of BL Lac objects with a synchrotron peak located at low energy.