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

TL;DR

This paper investigates the origin of TeV emission in M87 by modeling a structured jet with a fast spine and slower layer, proposing that the layer produces the TeV radiation while the spine accounts for radio to GeV emission.

Contribution

It introduces a structured jet model with a spine and layer to explain M87's TeV emission, addressing limitations of previous one-zone models.

Findings

The TeV emission likely originates from the jet's slower layer.

The spine dominates radio to GeV emission, resembling low-energy BL Lac objects.

The model predicts complex correlations between different emission components.

Abstract

Simple one-zone homogeneous synchrotron self-Compton models have severe difficulties in explaining the TeV emission observed in the radiogalaxy M87. Also the site of the TeV emission region is uncertain: it could be the unresolved jet close to the nucleus, analogously to what proposed for blazars, or an active knot, called HST-1, tens of parsec away. We explore the possibility that the TeV emission of M87 is produced in the misaligned subpc 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 (LBLs).