Particle Acceleration and Magnetic Field Structure in PKS 2155-304: Optical Polarimetric Observations

TL;DR

This study uses multiband optical polarimetric observations of the blazar PKS 2155-304 during a low state to analyze particle acceleration, magnetic field structure, and their relation to gamma-ray emission.

Contribution

It presents a detailed temporal analysis of optical polarization and proposes a model linking polarization properties to shock-driven particle acceleration in the source.

Findings

Particle acceleration timescales are decoupled from polarimetric changes.

Optical emission originates from a polarized mm-wave core with shock-induced magnetic field organization.

The quiescent gamma-ray flux may be linked to the inhomogeneous synchrotron source structure.

Abstract

In this paper we present multiband optical polarimetric observations of the VHE blazar PKS 2155-304 made simultaneously with a H.E.S.S./Fermi high-energy campaign in 2008, when the source was found to be in a low state. The intense daily coverage of the dataset allowed us to study in detail the temporal evolution of the emission and we found that the particle acceleration timescales are decoupled from the changes in the polarimetric properties of the source. We present a model in which the optical polarimetric emission originates at the polarised mm-wave core and propose an explanation for the lack of correlation between the photometric and polarimetric fluxes. The optical emission is consistent with an inhomogeneous synchrotron source in which the large scale field is locally organised by a shock in which particle acceleration takes place. Finally, we use these optical polarimetric observations of PKS 2155-304 at a low state to propose an origin for the quiescent gamma-ray flux of the object, in an attempt to provide clues for the source of its recently established persistent TeV emission.