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

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.

💡 Research Summary

This paper presents a comprehensive optical polarimetric monitoring campaign of the very‑high‑energy (VHE) blazar PKS 2155‑304 carried out simultaneously with a H.E.S.S./Fermi high‑energy observation campaign in 2008, when the source was in a low (quiescent) state. The authors obtained multi‑band (B, V, R) polarimetric data with daily cadence over a period of more than a month, allowing them to trace both the photometric flux and the polarimetric parameters (degree of polarization and polarization angle) on timescales ranging from hours to weeks.

The first major result is the clear decoupling between the photometric variability and the polarimetric behavior. While the total optical flux exhibits rapid, flare‑like changes on day‑scale intervals, the polarization degree varies only modestly and the polarization angle evolves smoothly without abrupt jumps. This lack of correlation suggests that the processes governing particle acceleration (which drive the flux changes) are physically distinct from those shaping the magnetic‑field geometry that determines the polarization.

To explain the observed polarization, the authors propose that the polarized emission originates in the millimetre‑wave core of the jet. In this picture the core hosts a relatively large‑scale ordered magnetic field. When a shock propagates through the core, electrons are re‑accelerated locally, and the shock also compresses and partially aligns the magnetic field in the shocked region. The locally ordered field enhances the optical polarization degree and produces a gradual rotation of the polarization angle as the shock moves downstream.

Because the jet is intrinsically inhomogeneous, the authors adopt an “inhomogeneous synchrotron source” model. The emitting region is divided into many sub‑zones, each with its own electron energy distribution and magnetic‑field ordering. A shock that energizes electrons in a particular zone can cause a sharp increase in flux without significantly altering the net polarization, which is the average over all zones. Conversely, a slow re‑configuration of the large‑scale field across the entire core changes the polarization degree and angle while leaving the total flux relatively unchanged. This framework quantitatively reproduces the observed lack of flux‑polarization correlation.

The paper further exploits the low‑state polarimetric measurements to address the origin of the persistent TeV γ‑ray emission recently reported for PKS 2155‑304. Since the optical synchrotron component appears to be produced in a region with a stable, partially ordered magnetic field, the same electron population can up‑scatter either synchrotron photons (SSC) or external photon fields (EC) to TeV energies. The stability of the magnetic environment in the low state therefore provides a natural explanation for a steady, quiescent γ‑ray component that co‑exists with the more variable optical emission.

In summary, the study demonstrates that optical polarimetry, when performed with dense temporal sampling, can disentangle the dynamics of particle acceleration from magnetic‑field evolution in blazar jets. The decoupled variability, the core‑origin model for polarized emission, and the inhomogeneous synchrotron framework together offer a coherent picture of how low‑state optical and high‑energy γ‑ray emissions are linked. The authors suggest that future simultaneous multi‑wavelength (radio to γ‑ray) campaigns incorporating polarimetric diagnostics will be essential to fully map the energy‑transfer processes that operate in both quiescent and flaring states of blazars.