Unveiling blazar synchrotron emission: a multiwavelength polarimetric study of HSP and LSP populations

TL;DR

This study uses multiwavelength polarimetric observations to compare the emission mechanisms and magnetic field structures of HSP and LSP blazars, revealing similarities in magnetic field alignment and differences in polarization degree across wavelengths.

Contribution

It provides new multiwavelength polarimetric data, including four new IXPE observations of Mrk 501, and demonstrates that HSP and LSP blazars share similar magnetic field structures near their acceleration regions.

Findings

HSP blazars show higher polarization degrees in X-rays than in optical and radio wavelengths.

The electric vector position angles are aligned with the jet axis in both HSP and LSP populations.

The magnetic field structure near the acceleration region appears similar for both blazar types.

Abstract

Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets.