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

Polarimetry offers a powerful window into the physical conditions and particle acceleration processes within blazar jets. In this study, we investigate the multiwavelength polarimetric behaviour of High Synchrotron Peaked (HSP) and Low Synchrotron Peaked (LSP) blazars to explore differences in their emission mechanisms and magnetic field structure near the acceleration region. We take advantage of several X-ray polarisation observations of HSP by the Imaging X-ray Polarimetry Explorer, including four new observations of Mrk501, and optical polarisation observations of LSP from RoboPol and other monitoring programs. We find that the polarisation degree (PD) of HSPs in X-rays is systematically higher than in the optical and mm-radio bands, consistent with earlier IXPE findings. Moreover, the electric vector position angles (PA) in the X-rays are predominantly aligned with the jet axis, with most measurements showing minimal offset within uncertainties. When comparing the offset of the polarisation angle from the jet axis between the LSP and HSP populations—measured in optical and X-rays respectively—we find a consistent distribution. This points toward a shared magnetic field structure near the particle acceleration region and lends strong support to the emerging picture of energy-stratified acceleration followed by radiative energy loss in blazar jets.