High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

TL;DR

This study uses multiwavelength polarimetry to show that BL Lacertae's high optical polarization and low X-ray polarization support a leptonic Compton scattering origin of its high-energy emission, ruling out hadronic models.

Contribution

It provides the first multiwavelength polarization measurements of BL Lacertae, demonstrating the power of polarimetry to distinguish emission mechanisms in blazar jets.

Findings

Optical polarization reached 47.5%, the highest ever observed.

X-ray polarization was less than 7.4%, ruling out hadronic models.

Results favor a leptonic Compton scattering origin for high-energy emission.

Abstract

Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.