X-ray Polarization Observations of BL Lacertae

TL;DR

This paper presents the first X-ray polarization observations of BL Lacertae, providing insights into the emission mechanisms and supporting a leptonic origin for the high-energy X-ray emission.

Contribution

It reports the first X-ray polarization measurement of BL Lacertae and combines multiwavelength data to constrain the emission models.

Findings

X-ray polarization degree upper limit <12.6% in 2-8 keV band

Multiwavelength polarization analysis favors leptonic emission models

Proton synchrotron contribution to X-ray emission is disfavored

Abstract

Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus the Synchrotron emission to be responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae performed with the Imaging X-ray Polarimetry Explorer ({IXPE}), from which an upper limit to the polarization degree $\Pi_X<$12.6\% was found in the 2-8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.