X-ray Polarization Observations of BL Lacertae by Riccardo Middei et al. on Sunday 27 November

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.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.13764v1