Multiwavelength Polarization Observations of Mrk 501

Abstract

Mrk 501 is a prototypical high-synchrotron-peaked blazar (HBL) and serves as one of the primary targets for the {\it Imaging X-ray Polarimetry Explorer} ({\it IXPE}). In this study, we report X-ray polarization measurements of Mrk 501 based on six {\it IXPE} observations. The detection of X-ray polarization at a confidence level exceeding 99\% is achieved in four out of the six observations conducted across the entire energy range (2--8 keV) of {\it IXPE}. The maximum polarization degree (${\hat{I}}_{\mathrm{X}}$) is measured to be $15.8\%\pm 2.8\%$, accompanied by a polarization angle ($\ddot{I}{ˆ}_{\mathrm{X}}$) of $98.0\hat{A}°\pm 5.1\hat{A}°$ at a confidence level of $5.6\ddot{I}ƒ$. During the remaining two observations, only an upper limit of ${\hat{I}}_{\mathrm{X}}<$12\% could be derived at the 99\% confidence level. No temporal variability in polarization is observed throughout all six {\it IXPE} observations for Mrk 501. A discernible trend of energy-dependent variation in the polarization degree is detected in optical spectropolarimetry; however, no analogous indication is observed in ${\hat{I}}_{\mathrm{X}}$. The chromatic behavior of $\hat{I}$ and the consistent values of $\ddot{I}ˆ$ across different frequencies from X-rays to radio waves, along with the agreement between $\ddot{I}ˆ$ and jet position angle, strongly support the interpretation of the energy-stratified model with shock-accelerated particles in the jet of Mrk 501. Additionally, the possibility of the presence of a global helical magnetic field in the jet of Mrk 501 is discussed.

20 pages, 8 figures, 4 tables, accepted for publication in ApJL