2024ApJ...970L..22H

Mrk 501 is a prototypical high-synchrotron-peaked blazar and serves as one of the primary targets for the Imaging X-ray Polarimetry Explorer (IXPE). In this study, we report X-ray polarization measurements of Mrk 501 based on six 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 IXPE. The maximum polarization degree (Π_X) is measured to be 15.8% ± 2.8%, accompanied by a polarization angle (ψ_X) of 98.°0 ± 5.°1 at a confidence level of 5.6σ. During the remaining two observations, only an upper limit of Π_X < 12% could be derived at the 99% confidence level. No temporal variability in polarization is observed throughout all six 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 Π_X. The chromatic behavior of Π and the consistent values of ψ across different frequencies from X-ray to radio bands, along with the agreement between ψ 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.