X-ray and multiwavelength polarization of Mrk 501 from 2022 to 2023

TL;DR

This study presents multiwavelength polarization measurements of blazar Mrk 501 over 14 months, revealing stable polarization in X-ray, optical, and radio bands with a notable increase in X-ray polarization in the last observation, supporting an energy-stratified shock emission model.

Contribution

First multiwavelength polarization dataset of Mrk 501 spanning 14 months, highlighting energy-dependent polarization behavior and variability.

Findings

X-ray polarization remained stable in five observations, increased significantly in the sixth.

Optical and radio polarization showed no correlation with X-ray polarization.

X-ray polarization degree was higher than optical, which was higher than radio, consistent with energy-stratified shock scenario.

Abstract

We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical-infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at $>3\sigma$ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions.