Kavli Affiliate: Roger W. Romani
| First 5 Authors: Chien-Ting J. Chen, Ioannis Liodakis, Riccardo Middei, Dawoon E. Kim, Laura Di Gesu
| Summary:
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 $>3sigma$ 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.
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