Kavli Affiliate: Grzegorz M. Madejski
| First 5 Authors: Ioannis Liodakis, Alan P. Marscher, Iván Agudo, Andrei V. Berdyugin, Maria I. Bernardos
| Summary:
Blazars are active galactic nuclei that launch collimated, powerful jets of
magnetized relativistic plasma. Their primary jet, whose emission typically
spans from low-frequency radio to very high-energy ($gtrsim0.1$ TeV)
$gamma$-rays (Blandford et al., 2019), is aligned towards our line of sight.
Multiwavelength polarization is a crucial probe of the magnetic field structure
and emission processes in such jets. Until now, sensitive polarization
observations have been limited to the radio, infrared, and optical range,
thereby leaving a gap in our knowledge of the physical conditions experienced
by the most energetic particles. Here, we report the first-ever detection of
X-ray polarization from the jet in an accreting supermassive black hole system,
the blazar Markarian 501 (Mrk 501). The recently launched Imaging X-ray
Polarimetry Explorer ($IXPE$, Weisskopf et al., 2022) measures a linear
polarization degree ($Pi$) over the 2-8 keV X-ray energy range of 10$pm$2%
with an electric vector position angle of 134$^circpm$5$^circ$, parallel to
the radio jet. The X-ray $Pi$ is more than a factor of 2 higher than the
optical $Pi$. We conclude that an energy-stratified relativistic electron
population, i.e., an acceleration scenario where the higher energy particles
emit from more magnetically ordered regions closer to the acceleration site, is
the most likely explanation of the higher degree of polarization at X-ray
energies. A second $IXPE$ observation conducted 16 days later yielded similar
results, strengthening our conclusions.
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