Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Luigi Pacciani, Dawoon E. Kim, Riccardo Middei, Herman L. Marshall, Alan P. Marscher
| Summary:
We report the Imaging X-ray Polarimetry Explorer (IXPE) polarimetric and
simultaneous multiwavelength observations of the high-energy-peaked BL Lacertae
(HBL) object 1ES 1959+650, performed in 2022 October and 2023 August. In 2022
October IXPE measured an average polarization degree $Pi_{rm X}=9.4;!%pm
1.6;!%$ and an electric-vector position angle $psi_{rm X}=53^{circ}pm
5^{circ}$. The polarized X-ray emission can be decomposed into a constant
component, plus a rotating component, with rotation velocity $omega_{rm
EVPA}=(-117;!pm;!12)$ ${rm deg};!{rm d}^{-1}$. In 2023 August, during
a period of pronounced activity of the source, IXPE measured an average
$Pi_{rm X}=12.4;!%pm0.7;!%$ and $psi_X=20^{circ}pm2^{circ}$, with
evidence ($sim$0.4$;!%$ chance probability) for a rapidly rotating
component with $omega_{rm EVPA}=(1864;!pm;!34)$ ${rm deg};!{rm
d}^{-1}$. These findings suggest the presence of a helical magnetic field in
the jet of 1ES 1959+650 or stochastic processes governing the field in
turbulent plasma. Our multiwavelength campaigns from radio to X-ray reveal
variability in both polarization and flux from optical to X-rays. We interpret
the results in terms of a relatively slowly varying component dominating the
radio and optical emission, while rapidly variable polarized components
dominate the X-ray and provide minor contribution at optical wavelengths. The
radio and optical data indicate that on parsec scales the magnetic field is
primarily orthogonal to the jet direction. On the contrary, X-ray measurements
show a magnetic field almost aligned with the parsec jet direction. Confronting
with other IXPE observations, we guess that the magnetic field of HBLs on
sub-pc scale should be rather unstable, often changing its direction with
respect to the VLBA jet.
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