Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Pouya M. Kouch, Ioannis Liodakis, Francesco Fenu, Haocheng Zhang, Stella Boula
| Summary:
The X-ray polarization observations made possible with the Imaging X-ray
Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission
processes in astrophysical jets from blazars. Here we report on the first X-ray
polarization observation of the blazar S4 0954+65 in a high optical and X-ray
state. During our multi-wavelength campaign on the source, we detected an
optical flare whose peak coincided with the peak of an X-ray flare. This
optical-X-ray flare most likely took place in a feature moving along the
parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array. The 43 GHz
polarization angle of the moving component underwent a rotation near the time
of the flare. In the optical band, prior to the IXPE observation, we measured
the polarization angle to be aligned with the jet axis. In contrast, during the
optical flare the optical polarization angle was perpendicular to the jet axis;
after the flare, it reverted to being parallel to the jet axis. Due to the
smooth behavior of the optical polarization angle during the flare, we favor
shocks as the main acceleration mechanism. We also infer that the ambient
magnetic field lines in the jet were parallel to the jet position angle. The
average degree of optical polarization during the IXPE observation was
(14.3$pm$4.1)%. Despite the flare, we only detected an upper limit of 14% (at
3$sigma$ level) on the X-ray polarization degree; although a reasonable
assumption on the X-ray polarization angle results in an upper limit of 8.8%
($3sigma$). We model the spectral energy distribution (SED) and spectral
polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron
self-Compton) and hadronic (proton and pair synchrotron) models. The
constraints we obtain with our combined multi-wavelength polarization
observations and SED modeling tentatively disfavor hadronic models for the
X-ray emission in S4 0954+65.
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