Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Riccardo Middei, Matteo Perri, Simonetta Puccetti, Ioannis Liodakis, Laura Di Gesu
| Summary:
The lower energy peak of the spectral energy distribution of blazars has
commonly been ascribed to synchrotron radiation from relativistic particles in
the jets. Despite the consensus regarding jet emission processes, the particle
acceleration mechanism is still debated. Here, we present the first X-ray
polarization observations of PG 1553+113, a high-synchrotron-peak blazar
observed by the Imaging X-ray Polarimetry Explorer (IXPE). We detect an X-ray
polarization degree of $(10pm2)%$ along an electric-vector position angle of
$psi_X=86^{circ}pm8^{circ}$. At the same time, the radio and optical
polarization degrees are lower by a factor of $sim$3. During our IXPE
pointing, we observed the first orphan optical polarization swing of the IXPE
era, as the optical angle of PG 1553+113 underwent a smooth monotonic rotation
by about 125$^circ$, with a rate of $sim$17 degrees per day. We do not find
evidence of a similar rotation in either radio or X-rays, which suggests that
the X-ray and optically emitting regions are separate or, at most, partially
co-spatial. Our spectro-polarimetric results provide further evidence that the
steady-state X-ray emission in blazars originates in a shock-accelerated and
energy-stratified electron population.
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