IXPE and multi-wavelength observations of blazar PG 1553+113 reveal an orphan optical polarization swing

TL;DR

This study presents the first X-ray polarization measurements of blazar PG 1553+113, revealing an orphan optical polarization swing and providing insights into jet emission and particle acceleration mechanisms.

Contribution

First X-ray polarization observations of PG 1553+113, detecting an orphan optical polarization swing and suggesting separate emission regions in the jet.

Findings

X-ray polarization degree of (10±2)% with a specific electric-vector angle

Observation of a 125° optical polarization swing without similar X-ray or radio rotation

Evidence supporting shock-accelerated, energy-stratified electron populations in blazar jets

Abstract

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 $(10\pm2)\%$ 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.