Kouch, Pouya M; Liodakis, I; Wu, KInwah; Zane, S; Turolla, R; ...; (2024) IXPE observation of PKS 2155-304 reveals the most highly polarized blazar. Astronomy and Astrophysics: a European journal 10.1051/0004-6361/202449166. (In press).

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Abstract

We report the X-ray polarization properties of the southern, high-synchrotron-peaked (HSP) blazar PKS 2155−304 based on observations with the Imaging X-ray Polarimetry Explorer (IXPE). We observed the source between October 27 and November 7, 2023. We additionally conducted an extensive contemporaneous multiwavelength campaign to place the X-ray polarization results into a broader context. We find that during the first half (T1) of the IXPE pointing, the source exhibited the highest X-ray polarization degree (ΠX) detected for an HSP blazar thus far, (30.7±2.0)%, which dropped to (15.3±2.1)% during the second half (T2). The X-ray polarization angle (ΨX) remained stable during the IXPE pointing at 129.4◦±1.8◦ and 125.4◦±3.9◦ during T1 and T2, respectively. Meanwhile, the optical polarization degree (ΠO) remained stable within the uncertainties during the IXPE pointing, with average host-galaxy-corrected values of (4.3±0.7)% and (3.8±0.9)% during T1 and T2, respectively. The optical polarization angle (ΨO) changed achromatically from ∼140◦ to ∼90◦ during the IXPE pointing, and subsequently rotated back to ∼130◦ . Despite several attempts to observe the source in the radio band, the faintness and the weakly polarized nature of the source in the radio band resulted in only one detection (99.7% confidence) of the polarization at 225.5 GHz during T2, with degree ΠR=(1.7±0.4)% and angle ΨR=112.5◦±5.5◦ . The direction of the broad parsec-scale jet is rather ambiguous and has been found to point to the east and to the south at different epochs; however, on somewhat larger scales (> 1.5 pc) the jet points toward the southeast (∼135◦ ), similar to all of the multiwavelength polarization angles. Moreover, the X-ray to optical polarization degree ratios (ΠX/ΠO) of ∼7 and ∼4 during T1 and T2, respectively, are similar to previous IXPE results for several HSP blazars. These findings, combined with the lack of correlation of temporal variability between the polarization properties at different wavelengths, agree with an energy-stratified shock-acceleration scenario in HSP blazars.

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