IXPE observation of PKS 2155–304 reveals the most highly polarized blazar
Authors: Kouch Pouya M. , Liodakis Ioannis , Middei Riccardo , Kim Dawoon E. , Tavecchio Fabrizio , Marscher Alan P. , Marshall Herman L. , Ehlert Steven R. , Di Gesu Laura , Jorstad Svetlana G. , Agudo Iván , Madejski Grzegorz M. , Romani Roger W. , Errando Manel , Lindfors Elina , Nilsson Kari , Toppari Ella , Potter Stephen B. , Imazawa Ryo , Sasada Mahito , Fukazawa Yasushi , Kawabata Koji S. , Uemura Makoto , Mizuno Tsunefumi , Nakaoka Tatsuya , Akitaya Hiroshi , McCall Callum , Jermak Helen E. , Steele Iain A. , Myserlis Ioannis , Gurwell Mark , Keating Garrett K. , Rao Ramprasad , Kang Sincheol , Lee Sang-Sung , Kim Sang-Hyun , Cheong Whee Yeon , Jeong Hyeon-Woo , Angelakis Emmanouil , Kraus Alexander , Aceituno Francisco José , Bonnoli Giacomo , Casanova Víctor , Escudero Juan , Agís-González Beatriz , Husillos César , Morcuende Daniel , Otero-Santos Jorge , Sota Alfredo , Bachev Rumen , Antonelli Lucio Angelo , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Lalla Niccolò , Di Marco Alessandro , Donnarumma Immacolata , Doroshenko Victor , Dovčiak Michal , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Ferrazzoli Riccardo , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Kaaret Philip , Karas Vladimir , Kislat Fabian , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , La Monaca Fabio , Latronico Luca , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , Muleri Fabio , Negro Michela , Ng Chi-Yung , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel Lawrence , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Poutanen Juri , Puccetti Simonetta , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Roberts Oliver J. , Sgrò Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei , Zane Silvia 
Published: June 2024

We report the X-ray polarization properties of the high-synchrotron-peaked (HSP) blazar PKS 2155−304 based on observations with the Imaging X-ray Polarimetry Explorer (IXPE). We observed the source between Oct 27 and Nov 7, 2023. We also conducted an extensive contemporaneous multiwavelength (MW) campaign. We find that during the first half (T1) of the IXPE pointing, the source exhibited the highest X-ray polarization degree detected for an HSP blazar thus far, (30.7 ± 2.0)%; this dropped to (15.3 ± 2.1)% during the second half (T2). The X-ray polarization angle 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 remained stable during the IXPE pointing, with average host-galaxy-corrected values of (4.3 ± 0.7)% and (3.8 ± 0.9)% during the T1 and T2, respectively. During the IXPE pointing, the optical polarization angle changed achromatically from ∼140° to ∼90° and back to ∼130°. Despite several attempts, we only detected (99.7% conf.) the radio polarization once (during T2, at 225.5 GHz): with degree (1.7 ± 0.4)% and angle 112.5° ±5.5°. The direction of the broad pc-scale jet is rather ambiguous and has been found to point to the east and south at different epochs; however, on larger scales (> 1.5 pc) the jet points toward the southeast (∼135°), similarly to all of the MW polarization angles. Moreover, the X-ray-to-optical polarization degree ratios 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 MW polarization properties, agree with an energy-stratified shock-acceleration scenario in HSP blazars.