IXPE observation of the low-synchrotron peaked blazar S4 0954+65 during an optical-X-ray flare
Authors: Kouch Pouya M. , Liodakis Ioannis , Fenu Francesco , Zhang Haocheng , Boula Stella , Middei Riccardo , Di Gesu Laura , Paraschos Georgios F. , Agudo Iván , Jorstad Svetlana G. , Lindfors Elina , Marscher Alan P. , Krawczynski Henric , Negro Michela , Hu Kun , Kim Dawoon E. , Cavazzuti Elisabetta , Errando Manel , Blinov Dmitry , Gourni Anastasia , Kiehlmann Sebastian , Kourtidis Angelos , Mandarakas Nikos , Triantafyllou Nikolaos , Vervelaki Anna , Borman George A. , Kopatskaya Evgenia N. , Larionova Elena G. , Morozova Daria A. , Savchenko Sergey S. , Vasilyev Andrey A. , Troitskiy Ivan S. , Grishina Tatiana S. , Shishkina Ekaterina V. , Zhovtan Alexey V. , José Aceituno Francisco , Bonnoli Giacomo , Casanova Víctor , Escudero Juan , Agís-González Beatriz , Husillos César , Otero-Santos Jorge , Piirola Vilppu , Sota Alfredo , Myserlis Ioannis , Gurwell Mark , Keating Garrett K. , Rao Ramprasad , Angelakis Emmanouil , Kraus Alexander , Angelo Antonelli Lucio , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Capitanio Fiamma , Castellano Simone , Chen Chien-Ting , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Lalla Niccolò , Di Marco Alessandro , Donnarumma Immacolata , Doroshenko Victor , Dovciak Michal , Ehlert Steven R. , 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. , La Monaca Fabio , Latronico Luca , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Marshall Herman L. , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Muleri Fabio , Ng Chi-Yung , O'Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Lawrence Peirson Abel , 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 , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Romani Roger W. , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei , Zane Silvia 
Published: February 2025

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 the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength (MWL) campaign of 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 (VLBA). 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±4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3σ level) on the X-ray polarization degree; however, a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% ($3σ$). We modeled 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. Our combined MWL polarization observations and SED modeling tentatively disfavor the use of hadronic models for the X-ray emission in S4 0954+65.