IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare
Creators: name: Kouch, Pouya M.
name: Liodakis, Ioannis
name: Fenu, Francesco
name: Zhang, Haocheng
name: Boula, Stella
name: Middei, Riccardo
name: Di Gesu, Laura
name: Paraschos, Georgios F.
name: Agudo, Iván
name: Jorstad, Svetlana G.
name: Lindfors, Elina
name: Marscher, Alan P.
name: Krawczynski, Henric
name: Negro, Michela
name: Hu, Kun
name: Kim, Dawoon E.
name: Cavazzuti, Elisabetta
name: Errando, Manel
name: Blinov, Dmitry
name: Gourni, Anastasia
name: Kiehlmann, Sebastian
name: Kourtidis, Angelos
name: Mandarakas, Nikos
name: Triantafyllou, Nikolaos
name: Vervelaki, Anna
name: Borman, George A.
name: Kopatskaya, Evgenia N.
name: Larionova, Elena G.
name: Morozova, Daria A.
name: Savchenko, Sergey S.
name: Vasilyev, Andrey A.
name: Troitskiy, Ivan S.
name: Grishina, Tatiana S.
name: Zhovtan, Alexey V.
name: Aceituno, Francisco José
name: Bonnoli, Giacomo
name: Casanova, Víctor
name: Escudero, Juan
name: Agís-González, Beatriz
name: Husillos, César
name: Otero-Santos, Jorge
name: Piirola, Vilppu
name: Sota, Alfredo
name: Myserlis, Ioannis
name: Gurwell, Mark
name: Keating, Garrett K.
name: Rao, Ramprasad
name: Angelakis, Emmanouil
name: Kraus, Alexander
name: Antonelli, Lucio Angelo
name: Bachetti, Matteo
name: Baldini, Luca
name: Baumgartner, Wayne H.
name: Bellazzini, Ronaldo
name: Bianchi, Stefano
name: Bongiorno, Stephen D.
name: Bonino, Raffaella
name: Brez, Alessandro
name: Bucciantini, Niccolò
name: Capitanio, Fiamma
name: Castellano, Simone
name: Chen, Chien-Ting
name: Ciprini, Stefano
name: Costa, Enrico
name: De Rosa, Alessandra
name: Del Monte, Ettore
name: Di Lalla, Niccolò
name: Di Marco, Alessandro
name: Donnarumma, Immacolata
name: Doroshenko, Victor
name: Dovčiak, Michal
name: Ehlert, Steven R.
name: Enoto, Teruaki
name: Evangelista, Yuri
name: Fabiani, Sergio
name: Ferrazzoli, Riccardo
name: Garcia, Javier A.
name: Gunji, Shuichi
name: Hayashida, Kiyoshi
name: Heyl, Jeremy
name: Iwakiri, Wataru
name: Kaaret, Philip
name: Karas, Vladimir
name: Kislat, Fabian
name: Kitaguchi, Takao
name: Kolodziejczak, Jeffery J.
name: La Monaca, Fabio
name: Latronico, Luca
name: Maldera, Simone
name: Manfreda, Alberto
name: Marin, Frédéric
name: Marinucci, Andrea
name: Marshall, Herman L.
name: Massaro, Francesco
name: Matt, Giorgio
name: Mitsuishi, Ikuyuki
name: Mizuno, Tsunefumi
name: Muleri, Fabio
name: Ng, Chi-Yung
name: O'Dell, Stephen L.
name: Omodei, Nicola
name: Oppedisano, Chiara
name: Papitto, Alessandro
name: Pavlov, George G.
name: Peirson, Abel Lawrence
name: Perri, Matteo
name: Pesce-Rollins, Melissa
name: Petrucci, Pierre-Olivier
name: Pilia, Maura
name: Possenti, Andrea
name: Poutanen, Juri
name: Puccetti, Simonetta
name: Ramsey, Brian D.
name: Rankin, John
name: Ratheesh, Ajay
name: Roberts, Oliver J.
name: Sgrò, Carmelo
name: Slane, Patrick
name: Soffitta, Paolo
name: Spandre, Gloria
name: Swartz, Douglas A.
name: Tamagawa, Toru
name: Tavecchio, Fabrizio
name: Taverna, Roberto
name: Tawara, Yuzuru
name: Tennant, Allyn F.
name: Thomas, Nicholas E.
name: Tombesi, Francesco
name: Trois, Alessio
name: Tsygankov, Sergey S.
name: Turolla, Roberto
name: Romani, Roger W.
name: Vink, Jacco
name: Weisskopf, Martin C.
name: Wu, Kinwah
name: Xie, Fei
name: Zane, Silvia
Publisher: name: arXiv
Year: 2024
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on 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. 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$\pm$4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3$σ$ level) on the X-ray polarization degree; although a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% ($3σ$). We model 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. The constraints we obtain with our combined multi-wavelength polarization observations and SED modeling tentatively disfavor hadronic models for the X-ray emission in S4 0954+65.
description: Accepted for publication in A&amp;A, 18 pages, 5 figures, and 8 tables