TY - JOUR

T1 - IXPE observation of the low-synchrotron peaked blazar S4 0954+65 during an optical-X-ray flare

AU - Kouch, Pouya M.

AU - Liodakis, Ioannis

AU - Fenu, Francesco

AU - Zhang, Haocheng

AU - Boula, Stella

AU - Middei, Riccardo

AU - Di Gesu, Laura

AU - Paraschos, Georgios F.

AU - Agudo, Iván

AU - Jorstad, Svetlana G.

AU - Lindfors, Elina

AU - Marscher, Alan P.

AU - Krawczynski, Henric

AU - Negro, Michela

AU - Hu, Kun

AU - Kim, Dawoon E.

AU - Cavazzuti, Elisabetta

AU - Errando, Manel

AU - Blinov, Dmitry

AU - Gourni, Anastasia

AU - Kiehlmann, Sebastian

AU - Kourtidis, Angelos

AU - Mandarakas, Nikos

AU - Triantafyllou, Nikolaos

AU - Vervelaki, Anna

AU - Borman, George A.

AU - Kopatskaya, Evgenia N.

AU - Larionova, Elena G.

AU - Morozova, Daria A.

AU - Savchenko, Sergey S.

AU - Vasilyev, Andrey A.

AU - Troitskiy, Ivan S.

AU - Grishina, Tatiana S.

AU - Shishkina, Ekaterina V.

AU - Zhovtan, Alexey V.

AU - Aceituno, Francisco José

AU - Bonnoli, Giacomo

AU - Casanova, Víctor

AU - Escudero, Juan

AU - Agís-González, Beatriz

AU - Husillos, César

AU - Otero-Santos, Jorge

AU - Piirola, Vilppu

AU - Sota, Alfredo

AU - Myserlis, Ioannis

AU - Gurwell, Mark

AU - Keating, Garrett K.

AU - Rao, Ramprasad

AU - Angelakis, Emmanouil

AU - Kraus, Alexander

AU - Antonelli, Lucio Angelo

AU - Bachetti, Matteo

AU - Baldini, Luca

AU - Baumgartner, Wayne H.

AU - Bellazzini, Ronaldo

AU - Bianchi, Stefano

AU - Bongiorno, Stephen D.

AU - Bonino, Raffaella

AU - Brez, Alessandro

AU - Bucciantini, Niccolò

AU - Capitanio, Fiamma

AU - Castellano, Simone

AU - Chen, Chien-Ting

AU - Ciprini, Stefano

AU - Costa, Enrico

AU - De Rosa, Alessandra

AU - Del Monte, Ettore

AU - Di Lalla, Niccolò

AU - Di Marco, Alessandro

AU - Donnarumma, Immacolata

AU - Doroshenko, Victor

AU - Dovčiak, Michal

AU - Ehlert, Steven R.

AU - Enoto, Teruaki

AU - Evangelista, Yuri

AU - Fabiani, Sergio

AU - Ferrazzoli, Riccardo

AU - Garcia, Javier A.

AU - Gunji, Shuichi

AU - Hayashida, Kiyoshi

AU - Heyl, Jeremy

AU - Iwakiri, Wataru

AU - Kaaret, Philip

AU - Karas, Vladimir

AU - Kislat, Fabian

AU - Kitaguchi, Takao

AU - Kolodziejczak, Jeffery J.

AU - La Monaca, Fabio

AU - Latronico, Luca

AU - Maldera, Simone

AU - Manfreda, Alberto

AU - Marin, Frédéric

AU - Marinucci, Andrea

AU - Marshall, Herman L.

AU - Massaro, Francesco

AU - Matt, Giorgio

AU - Mitsuishi, Ikuyuki

AU - Mizuno, Tsunefumi

AU - Muleri, Fabio

AU - Ng, Chi-Yung

AU - O’Dell, Stephen L.

AU - Omodei, Nicola

AU - Oppedisano, Chiara

AU - Papitto, Alessandro

AU - Pavlov, George G.

AU - Peirson, Abel Lawrence

AU - Perri, Matteo

AU - Pesce-Rollins, Melissa

AU - Petrucci, Pierre-Olivier

AU - Pilia, Maura

AU - Possenti, Andrea

AU - Poutanen, Juri

AU - Puccetti, Simonetta

AU - Ramsey, Brian D.

AU - Rankin, John

AU - Ratheesh, Ajay

AU - Roberts, Oliver J.

AU - Sgrò, Carmelo

AU - Slane, Patrick

AU - Soffitta, Paolo

AU - Spandre, Gloria

AU - Swartz, Douglas A.

AU - Tamagawa, Toru

AU - Tavecchio, Fabrizio

AU - Taverna, Roberto

AU - Tawara, Yuzuru

AU - Tennant, Allyn F.

AU - Thomas, Nicholas E.

AU - Tombesi, Francesco

AU - Trois, Alessio

AU - Tsygankov, Sergey S.

AU - Turolla, Roberto

AU - Romani, Roger W.

AU - Vink, Jacco

AU - Weisskopf, Martin C.

AU - Wu, Kinwah

AU - Xie, Fei

AU - Zane, Silvia

PY - 2025/3/11

Y1 - 2025/3/11

N2 - 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.

AB - 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.

KW - BL Lacertae objects: individual: S4 0954+65

KW - Galaxies: jets

KW - Magnetic fields

KW - Polarization

KW - Relativistic processes

UR - https://www.mendeley.com/catalogue/04f1d239-c9f9-3de7-8e01-a6d624844118/

U2 - 10.1051/0004-6361/202453127

DO - 10.1051/0004-6361/202453127

M3 - Article

VL - 695

JO - ASTRONOMY & ASTROPHYSICS

JF - ASTRONOMY & ASTROPHYSICS

SN - 0004-6361

M1 - A99

ER -