Middei, RiccardoLiodakis, IoannisPerri, MatteoPuccetti, SimonettaCavazzuti, ElisabettaDi Gesu, LauraEhlert, Steven R.Madejski, GrzegorzMarscher, Alan P.Marshall, Herman L.Muleri, FabioNegro, MichelaJorstad, Svetlana G.Agís-González, BeatrizAgudo, IvánBonnoli, G.Bernardos, Maria I.Casanova, VíctorGarcía-Comas, MayaHusillos, CésarMarchini, AlessandroSota, AlfredoKouch, Pouya M.Lindfors, ElinaBorman, G.A.Kopatskaya, Evgenia N.Larionova, Elena G.Morozova, Daria A.Savchenko, Sergey S.Vasilyev, Andrey A.Zhovtan, Alexey V.Casadio, CarolinaEscudero, JuanMyserlis, IoannisHales, AntonioKameno, SeijiKneissl, RuedigerMessias, HugoNagai, HiroshiBlinov, DmitryBourbah, Ioakeim G.Kiehlmann, SebastianKontopodis, EvangelosMandarakas, NikosRomanopoulos, StylianosSkalidis, RaphaelVervelaki, AnnaMasiero, Joseph R.Mawet, DimitriMillar-Blanchaer, Maxwell A.Panopoulou, Georgia V.Tinyanont, SamapornBerdyugin, AndreiKagitani, MasatoKravtsov, VadimSakanoi, TakeshiImazawa, RyoSasada, MahitoFukazawa, YasushiKawabata, Koji S.Uemura, MakotoMizuno, TsunefumiNakaoka, TatsuyaAkitaya, HiroshiGurwell, MarkRao, RamprasadLalla, Niccoló DiCibrario, NicolóDonnarumma, ImmacolataKim, Dawoon E.Omodei, NicolaPacciani, LuigiPoutanen, JuriTavecchio, FabrizioAntonelli, L.A.Bachetti, MatteoBaldini, LucaBaumgartner, Wayne H.Bellazzini, RonaldoBianchi, StefanoBongiorno, Stephen D.Bonino, RaffaellaBrez, AlessandroBucciantini, NiccolòCapitanio, FiammaCastellano, SimoneCiprini, StefanoCosta, EnricoDe Rosa, AlessandraDel Monte, EttoreMarco, Alessandro DiDoroshenko, VictorDovčiak, MichalEnoto, TeruakiEvangelista, YuriFabiani, SergioFerrazzoli, RiccardoGarcia, Javier A.Gunji, ShuichiHayashida, KiyoshiHeyl, JeremyIwakiri, WataruKaras, VladimirKitaguchi, TakaoKolodziejczak, Jeffery J.Krawczynski, HenricMonaca, Fabio LaLatronico, LucaMaldera, SimoneManfreda, AlbertoMarin, FrédéricMarinucci, AndreaMassaro, FrancescoMatt, GiorgioMitsuishi, IkuyukiNg, C.-Y.O’Dell, Stephen L.Oppedisano, ChiaraPapitto, AlessandroPavlov, George G.Peirson, Abel L.Pesce-Rollins, MelissaPetrucci, Pierre-OlivierPilia, MauraPossenti, AndreaRamsey, Brian D.Rankin, JohnRatheesh, AjayRomani, Roger W.Sgró, CarmeloSlane, PatrickSoffitta, PaoloSpandre, GloriaTamagawa, ToruTaverna, RobertoTawara, YuzuruTennant, Allyn F.Thomas, Nicholas E.Tombesi, FrancescoTrois, AlessioTsygankov, SergeyTurolla, RobertoVink, JaccoWeisskopf, Martin C.Wu, KinwahXie, FeiZane, Silvia2023-06-072023-06-072023-01-01R. Middei, I. Liodakis, M. Perri, S. Puccetti, E. Cavazzuti, L. Di Gesu, S.R. Ehlert, G. Madejski, A.P. Marscher, H.L. Marshall, F. Muleri, M. Negro, S.G. Jorstad, B. Agís-González, I. Agudo, G. Bonnoli, M.I. Bernardos, V. Casanova, M. García-Comas, C. Husillos, A. Marchini, A. Sota, P.M. Kouch, E. Lindfors, G.A. Borman, E.N. Kopatskaya, E.G. Larionova, D.A. Morozova, S.S. Savchenko, A.A. Vasilyev, A.V. Zhovtan, C. Casadio, J. Escudero, I. Myserlis, A. Hales, S. Kameno, R. Kneissl, H. Messias, H. Nagai, D. Blinov, I.G. Bourbah, S. Kiehlmann, E. Kontopodis, N. Mandarakas, S. Romanopoulos, R. Skalidis, A. Vervelaki, J.R. Masiero, D. Mawet, M.A. Millar-Blanchaer, G.V. Panopoulou, S. Tinyanont, A.V. Berdyugin, M. Kagitani, V. Kravtsov, T. Sakanoi, R. Imazawa, M. Sasada, Y. Fukazawa, K.S. Kawabata, M. Uemura, T. Mizuno, T. Nakaoka, H. Akitaya, M. Gurwell, R. Rao, N.D. Lalla, N. Cibrario, I. Donnarumma, D.E. Kim, N. Omodei, L. Pacciani, J. Poutanen, F. Tavecchio, L.A. Antonelli, M. Bachetti, L. Baldini, W.H. Baumgartner, R. Bellazzini, S. Bianchi, S.D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, F. Capitanio, S. Castellano, S. Ciprini, E. Costa, A. De Rosa, E. Del Monte, A.D. Marco, V. Doroshenko, M. Dovčiak, T. Enoto, Y. Evangelista, S. Fabiani, R. Ferrazzoli, J.A. Garcia, S. Gunji, K. Hayashida, J. Heyl, W. Iwakiri, V. Karas, T. Kitaguchi, J.J. Kolodziejczak, H. Krawczynski, F.L. Monaca, L. Latronico, S. Maldera, A. Manfreda, F. Marin, A. Marinucci, F. Massaro, G. Matt, I. Mitsuishi, C.-Y. Ng, S.L. O’Dell, C. Oppedisano, A. Papitto, G.G. Pavlov, A.L. Peirson, M. Pesce-Rollins, P.-O. Petrucci, M. Pilia, A. Possenti, B.D. Ramsey, J. Rankin, A. Ratheesh, R.W. Romani, C. Sgró, P. Slane, P. Soffitta, G. Spandre, T. Tamagawa, R. Taverna, Y. Tawara, A.F. Tennant, N.E. Thomas, F. Tombesi, A. Trois, S. Tsygankov, R. Turolla, J. Vink, M.C. Weisskopf, K. Wu, F. Xie, S. Zane. 2023. "X-Ray Polarization Observations of BL Lacertae" Letters of the Astrophysical Journal, Volume 942, Issue 1, pp.L10-L10. https://doi.org/10.3847/2041-8213/aca2812041-82052041-8213https://hdl.handle.net/2144/46302Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus that the synchrotron emission is responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae (BL Lac) performed with the Imaging X-ray Polarimetry Explorer, from which an upper limit to the polarization degree Π X < 12.6% was found in the 2–8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton-synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.L10-L10en-US© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.https://creativecommons.org/licenses/by/4.0/0201 Astronomical and Space SciencesAstronomy & Astrophysics5101 Astronomical sciences5109 Space sciencesArticle2023-02-0710.3847/2041-8213/aca2810000-0001-9815-9092 (Middei, Riccardo)0000-0001-9200-4006 (Liodakis, Ioannis)0000-0003-3613-4409 (Perri, Matteo)0000-0002-2734-7835 (Puccetti, Simonetta)0000-0001-7150-9638 (Cavazzuti, Elisabetta)0000-0003-1111-4292 (Ehlert, Steven R)0000-0001-7396-3332 (Marscher, Alan P)0000-0002-6492-1293 (Marshall, Herman L)0000-0003-3331-3794 (Muleri, Fabio)0000-0002-6548-5622 (Negro, Michela)0000-0001-6158-1708 (Jorstad, Svetlana G)0000-0001-7702-8931 (Agís-González, Beatriz)0000-0002-3777-6182 (Agudo, Iván)0000-0003-2464-9077 (Bonnoli, Giacomo)0000-0001-8286-5443 (Husillos, César)0000-0003-3779-6762 (Marchini, Alessandro)0000-0002-9328-2750 (Kouch, Pouya M)0000-0001-9518-337X (Kopatskaya, Evgenia N)0000-0002-2471-6500 (Larionova, Elena G)0000-0003-4147-3851 (Savchenko, Sergey S)0000-0002-8293-0214 (Vasilyev, Andrey A)0000-0003-1117-2863 (Casadio, Carolina)0000-0002-4131-655X (Escudero, Juan)0000-0003-3025-9497 (Myserlis, Ioannis)0000-0002-5158-0063 (Kameno, Seiji)0000-0002-5580-006X (Kneissl, Ruediger)0000-0003-0292-3645 (Nagai, Hiroshi)0000-0002-8597-6154 (Blinov, Dmitry)0000-0002-8597-6154 (Bourbah, Ioakeim G)0000-0001-6314-9177 (Kiehlmann, Sebastian)0000-0002-2567-2132 (Mandarakas, Nikos)0000-0002-2897-2448 (Romanopoulos, Stylianos)0000-0003-2337-0277 (Skalidis, Raphael)0000-0003-2638-720X (Masiero, Joseph R)0000-0002-8895-4735 (Mawet, Dimitri)0000-0001-6205-9233 (Millar-Blanchaer, Maxwell A)0000-0001-7482-5759 (Panopoulou, Georgia V)0000-0002-1481-4676 (Tinyanont, Samaporn)0000-0002-9353-5164 (Berdyugin, Andrei V)0000-0002-7502-3173 (Kravtsov, Vadim)0000-0002-0643-7946 (Imazawa, Ryo)0000-0002-0921-8837 (Fukazawa, Yasushi)0000-0001-6099-9539 (Kawabata, Koji S)0000-0002-7375-7405 (Uemura, Makoto)0000-0001-7263-0296 (Mizuno, Tsunefumi)0000-0001-6156-238X (Akitaya, Hiroshi)0000-0003-0685-3621 (Gurwell, Mark)0000-0002-7574-1298 (Lalla, Niccoló Di)0000-0003-3842-4493 (Cibrario, Nicoló)0000-0002-4700-4549 (Donnarumma, Immacolata)0000-0001-5717-3736 (Kim, Dawoon E)0000-0002-5448-7577 (Omodei, Nicola)0000-0001-6897-5996 (Pacciani, Luigi)0000-0002-0983-0049 (Poutanen, Juri)0000-0003-0256-0995 (Tavecchio, Fabrizio)0000-0002-5037-9034 (Antonelli, Lucio A)0000-0002-4576-9337 (Bachetti, Matteo)0000-0002-9785-7726 (Baldini, Luca)0000-0002-5106-0463 (Baumgartner, Wayne H)0000-0002-2469-7063 (Bellazzini, Ronaldo)0000-0002-4622-4240 (Bianchi, Stefano)0000-0002-0901-2097 (Bongiorno, Stephen D)0000-0002-4264-1215 (Bonino, Raffaella)0000-0002-9460-1821 (Brez, Alessandro)0000-0002-8848-1392 (Bucciantini, Niccoló)0000-0002-6384-3027 (Capitanio, Fiamma)0000-0003-1111-4292 (Castellano, Simone)0000-0002-0712-2479 (Ciprini, Stefano)0000-0003-4925-8523 (Costa, Enrico)0000-0001-5668-6863 (De Rosa, Alessandra)0000-0002-3013-6334 (Del Monte, Ettore)0000-0003-0331-3259 (Marco, Alessandro Di)0000-0001-8162-1105 (Doroshenko, Victor)0000-0003-0079-1239 (Dovčiak, Michal)0000-0003-1244-3100 (Enoto, Teruaki)0000-0001-6096-6710 (Evangelista, Yuri)0000-0003-1533-0283 (Fabiani, Sergio)0000-0003-1074-8605 (Ferrazzoli, Riccardo)0000-0003-3828-2448 (Garcia, Javier A)0000-0002-5881-2445 (Gunji, Shuichi)0000-0001-9739-367X (Heyl, Jeremy)0000-0002-0207-9010 (Iwakiri, Wataru)0000-0002-5760-0459 (Karas, Vladimir)0000-0002-0110-6136 (Kolodziejczak, Jeffery J)0000-0002-1084-6507 (Krawczynski, Henric)0000-0001-8916-4156 (Monaca, Fabio La)0000-0002-0984-1856 (Latronico, Luca)0000-0002-0698-4421 (Maldera, Simone)0000-0002-0998-4953 (Manfreda, Alberto)0000-0003-4952-0835 (Marin, Frédéric)0000-0002-2055-4946 (Marinucci, Andrea)0000-0002-1704-9850 (Massaro, Francesco)0000-0002-2152-0916 (Matt, Giorgio)0000-0002-5847-2612 (Ng, C-Y)0000-0002-1868-8056 (O’Dell, Stephen L)0000-0001-6194-4601 (Oppedisano, Chiara)0000-0001-6289-7413 (Papitto, Alessandro)0000-0002-7481-5259 (Pavlov, George G)0000-0001-6292-1911 (Peirson, Abel L)0000-0003-1790-8018 (Pesce-Rollins, Melissa)0000-0001-6061-3480 (Petrucci, Pierre-Olivier)0000-0001-7397-8091 (Pilia, Maura)0000-0001-5902-3731 (Possenti, Andrea)0000-0002-9774-0560 (Rankin, John)0000-0003-0411-4243 (Ratheesh, Ajay)0000-0001-6711-3286 (Romani, Roger W)0000-0001-5676-6214 (Sgró, Carmelo)0000-0002-6986-6756 (Slane, Patrick)0000-0001-8916-4156 (Soffitta, Paolo)0000-0003-0802-3453 (Spandre, Gloria)0000-0002-8801-6263 (Tamagawa, Toru)0000-0002-1768-618X (Taverna, Roberto)0000-0002-9443-6774 (Tennant, Allyn F)0000-0003-0411-4606 (Thomas, Nicholas E)0000-0002-6562-8654 (Tombesi, Francesco)0000-0002-3180-6002 (Trois, Alessio)0000-0002-9679-0793 (Tsygankov, Sergey)0000-0003-3977-8760 (Turolla, Roberto)0000-0002-4708-4219 (Vink, Jacco)0000-0002-5270-4240 (Weisskopf, Martin C)0000-0002-7568-8765 (Wu, Kinwah)0000-0002-0105-5826 (Xie, Fei)0000-0001-5326-880X (Zane, Silvia)777977