X-ray Polarization of BL Lacertae in Outburst
Creators: name: Peirson, Abel L.
name: Negro, Michela
name: Liodakis, Ioannis
name: Middei, Riccardo
name: Kim, Dawoon E.
name: Marscher, Alan P.
name: Marshall, Herman L.
name: Pacciani, Luigi
name: Romani, Roger W.
name: Wu, Kinwah
name: Di Marco, Alessandro
name: Di Lalla, Niccolo
name: Omodei, Nicola
name: Jorstad, Svetlana G.
name: Agudo, Ivan
name: Kouch, Pouya M.
name: Lindfors, Elina
name: Aceituno, Francisco Jose
name: Bernardos, Maria I.
name: Bonnoli, Giacomo
name: Casanova, Victor
name: Garcia-Comas, Maya
name: Agis-Gonzalez, Beatriz
name: Husillos, Cesar
name: Marchini, Alessandro
name: Sota, Alfredo
name: Casadio, Carolina
name: Escudero, Juan
name: Myserlis, Ioannis
name: Sievers, Albrecht
name: Gurwell, Mark
name: Rao, Ramprasad
name: Imazawa, Ryo
name: Sasada, Mahito
name: Fukazawa, Yasushi
name: Kawabata, Koji S.
name: Uemura, Makoto
name: Mizuno, Tsunefumi
name: Nakaoka, Tatsuya
name: Akitaya, Hiroshi
name: Cheong, Whee Yeon
name: Jeong, Hyeon-Woo
name: Kang, Sincheol
name: Kim, Sang-Hyun
name: Lee, Sang-Sung
name: Angelakis, Emmanouil
name: Kraus, Alexander
name: Cibrario, Nicolo
name: Donnarumma, Immacolata
name: Poutanen, Juri
name: Tavecchio, Fabrizio
name: Antonelli, Lucio A.
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, Niccolo
name: Capitanio, Fiamma
name: Castellano, Simone
name: Cavazzuti, Elisabetta
name: Chen, Chien-Ting
name: Ciprini, Stefano
name: Costa, Enrico
name: De Rosa, Alessandra
name: Del Monte, Ettore
name: Di Gesu, Laura
name: Doroshenko, Victor
name: Dovciak, 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: Kitaguchi, Takao
name: Kolodziejczak, Jeffery J.
name: Krawczynski, Henric
name: La Monaca, Fabio
name: Latronico, Luca
name: Madejski, Grzegorz
name: Maldera, Simone
name: Manfreda, Alberto
name: Marin, Frederic
name: Marinucci, Andrea
name: Massaro, Francesco
name: Matt, Giorgio
name: Mitsuishi, Ikuyuki
name: Muleri, Fabio
name: Ng, C. -Y.
name: O'Dell, Stephen L.
name: Oppedisano, Chiara
name: Papitto, Alessandro
name: Pavlov, George G.
name: Perri, Matteo
name: Pesce-Rollins, Melissa
name: Petrucci, Pierre-Olivier
name: Pilia, Maura
name: Possenti, Andrea
name: Puccetti, Simonetta
name: Ramsey, Brian D.
name: Rankin, John
name: Ratheesh, Ajay
name: Roberts, Oliver J.
name: Sgro, Carmelo
name: Slane, Patrick
name: Soffitta, Paolo
name: Spandre, Gloria
name: Swartz, Douglas A.
name: Tamagawa, Toru
name: Taverna, Roberto
name: Tawara, Yuzuru
name: Tennant, Allyn F.
name: Thomas, Nicholas E.
name: Tombesi, Francesco
name: Trois, Alessio
name: Tsygankov, Sergey
name: Turolla, Roberto
name: Vink, Jacco
name: Weisskopf, Martin C.
name: Xie, Fei
name: Zane, Silvia
Publisher: name: arXiv
Year: 2023
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report the first $&gt; 99\%$ confidence detection of X-ray polarization in BL Lacertae. During a recent X-ray/$γ$-ray outburst, a 287 ksec observation (2022 November 27-30) was taken using the Imaging X-ray Polarimetry Explorer ({\it IXPE}), together with contemporaneous multiwavelength observations from the Neil Gehrels {\it Swift} observatory and {\it XMM-Newton} in soft X-rays (0.3--10~keV), {\it NuSTAR} in hard X-rays (3--70~keV), and optical polarization from the Calar Alto, and Perkins Telescope observatories. Our contemporaneous X-ray data suggest that the {\it IXPE} energy band is at the crossover between the low- and high-frequency blazar emission humps. The source displays significant variability during the observation, and we measure polarization in three separate time bins. Contemporaneous X-ray spectra allow us to determine the relative contribution from each emission hump. We find $&gt;99\%$ confidence X-ray polarization $Π_{2-4{\rm keV}} = 21.7^{+5.6}_{-7.9}\%$ and electric vector polarization angle $ψ_{2-4{\rm keV}} = -28.7 \pm 8.7^{\circ}$ in the time bin with highest estimated synchrotron flux contribution. We discuss possible implications of our observations, including previous {\it IXPE} BL Lacertae pointings, tentatively concluding that synchrotron self-Compton emission dominates over hadronic emission processes during the observed epochs.
description: Accepted to ApJL. 16 pages, 8 figures