X-Ray Polarization of BL Lacertae in Outburst
Authors: Peirson Abel L. , Negro Michela , Liodakis Ioannis , Middei Riccardo , Kim Dawoon E. , Marscher Alan P. , Marshall Herman L. , Pacciani Luigi , Romani Roger W. , Wu Kinwah , Di Marco Alessandro , Di Lalla Niccoló , Omodei Nicola , Jorstad Svetlana G. , Agudo Iván , Kouch Pouya M. , Lindfors Elina , Aceituno Francisco José , Bernardos Maria I. , Bonnoli Giacomo , Casanova Víctor , García-Comas Maya , Agís-González Beatriz , Husillos César , Marchini Alessandro , Sota Alfredo , Casadio Carolina , Escudero Juan , Myserlis Ioannis , Sievers Albrecht , Gurwell Mark , Rao Ramprasad , Imazawa Ryo , Sasada Mahito , Fukazawa Yasushi , Kawabata Koji S. , Uemura Makoto , Mizuno Tsunefumi , Nakaoka Tatsuya , Akitaya Hiroshi , Cheong Yeon , Jeong Hyeon-Woo , Kang Sincheol , Kim Sang-Hyun , Lee Sang-Sung , Angelakis Emmanouil , Kraus Alexander , Cibrario Nicoló , Donnarumma Immacolata , Poutanen Juri , Tavecchio Fabrizio , Antonelli Lucio A. , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccoló , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Doroshenko Victor , Dovčiak 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 , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , La Monaca Fabio , Latronico Luca , Madejski Grzegorz , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , Muleri Fabio , Ng C.-Y. , O’Dell Stephen L. , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Puccetti Simonetta , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Roberts Oliver J. , Sgró Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Xie Fei , Zane Silvia 
Published: May 2023

Abstract
We report the first >99% confidence detection of X-ray polarization in BL Lacertae. During a recent X-ray/γ-ray outburst, a 287 ks observation (2022 November 27–30) was taken using the Imaging X-ray Polarimetry Explorer (IXPE), together with contemporaneous multiwavelength observations from the Neil Gehrels Swift observatory and XMM-Newton in soft X-rays (0.3–10 keV), 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 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 >99% confidence X-ray polarization

Π

2
–
4
keV

=

21.7

−
7.9

+
5.6

%

and electric vector polarization angle ψ
2–4keV = −28.°7 ± 8.°7 in the time bin with highest estimated synchrotron flux contribution. We discuss possible implications of our observations, including previous IXPE BL Lacertae pointings, tentatively concluding that synchrotron self-Compton emission dominates over hadronic emission processes during the observed epochs.