X-Ray Polarization of BL Lacertae in Outburst
Kaaret, Philip; Wu, Kinwah; Enoto, Teruaki; Turolla, Roberto; Lee, Sang-Sung; Rao, Ramprasad; Pavlov, George G.; Fukazawa, Yasushi; Weisskopf, Martin C.; Tennant, Allyn F.; Swartz, Douglas A.; Slane, Patrick; Papitto, Alessandro; Hayashida, Kiyoshi; Sasada, Mahito; Kawabata, Koji S.; Uemura, Makoto; Vink, Jacco; Bianchi, Stefano; Matt, Giorgio; Kitaguchi, Takao; Doroshenko, Victor; Poutanen, Juri; Tombesi, Francesco; Ng, C. -Y.; Mitsuishi, Ikuyuki; Mizuno, Tsunefumi; Tamagawa, Toru; Tawara, Yuzuru; Zane, Silvia; Dovčiak, Michal; Iwakiri, Wataru; Akitaya, Hiroshi; Nakaoka, Tatsuya; Angelakis, Emmanouil; Bachetti, Matteo; Tavecchio, Fabrizio; Thomas, Nicholas E.; Pesce-Rollins, Melissa; Marshall, Herman L.; Heyl, Jeremy; Ratheesh, Ajay; García-Comas, Maya; Imazawa, Ryo; Marinucci, Andrea; De Rosa, Alessandra; Middei, Riccardo; Marin, Frédéric; Petrucci, Pierre-Olivier; Baumgartner, Wayne H.; Negro, Michela; Perri, Matteo; Taverna, Roberto; Madejski, Grzegorz; Muleri, Fabio; Di Marco, Alessandro; Krawczynski, Henric; Possenti, Andrea; Baldini, Luca; Agudo, Iván; Antonelli, Lucio A.; Bellazzini, Ronaldo; Bongiorno, Stephen D.; Bonino, Raffaella; Brez, Alessandro; Capitanio, Fiamma; Castellano, Simone; Cavazzuti, Elisabetta; Ciprini, Stefano; Costa, Enrico; Del Monte, Ettore; Di Gesu, Laura; Donnarumma, Immacolata; Ehlert, Steven R.; Evangelista, Yuri; Fabiani, Sergio; Ferrazzoli, Riccardo; Garcia, Javier A.; Gunji, Shuichi; Jorstad, Svetlana G.; Karas, Vladimir; Kolodziejczak, Jeffery J.; La Monaca, Fabio; Latronico, Luca; Liodakis, Ioannis; Maldera, Simone; Manfreda, Alberto; Marscher, Alan P.; Massaro, Francesco; O'Dell, Stephen L.; Omodei, Nicola; Oppedisano, Chiara; Peirson, Abel L.; Pilia, Maura; Puccetti, Simonetta; Ramsey, Brian D.; Rankin, John; Roberts, Oliver J.; Romani, Roger W.; Sgró, Carmelo; Soffitta, Paolo; Spandre, Gloria; Trois, Alessio; Xie, Fei; Chen, Chien-Ting; Sievers, Albrecht; Sota, Alfredo; Marchini, Alessandro; Pacciani, Luigi; Kim, Dawoon E.; Aceituno, Francisco José; Bonnoli, Giacomo; Casanova, Víctor; Agís-González, Beatriz; Casadio, Carolina; Escudero, Juan; Myserlis, Ioannis; Kouch, Pouya M.; Lindfors, Elina; Gurwell, Mark; Kang, Sincheol; Kim, Sang-Hyun; Jeong, Hyeon-Woo; Kraus, Alexander; Husillos, César; Bucciantini, Niccoló; Di Lalla, Niccoló; Tsygankov, Sergey; Bernardos, Maria I.; Cibrario, Nicoló; Cheong, Yeon
United States, Finland, Italy, United Kingdom, Russia, Spain, Greece, Germany, Japan, Republic of Korea, Czech Republic, Canada, France, Hong Kong SAR, Netherlands, China
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 ${{\rm{\Pi }}}_{2\mbox{--}4\mathrm{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.