X-Ray Polarization of BL Lacertae in Outburst

Peirson, A. L.; Negro, M.; Liodakis, I.; Middei, R.; Kim, D. E.; Marscher, A. P.; Marshall, H. L.; Pacciani, L.; Romani, R. W.; Wu, K.; Di Marco, A.; Di Lalla, N.; Omodei, N.; Jorstad, S. G.; Agudo, I.; Kouch, P. M.; Lindfors, E.; Aceituno, F. J.; Bernardos, M. I.; Bonnoli, G.; Casanova, V.; Garcia-Comas, M.; Agis-Gonzalez, B.; Husillos, C.; Marchini, A.; Sota, A.; Casadio, C.; Escudero, J.; Myserlis, I.; Sievers, A.; Gurwell, M.; Rao, R.; Imazawa, R.; Sasada, M.; Fukazawa, Y.; Kawabata, K. S.; Uemura, M.; Mizuno, T.; Nakaoka, T.; Akitaya, H.; Cheong, Y.; Jeong, H. -W.; Kang, S.; Kim, S. -H.; Lee, S. -S.; Angelakis, E.; Kraus, A.; Cibrario, N.; Donnarumma, I.; Poutanen, J.; Tavecchio, F.; Antonelli, L. A.; Bachetti, M.; Baldini, L.; Baumgartner, W. H.; Bellazzini, R.; Bianchi, S.; Bongiorno, S. D.; Bonino, R.; Brez, A.; Bucciantini, N.; Capitanio, F.; Castellano, S.; Cavazzuti, E.; Chen, C. -T.; Ciprini, S.; Costa, E.; De Rosa, A.; Del Monte, E.; Di Gesu, L.; Doroshenko, V.; Dovciak, M.; Ehlert, S. R.; Enoto, T.; Evangelista, Y.; Fabiani, S.; Ferrazzoli, R.; Garcia, J. A.; Gunji, S.; Hayashida, K.; Heyl, J.; Iwakiri, W.; Kaaret, P.; Karas, V.; Kitaguchi, T.; Kolodziejczak, J. J.; Krawczynski, H.; La Monaca, F.; Latronico, L.; Madejski, G.; Maldera, S.; Manfreda, A.; Marin, F.; Marinucci, A.; Massaro, F.; Matt, G.; Mitsuishi, I.; Muleri, F.; C. -Y., Ng; O'Dell, S. L.; Oppedisano, C.; Papitto, A.; Pavlov, G. G.; Perri, M.; Pesce-Rollins, M.; Petrucci, P. -O.; Pilia, M.; Possenti, A.; Puccetti, S.; Ramsey, B. D.; Rankin, J.; Ratheesh, A.; Roberts, O. J.; Sgro, C.; Slane, P.; Soffitta, P.; Spandre, G.; Swartz, D. A.; Tamagawa, T.; Taverna, R.; Tawara, Y.; Tennant, A. F.; Thomas, N. E.; Tombesi, F.; Trois, A.; Tsygankov, S.; Turolla, R.; Vink, J.; Weisskopf, M. C.; Xie, F.; Zane, S.

doi:10.3847/2041-8213/acd242

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.