X-ray Polarization of BL Lacertae in Outburst

Abstract

We report the first $>99\%$ confidence detection of X-ray polarization in BL Lacertae. During a recent X-ray/$\gamma$-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 $>99\%$ confidence X-ray polarization ${\Pi }_{2-4\mathrm{keV}}=21.7_{-7.9}^{+5.6}\%$ and electric vector polarization angle ${\psi }_{2-4\mathrm{keV}}=-28.7\pm 8.7^{\circ }$ 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.

Accepted to ApJL. 16 pages, 8 figures

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• IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare
• Multifrequency polarimetry of High-Synchrotron Peaked blazars probes the shape of their jets
• Determining the origin of the X-ray emission in blazars through multiwavelength polarization
• Axion-like Particle Effects on Photon Polarization in High-Energy Astrophysics
• X-ray Polarization of the High-Synchrotron-Peak BL Lacertae Object 1ES 1959+650 during Intermediate and High X-ray Flux States
• Revisiting radio variability of the blazar 3C 454.3
• Optical Spectropolarimetric Variability Properties in Blazars PKS 0637-75 and PKS 1510-089
• Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations
• Spectral Energy Distribution Modeling of BL Lacertae During a Large Submillimeter Outburst and Low X-Ray Polarization State
• Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI
• Detection of Compton scattering in the jet of 3C 84