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 $\Pi_{ 2-4{ \rm keV}} = 21.7^{ +5.6}_{ -7.9}\%$ and electric vector polarization angle $\psi_{ 2-4{ \rm keV}} = -28.7 \pm 8.7^{ \circ}$ 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.
Categories: astro-ph.HE astro-ph.GA