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/${\hat{I}}^3$-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 ${\hat{I}}_{2-4\mathrm{keV}}=21.7_{-7.9}^{+5.6}\%$ and electric vector polarization angle $\ddot{I}{ˆ}_{2-4\mathrm{keV}}=-28.7\pm 8.7^{\circ }$ 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