X-Ray Polarization of BL Lacertae in Outburst

Abstract

<p>We report the first >99% confidence detection of X-ray polarization in BL Lacertae. During a recent X-ray/<em>γ</em>-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 <img alt="${{\rm{\Pi }}}_{2\mbox{--}4\mathrm{keV}}={21.7}_{-7.9}^{+5.6} \% $" src="https://cfn-live-content-bucket-iop-org.s3.amazonaws.com/journals/2041-8205/948/2/L25/revision2/apjlacd242ieqn1.gif?AWSAccessKeyId=AKIAYDKQL6LTV7YY2HIK&Expires=1692843820&Signature=eFDzWFawEVUm9sHTSS%2F%2FxRrCCMM%3D"> and electric vector polarization angle <em>ψ</em>_2–4keV = −28<img src="https://cdn.images.iop.org/Entities/fdg.gif" alt="fdg">7 ± 8<img src="https://cdn.images.iop.org/Entities/fdg.gif" alt="fdg">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.<br></p>