Kavli Affiliate: Roger W. Romani
| First 5 Authors: Abel L. Peirson, Michela Negro, Ioannis Liodakis, Riccardo Middei, Dawoon E. Kim
| Summary:
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.
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