Detection of Compton Scattering in the Jet of 3C 84

3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low-frequency radio up to TeV γ-rays. Here we report on the first X-ray polarization observation of 3C 84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Ms coinciding with a flare in γ-rays. This is the longest observation for a radio-loud active galaxy, which allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of Π_X = 4.2% ± 1.3% (∼3.2σ confidence) at an X-ray electric vector polarization angle of ψ_X = 163° ± 9°, which is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons—specifically synchrotron self-Compton—that takes place downstream, away from the supermassive black hole.