Detection of X-ray Polarization in the Supermassive Black Hole Jet of Mrk 501

X-ray polarization is a crucial probe of the magnetic field structure and emission processes in astrophysical systems. This is particularly true for blazars, active galactic nuclei with powerful jets aligned towards our line of sight. In blazars, X-ray polarimetry allows us to investigate the freshly accelerated particles at the heart of the accelerator. Until now, polarization observations have been limited to the radio-to-optical range, thereby leaving a gap in our knowledge of the physical conditions experienced by the most energetic particles. The recently launched Imaging X-ray Polarimetry Explorer — IXPE, the first X-ray polarization mission to target blazars, thus offers a radically new way of studying particle acceleration in relativistic jets. I will discuss the multiwavelength polarization observations and first results from the first IXPE blazar campaign targeting Markarian 501. Our results demonstrate the importance of X-ray polarization, and IXPE, in understanding high-energy processes in the Universe.