Polarization over wide bandwidths - impact of ALMA WSU on studies of extreme Faraday rotation

Relativistic jets launched by supermassive black holes in the centers of active galaxies are some of the most energetic phenomena in the Universe. These jets shine brightly over the entire electromagnetic spectrum and are capable of accelerating particles to extreme energies. Although the exact jet launching mechanism is not yet understood, it is believed that magnetic fields anchored to the black hole or the accretion disk extract energy from the rotating system, launching relativistic bipolar outflows. One way to study the magnetic field structure in these jets is via modeling their linear polarization as a function of wavelength, which provides, for example, the Faraday rotation measure that is related to the line-of-sight magnetic field strength in the rotating medium. To probe the regions relevant for jet launching, one needs to observe at high millimeter wavelengths where the jets are fully opaque unlike in the centimeter wavelengths where synchrotron self-absorption is prominent. In 2016 we observed the archetypal quasar 3C273 in full polarization with ALMA at band 6 and detected a very high Faraday rotation measure of 5 $\times$ 10$^5$ rad/m$^2$ over the available instantaneous bandwidth. In our follow-up observations in 2018 the rotation measure is much lower and it cannot be detected over band 6 alone. In my talk, I will discuss the improvements expected in the modeling of the polarization of relativistic jets by the increase in the instantaneous bandwidth offered by the WSU.