Measuring Black Hole Spin from Polarized Images of Jets

The Event Horizon Telescope (EHT) will soon be able to observe the large-scale jet of M87 at optically thin frequencies with unprecedented resolution and dynamic range. In this talk, I will discuss how polarized observations of the jet can be used to constrain the spin of the central black hole. In particular, I will show that the polarization undergoes large spatial swings at several characteristic locations along the jet. One such location – the light cylinder – is strongly tied to the black hole’s angular momentum and can therefore serve as an observational signature of spin. I will show how this signature can be measured in a way that bypasses many astrophysical uncertainties, thus providing an exciting new goal for future EHT observations.