Polarimetric signature of electromagnetic energy extraction from a black hole

In 1977, Blandford and Znajek showed that the electromagnetic field surrounding a rotating black hole can harvest its spin energy and use it to power a collimated astrophysical jet, such as the one launched from the center of the elliptical galaxy M87. Today, interferometric observations with the Event Horizon Telescope are delivering high-resolution, event-horizon-scale, polarimetric images of the supermassive black hole M87* at the jet launching point. These polarimetric images offer an unprecedented window into the electromagnetic field structure around a black hole. I will present a simple polarimetric observable––the sense of rotation of the linear polarization in a near-horizon image––that depends on the sign of the electromagnetic energy flux and therefore provides a direct probe of black hole energy extraction. Currently envisioned multi-frequency observations of M87* will achieve the dynamic range and angular resolution needed to detect this signature, enabling a definitive test for energy extraction from the black hole in M87.