October 9 -- Sam Gralla (University of Arizona)

Black Holes: From the Image to the Photon Ring

"The EHT collaboration has released millmeter-wave interferometric observations of the core of the galaxy M87 (henceforth M87). The data are consistent with expectations for an accreting supermassive black hole, but lack the precision and coverage required to distinguish between models of accretion flow or test theories of gravity. However, the observations establish the source as a very promising target for future precision measurements. After reviewing the EHT observations and data analysis, I will discuss a proposal for a space mission targeing M87*. The idea is to observe the predicted "photon ring" due to photons that orbit the black hole before arriving at the detector. We find that the GR-predicted shape of the photon ring is remarkably independent of astrophysical assumptions, and forcast that the kerr-black-hole nature of M87* can be tested with an estimated 0.05% precision."