Autocorrelation in black hole flare movies: Departures from Kerr
The study uses autocorrelation in black hole movie images to test spacetime geometry, specifically looking for deviations from Kerr geometry using the Johannsen-Psaltis spacetime with a deformation parameter. By analyzing secondary ridge observables—slope and intercept—the researchers found that spin and deformation affect these observables differently, allowing them to jointly constrain both parameters. This method provides a consistency test against Kerr models.