Events at Physics
Active Galactic Nuclei (AGN) are optimal laboratories for tests of general relativity (GR) in the "strong field" regime. In particular their X-ray emission comes from a few gravitational radii from the horizon event of the supermassive black hole, and is strongly affected by GR effects. However, testing GR has been difficult so far because of a strong degeneracy in the X-ray spectra of AGN between the (relativistically distorted) intrinsic emission and the complex absorption/reflection components due to the circumnuclear medium. I will review the main diagnostic methods developed in the past few years to investigate this issue, focusing on time-resolved spectral analysis of variable sources, and I will discuss the impact in this field of the recently launched hard X-ray telescope NuSTAR. I will show how the availability of high quality broad-band X-ray spectra, and a time-resolved spectral analysis can remove the main systematic uncertainties in GR tests, provide precise measurements of black hole spins, and reveal the structure of the circumnuclear X-ray absorber.