Description
Geometric constraints on the inner regions of active galactic nuclei (AGN) are crucial for uncovering the behaviour of matter in extreme gravity and for testing general relativity. A time lag between lightcurves in different energy bands is imprinted by the different path lengths between direct coronal photons and those reflected from the surrounding accretion disc. The lag-energy spectrum therefore encodes information regarding the otherwise unresolvable geometry of the system. Use of the lag spectrum is impacted by instrumental noise and stochastic variability intrinsic to the accretion process, which can decrease the coherence between bands and result in larger uncertainties on the lag and therefore inferred geometry. In our work, we are investigating lag spectra isolated at the frequencies of Quasi-periodic oscillations (QPOs). We show that the extremely high coherence of QPOs results in better constraints on the geometric parameters of the inner regions of AGN as compared to using the full broad band noise.