Extraction Method for Response Functions from X-Ray Light Curves of Active Galactic Nuclei by an Optimization Algorithm

Abstract We introduce a numerical optimization method to extract the X-ray reverberation response functions from multiband light curves of active galactic nuclei. This approach does not require prior assumptions about the geometry of the accretion disk and corona, provided that the light curves result from the superposition of direct and singly convolved signals, consistently across all bands. By reformulating the light-curve equations into matrix form, the optimal response matrix is derived by minimizing the squared difference between the observed and reconstructed light curves using a gradient-based optimization algorithm. We demonstrate that the method can robustly accommodate up to two convolution processes, such as reverberation and propagation, simultaneously. When tested on the synthesized light curves, the method demonstrates robustness of the solutions to variations in the relative contributions of each light-curve component as the recovered response kernel remains acceptably close to the ground truth, as evaluated by both the response geometry and the reconstructed light curves. The method’s tolerance to random noise was also assessed. With appropriate denoising, the response kernel can be reliably recovered when the signal-to-noise ratio is at least 100. We show, as a proof of concept, that the proposed method is independent of the geometrical model and has the potential to offer a flexible complement to traditional approaches.