Energy dependence of the X-ray power spectrum in NGC4051 and NGC4395

Abstract

Active galactic nuclei (AGNs) exhibit strong variability across the electromagnetic spectrum on a wide range of timescales, particularly in X-rays where fluctuations are both rapid and high amplitude. Power spectral density (PSD) analysis is commonly used to characterise this variability. Although AGN PSDs are typically well described by a bending power-law model, the dependence of the model parameters on photon energy has not been systematically investigated. We examine whether PSD parameters depend on energy using two highly variable, low-mass Seyfert galaxies, NGC4051 and NGC4395, as case studies. Using archival observations from XMM-Newton, Suzaku, and NuSTAR, we computed power spectra in six energy bands spanning 0.3-20 keV and fitted them with a bending power-law model to study the energy dependence of the PSD parameters. Power spectra derived from light curves obtained with different satellites and at different epochs are consistent within uncertainties, indicating that the X-ray variability process in both sources is stationary. For both AGNs we find that (i) the PSD bending frequency is consistent with being independent of energy, (ii) the high-frequency slope becomes flatter at higher energies, and (iii) the PSD amplitude decreases with increasing energy. These results place important constraints on models of AGN X-ray variability, such as the fluctuating accretion-rate model. Similar analyses of larger AGN samples are required to fully characterise the energy dependence of AGN power spectra.