Explaining the UV to X-ray correlation in AGN within the framework of X-ray illumination of accretion discs

Abstract

It is established that the ultraviolet (UV) and X-ray emissions in active galactic nuclei (AGN) are tightly correlated. This correlation is observed both in low- and high-redshift sources. In particular, observations of large samples of quasars revealed the presence of a non-linear correlation between UV and X-rays. The physical origin of this correlation is poorly understood. In this work, we explore this observed correlation in the framework of the X-ray illumination of the accretion disc by a central source. We have shown in previous works that this model successfully explains the continuum UV/optical time delays, variability, and the broadband spectral energy distribution in AGN. We use this model to produce $150,000$ model SEDs assuming a uniform distribution of model parameters. We compute the corresponding UV ($2500\tilde{A}$) and X-ray (2 keV) monochromatic luminosities and select only the model data points that agree with the observed UV-to-X-ray correlation. Our results show that the X-ray illumination of accretion disc model can reproduce the observed correlation for a subset of model configurations with a non-uniform distribution of black hole mass ($M_{\mathrm{BH}}$), accretion rate ($\dot{m}/{\dot{m}}_{\mathrm{Edd}}$), and power transferred from the accretion disc to the corona ($L_{\mathrm{transf}}/L_{\mathrm{disc}}$). In addition, our results reveal the presence of a correlation between $M_{\mathrm{BH}}$ and $\dot{m}/{\dot{m}}_{\mathrm{Edd}}$, and between $\dot{m}/{\dot{m}}_{\mathrm{Edd}}$ and $L_{\mathrm{transf}}/L_{\mathrm{disc}}$, to explain the observed X-ray-UV correlation. We also present evidence based on observed luminosities supporting our findings. We finally discuss the implications of our results.

Accepted for publication in A&A