Association of Optical, Ultraviolet and Soft X-ray excess Emissions in AGNs

Abstract

Soft X-ray emission (0.5--2.0 keV) plays a pivotal role in regulating the optical and UV emission in the AGNs. We collected a sample of 1811 AGNs from the SDSS database and obtained various parameters of Balmer lines, optical continuum, MgII line \& UV continuum and studied their dependencies on soft X-ray luminosity. Based on the linear regression analysis, we found that FWHM$_{\text{MgII}}$ $\propto$ FWHM$_{\text{H}\beta}^{0.554}$ suggesting that UV emission is arising from a region relatively outside the broad line region (BLR) associated to the H$\beta$ emission and found a strong correlation between optical and UV luminosities (L$_{\text{MgII}}$ $\propto$ L$_{\text{H}\beta}^{0.822}$). It was noticed that the dependency of optical continuum luminosities on soft excess changes with the redshift (L$_{\text{X}}$ $\propto$ L$^{0.596}_{5100\text{\AA}}$ for z < 0.5 and L$_{\text{X}}$ $\propto$ L$^{0.429}_{5100\text{\AA}}$ for z > 0.5). The FWHM components of H${\beta}$ and MgII core components were found to be virialized and is not affected by the soft excess emission whereas the wings of MgII display a dependency. We estimated a relation viz. L$_{\text{X}}$ $\propto$L$^{0.520}_{3000\text{\AA}}$ FWHM$^{0.525}_{\text{MgII}}$ and found to be well in agreement with a proposed physical scenario. All the derived relations were used to understand the inter-modulating association of the BLR and disc in the AGNs.