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We present an investigation of the interdependence of the optical-to-X-ray spectral slope ($\alpha_{\rm ox}$), the HeII equivalent-width (EW), and the monochromatic luminosity at 2500 Angstroms ($L_{2500}$). The values of $\alpha_{\rm ox}$…
The variability of quasars across multiple wavelengths is a useful probe of physical conditions in active galactic nuclei. In particular, variable accretion rates, instabilities, and reverberation effects in the accretion disk of a…
Active galaxies form a clear pattern in the optical plane showing the correlation between the Full Width Half Maximum (FWHM) of the H$\beta$ line and the ratio of the Equivalent Width (EW) of the optical FeII emission and the EW(H$\beta$).…
Observations have shown that UV/optical variation amplitude of quasars depend on several physi- cal parameters including luminosity, Eddington ratio, and likely also black hole mass. Identifying new factors which correlate with the…
The radio emission in radio-quiet quasars (RQQs) has been a long mystery and its physical origin remains unclear. In a previous work we find UV/optical more variable quasars have stronger X-ray emission, indicating a link between disc…
By comparing the ratio of flux densities in the X-ray and UV wavebands by way of the spectral optical-X-Ray index, $\alpha_{ox}$, we explore the relation between the emissions in the respective wavebands for a number of ULXs with known…
The observed tight radio/X-ray correlation in the low spectral state of some black hole X-ray binaries implies the strong coupling of the accretion and jet. The correlation of $L_{\rm R} \propto L_{\rm X}^{\sim 0.5-0.7}$ was well explained…
We discuss a probe of the contribution of wind-related shocks to the radio emission in otherwise radio-quiet quasars. Given 1) the non-linear correlation between UV and X-ray luminosity in quasars, 2) that such correlation leads to higher…
The low frequency radio luminosity is believed to be an indicator of jet power, while the optical/UV emission is probably from accretion discs in the nuclei of steep-spectrum radio quasars. We present a correlation between the ratio of…
We outline a science case for next-generation wide-field spectroscopic surveys aimed at connecting the physics of the accretion disc and X-ray corona in active galactic nuclei (AGN) with the properties of their nebular regions and…
The non-linear relation between the X-ray and ultraviolet (UV) luminosity in quasars has been used to derive quasar distances and to build a Hubble diagram at redshifts up to $z\sim$ 7. This cosmological application is based on the…
The relation between X-ray and UV/optical variability in AGNs has been explored in many individual sources, however a large sample study is yet absent. Through matching the XMM-Newton serendipitous X-ray and UV source catalogs with SDSS…
We investigate the correlation between the accretion disk (UV) luminosity and the radio core emission of a quasar sample. In a radio/$L_{\rm disk}$ plot we find the quasars to be separated into four classes: core dominated quasars (CDQ),…
The relation between accretion disc (thermal emission) and jet (non-thermal emission) in blazars is still a mystery as, typically, the beamed jet emission swamps the disc even in the ultraviolet band where disc emission peaks. In this paper…
We present a new catalogue of ~2,400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM-Newton. The sample can be used to investigate the non-linear relation between the UV and X-ray…
A new technique is presented for determining the black-hole masses of high-redshift quasars from optical spectroscopy. The new method utilizes the full-width half maximum (FWHM) of the low-ionization MgII emission line and the correlation…
We construct a three-dimensional and redshift-evolutionary X-ray and ultraviolet ($L_X-L_{UV}$) luminosity relation for quasars from the powerful statistic tool called copula, and find that the constructed $L_X-L_{UV}$ relation from copula…
This work studies the relationship between accretion-disk size and quasar properties, using a sample of 95 quasars from the SDSS-RM project with measured lags between the $g$ and $i$ photometric bands. Our sample includes disk lags that are…
Quasars serve as important cosmological probes and constructing accurate luminosity relations for them is essential for their use in cosmology. If the coefficients of quasar's luminosity relation vary with redshift, it could introduce…
The optical/ultraviolet (UV) variability of quasars has been discovered to be correlated with other quasar properties, such as luminosity, black hole mass and rest-frame wavelength. However,the origin of variability has been a puzzle so…