The Link Between UV and X-ray Emission in Quasars

Quasars are well known to exhibit a non-linear relationship between the UV emission from their accretion disks and the X-ray emission from the surrounding corona. This relationship has recently been used to include quasars as standard candles in a Hubble diagram in order to estimate the cosmological parameters ΩM and ΩΛ (Risaliti and Lusso 2015). This project aims to improve this method by adding a large sample of quasars detected by ROSAT. These sources have been matched to UV and IR counterparts using GALEX and WISE data, and have optical spectra available from the Sloan Digital Sky Survey, which resulted in a catalogue of 5500 sources in the redshift range 0 ≤ z ≤ 2. This project will investigate whether the L(X-ray) - L(UV) relationship is universal to quasars of all types. Optical spectra and spectral energy distribution fitting will be used to test the L(X-ray) - L(UV) relationship for signs of evolution with quasar properties such as redshift, black hole mass, and Eddington ratio. A large number of sources from this sample will be added to the Hubble diagram in the redshift range currently populated by type 1a supernovae (z ≤ 1.4). This will allow the cosmological parameter results obtained from quasars to be compared directly with those obtained from supernovae