Using quasars as standard candles : measuring distances up to redshifts4

Abstract

In this thesis, we have restudied a sample chosen by (Lusso & Risaliti, 2016) consisting of a set of 2,153 unique quasars given by two source catalogues: 3XMM-DR5 (Rosen et al., 2016) and DR7 catalogue (Shen et al., 2011). Quasars serve science in two different ways: 1) Understanding accretion physics by studying the physical mechanism occuring between the disk and cloud corona, and 2) they are good cosmological probes in distance estimation reaching redshifts ~ 7 (Mortlock et al., 2011). A physical relation has been observed between the optical-UV disk and the X-ray corona through a log-log relation between their respective fluxes (or luminosities). From previous studies, Lusso et al. showed a relation between X-ray and UV luminosities with a dispersion varying between 0.35 to 0.4 dex. That sample was further reduced by eliminating biases including quasars with host galaxy contamination, reddening, X-ray obscured objects and radio loudness in (Lusso & Risaliti, 2016) to reach a dispersion of 0.21-0.24 dex. The least dispersion achieved helps us to better understand the nature of this dispersion and better understand what kind of mechanism is happening between the disk and the corona. In this thesis, the study is performed using a different statistical method, (BCES), presented by Akritas and Bershady to achieve a tight relation with a dispersion of ~0.23 dex. For now, very little is known about the physics behind the relation, αOX － log L2500˚A. It is a by-product of the luminosity log-log relation. It is further used to calculate luminosity distances (DL) and build the Hubble Diagram (HD) of quasars’ distances versus redshift z. The HD allowed us to constrain the Dark Energy equation of state (EOS) and modify its parameters (ΩM and ΩΛ). In this study, we assume a flat ΛCDM model and we obtained ΩM = 0.28 ± 0.04 and ΩΛ = 0.72