Draft version February 25, 2021 Typeset using L A T E X twocolumn style in AASTeX62 Time Delay of MgII Emission Response for the Luminous Quasar HE 0435-4312: Towards Application of High-Accretor Radius-Luminosity Relation in Cosmology Michal Zajaˇ cek, 1 Bo˙ zena Czerny, 1 Mary Loli Martinez–Aldama, 1 MateuszRalowski, 2 Aleksandra Olejak, 3 Robert Przy luski, 1 Swayamtrupta Panda, 1, 3 Krzysztof Hryniewicz, 4 Marzena ´ Sniegowska, 1, 3 Mohammad-Hassan Naddaf, 1 Raj Prince, 1 Wojtek Pych, 3 Grzegorz Pietrzy´ nski, 3 Catalina Sobrino Figaredo, 5 Martin Haas, 5 Justyna ´ Sredzi´ nska, 3 Magdalena Krupa, 2 Agnieszka Kurcz, 2 Andrzej Udalski, 6 Vladim´ır Karas, 7 Marek Sarna, 3 Hannah L. Worters, 8 Ramotholo R. Sefako, 8 and Anja Genade 8 1 Center for Theoretical Physics, Polish Academy of Sciences, Al. Lotnik´ ow 32/46, 02-668 Warsaw, Poland 2 Astronomical Observatory of the Jagiellonian University, Orla 171, 30-244 Cracow, Poland 3 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland 4 National Centre for Nuclear Research, Pasteura 7, 02-093 Warsaw, Poland 5 Astronomisches Institut - Ruhr Universitaet Bochum, Germany 6 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland 7 Astronomical Institute of the Czech Academy of Sciences, Boˇcn´ı II 1401, CZ-14100 Prague, Czech Republic 8 South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa (Received January 1, 2018; Revised January 7, 2018; Accepted February 25, 2021) Submitted to ApJ ABSTRACT Using the six years of the spectroscopic monitoring of the luminous quasar HE 0435-4312 (z = 1.2231) with the Southern African Large Telescope (SALT), in combination with the photometric data (CATALINA, OGLE, SALTICAM, and BMT), we determined the rest-frame time-delay of 296 +13 -14 days between the MgII broad-line emission and the ionizing continuum using seven different time-delay inference methods. Artefact time-delay peaks and aliases were mitigated using the bootstrap method, prior weighting probability function as well as by analyzing unevenly sampled mock light curves. The MgII emission is considerably variable with the fractional variability of ∼ 5.4%, which is comparable to the continuum variability (∼ 4.8%). Because of its high luminosity (L 3000 = 10 46.4 erg s -1 ), the source is beneficial for a further reduction of the scatter along the MgII-based radius-luminosity relation and its extended versions, especially when the high-accreting subsample that has an RMS scatter of ∼ 0.2 dex is considered. This opens up a possibility to use the high-accretor MgII-based radius-luminosity relation for constraining cosmological parameters. With the current sample of 27 reverberation-mapped sources, the best-fit cosmological parameters (Ω m , Ω Λ ) = (0.19; 0.62) are consistent with the standard cosmological model within 1σ confidence level. Keywords: accretion, accretion disks — galaxies: active — quasars: individual (HE 0435-4312) — quasars: emission lines — techniques: spectroscopic, photometric 1. INTRODUCTION Broad emission lines with line widths of several 1000 km s -1 are one of the main characteristic features of the optical and UV spectra of active galactic nu- clei (AGN; Seyfert 1943; Woltjer 1959; Schmidt 1963), Corresponding author: Michal Zajaˇcek zajacek@cft.edu.pl specifically of type I where the broad line region (BLR) is not obscured by the dusty molecular torus (Antonucci 1993; Urry & Padovani 1995). However, the scattered polarized light can reveal broad lines even for obscured type II AGN (type II NGC 1068 as the first case, An- tonucci & Miller 1985), which implies the universal presence of the BLR for accreting supermassive black holes (SMBH). The low-luminous systems, such as the Galactic Center (Genzel et al. 2010; Eckart et al. 2017; arXiv:2012.12409v2 [astro-ph.GA] 24 Feb 2021