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  1. OA@INAF
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  3. 1 CONTRIBUTI IN RIVISTE (Journal articles)
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Please use this identifier to cite or link to this item: http://hdl.handle.net/20.500.12386/29747
Title: Quasars as standard candles II: The non linear relation between UV and X-ray emission at high redshifts
Authors: SALVESTRINI, FRANCESCO 
Guido Risaliti
Bisogni, Susanna 
LUSSO, ELISABETA 
VIGNALI, CRISTIAN 
Issue Date: 2019
Journal: ASTRONOMY & ASTROPHYSICS 
Number: 631
First Page: A120
Abstract: A tight non-linear relation between the X-ray and the optical-ultraviolet (UV) emission has been observed in Active Galactic Nuclei (AGN) over a wide range of redshift and several orders of magnitude in luminosity, suggesting the existence of an ubiquitous physical mechanism regulating the energy transfer between the accretion disc and the X-ray emitting corona. Recently, our group developed a method to use this relation in the observational cosmology, turning quasars into standardizable candles. This work has the main aim to investigate the potential evolution of this correction at high redshifts. We thus studied the $L_{\rm X}-L_{\rm UV}$ relation for a sample of quasars in the redshift range 4<$z$<7, adopting the selection criteria proposed in our previous work regarding their spectral properties. The resulting sample consists of 53 Type 1 (unobscured) quasars, observed either with Chandra or XMM-Newton, for which we performed a full spectral analysis, determining the rest-frame 2 keV flux density, as well as more general X-ray properties such as the estimate of photon index, and the soft (0.5-2 keV) and hard (2-10 keV) unabsorbed luminosities. We find that the relation shows no evidence for evolution with redshift. The intrinsic dispersion of the L$_X$-L$_{UV}$ for a sample free of systematics/contaminants is of the order of 0.22 dex, which is consistent with previous estimates from our group on quasars at lower redshift.
URI: http://hdl.handle.net/20.500.12386/29747
URL: https://www.aanda.org/articles/aa/full_html/2019/11/aa35491-19/aa35491-19.html
ISSN: 0004-6361
DOI: 10.1051/0004-6361/201935491
Fulltext: open
Appears in Collections:1.01 Articoli in rivista

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