Exploratory X-Ray monitoring of luminous radio-quiet quasars at high redshift: extended time-series analyses and stacked imaging spectroscopy

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Exploratory X-Ray Monitoring of Luminous Radio-quiet Quasars at High Redshift: Extended Time-series Analyses and Stacked Imaging Spectroscopy Marcus O. Thomas1, Ohad Shemmer1, W. N. Brandt2,3,4, Maurizio Paolillo5,6,7, Shai Kaspi8, Cristian Vignali9,10, Paulina Lira11, and Donald P. Schneider2,3

Published 2021 December 15 • © 2021. The American Astronomical Society. All rights reserved. The Astrophysical Journal, Volume 923, Number 1 Citation Marcus O. Thomas et al 2021 ApJ 923 111 DownloadArticle PDF DownloadArticle ePub Figures Tables References 85 Total downloads Turn on MathJax Get permission to re-use this article

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Share this content via email Share on Facebook (opens new window) Share on Twitter (opens new window) Share on Mendeley (opens new window) Article information Abstract We present three new Chandra X-ray epochs along with new ground-based optical–UV observations as the third installment in a time-series analysis of four high-redshift (z ≈ 4.1–4.4) radio-quiet quasars. In total, we present nine epochs for these sources with rest-frame temporal baselines of ∼1300–2000 days. We utilize the X-ray data to determine basic variability properties, as well as produce mean spectra and stacked images based on effective exposure times of ∼40–70 ks per source. We perform time-series analyses in the soft and hard bands, separately, and compare variability properties to those of sources at lower redshifts and luminosities. The magnitude of X-ray variability of our sources remains consistent with or lower than that of similar sources at lower redshifts, in agreement with the variability–luminosity anticorrelation. The mean power-law photon indices in the stacked Chandra spectra of our sources are consistent with the values measured from their archival XMM-Newton spectra separated by about 3 yr in the rest frame. Along with the X-ray observations, we provide near-simultaneous optical monitoring of the sources in the optical–UV regime. The overall variability in the optical-to-X-ray spectral slope is consistent with sources at lower redshifts, and the optical–UV observations display mild variability on monthly timescales.