X-ray properties of z ≳ 6.5 quasars. Issue 3 (22nd November 2019)

• Record Type: Journal Article
• Authors: Pons, E; McMahon, R G; Banerji, M; Reed, S L
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 491:Issue 3(2020)
• Page Start: 3884
• Page End: 3890
• Publication Date: 2019-11-22
• Subjects: galaxies: active -- galaxies: high redshift -- dark ages -- reionization -- X-rays: galaxies; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz3275
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We present XMM–Newton X-ray observations and analysis of three Dark Energy Survey z > 6.5 quasars (VDES J0020−3653 at z = 6.824, VDES J0244−5008 at z = 6.724, and VDES J0224−4711 at z = 6.526) and six other quasars with 6.438 < z < 6.747 from the XMM–Newton public archive. Two of the nine quasars are detected at a high (>4σ) significance level: VDES J0224−4711( z = 6.53) at 9σ and PSO J159−02 ( z = 6.38) at 8σ. They have a photon index of $\Gamma =1.82^{+0.29}_{-0.27}$ and $1.94^{+0.31}_{-0.29}$, respectively, which is consistent with the mean value of ∼1.9 found for quasars at all redshifts. The rest-frame 2–10 keV luminosity of VDES J0224−4711 is $L_{2\!-\!10\, \mathrm{keV}} = (2.92\pm 0.43)\times 10^{45}\, \mathrm{erg\, s^{-1}}$, which makes this quasar one of the most X-ray luminous quasars at z > 5.5 and the most X-ray luminous quasar at z > 6.5, with a luminosity 6 times and 2.5 times larger than ULAS J1120+0641 ( z = 7.08) and ULAS J1342+0928 ( z = 7.54), respectively. The X-ray-to-optical power-law slopes of the nine quasars are consistent with the previously observed anticorrelation of αox with UV luminosity $L_{2500\, \mathrm{\mathring{\rm A} }}$ . We find no evidence for evolution of αox with redshift when the anticorrelation with UV luminosity is taken into account. Similar to previous studies at z ∼ 6, we have found remarkably consistent X-ray spectral properties between low-redshift quasars ( z ∼ 1) and high-redshift quasars. Our results add further evidence to the picture that the observable properties of high-luminosity quasars over the UV-to-X-ray spectral region have not evolved significantly from z ∼ 7 to the present day and that quasars comparable to local versions existed 800 Myr after the big bang.