The X-ray properties of z ~ 6 luminous quasars

¹ INAF, Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
e-mail: riccardo.nanni7@unibo.it
² Dipartimento di Astronomia, Università degli Studi di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
³ INAF, Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁴ Department of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
⁵ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁶ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA

Received: 23 January 2017
Accepted: 12 April 2017

Abstract

We present a systematic analysis of X-ray archival data of all the 29 quasars (QSOs) at z> 5.5 observed so far with Chandra, XMM-Newton and Swift-XRT, including the most-distant quasar ever discovered, ULAS J1120+0641 (z = 7.08). This study allows us to place constraints on the mean spectral properties of the primordial population of luminous Type 1 (unobscured) quasars. Eighteen quasars are detected in the X-ray band, and we provide spectral-fitting results for their X-ray properties, while for the others we provide upper limits to their soft (0.5–2.0 keV) X-ray flux. We measured the power-law photon index and derived an upper limit to the column density for the five quasars (J1306+0356, J0100+2802, J1030+0524, J1148+5251, J1120+0641) with the best spectra (>30 net counts in the 0.5–7.0 keV energy range) and find that they are consistent with values from the literature and lower-redshift quasars. By stacking the spectra of ten quasars detected by Chandra in the redshift range 5.7 ≤ z ≤ 6.1 we find a mean X-ray power-law photon index of Γ = 1.92_-0.27^+0.28 and a neutral intrinsic absorption column density of N_H ≤ 10²³ cm^-2. These results suggest that the X-ray spectral properties of luminous quasars have not evolved up to z ≈ 6. We also derived the optical-X-ray spectral slopes (α_ox) of our sample and combined them with those of previous works, confirming that α_ox strongly correlates with UV monochromatic luminosity at 2500 Å. These results strengthen the non-evolutionary scenario for the spectral properties of luminous active galactic nuclei (AGN).