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|Title:||The X-ray properties of z 6 luminous quasars||Authors:||Nanni, R.
Brandt, W. N.
|Issue Date:||2017||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||603||First Page:||A128||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<SUB>-0.27</SUB><SUP>+0.28</SUP> and a neutral intrinsic absorption column density of N<SUB>H</SUB> ≤ 10<SUP>23</SUP> cm<SUP>-2</SUP>. 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<SUB>ox</SUB>) of our sample and combined them with those of previous works, confirming that α<SUB>ox</SUB> 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).||URI:||http://hdl.handle.net/20.500.12386/27087||URL:||https://arxiv.org/abs/1704.08693
|Appears in Collections:||1.01 Articoli in rivista|
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