Thursday 06/06/2019 @ 11:30, Sala IV piano Battiferro
Fabio Vito (Pontificia Universidad Católica de Chile), "The X-ray properties of the z>6 QSO population"
I will present the results of new Chandra observations of a sample of 10 optically selected z>6 QSOs. Adding archival X-ray data, we studied the X-ray properties of a statistically significant sample of 25 QSOs in the first Gyr of the Universe. We focused in particular on the relation between UV and X-ray luminosity, as traced by the alpha_ox parameter, and on the typical slope of QSO X-ray spectra, which provide information on the SMBH accretion mechanisms. We found no significant evolution of these quantities from low redshift to z>6, implying that the accretion disk/hot corona physics does not change strongly at high redshift. Such a lack of evolution can be exploited to use QSOs as standard candles and infer cosmological parameters, extending the analysis of Risaliti & Lusso (2019) to z>6. One of the new 10 targets is detected in the 2-5 keV band, with no emission at softer energies. Such a hard X-ray spectrum spectrum suggests this is the first heavily obscured QSO candidate detected at z>6.