The WISSH quasars project III. X-ray properties of hyper-luminous quasars

S. Martocchia, E. Piconcelli, L. Zappacosta, F. Duras, G. Vietri, C. Vignali, S. Bianchi, M. Bischetti, A. Bongiorno, M. Brusa, G. Lanzuisi, A. Marconi, S. Mathur, G. Miniutti, F. Nicastro, G. Bruni, F. Fiore. 2017. The WISSH quasars project III. X-ray properties of hyper-luminous quasars. Astronomy and Astrophysics 608, DOI: 10.1051/0004-6361/201731314

We performed a survey of the X-ray properties of 41 objects from the WISE/SDSS selected hyper-luminous (WISSH) quasars sample, which includes 86 broad-line quasars with bolometric luminosity L-Bol greater than or similar to 2 x 10(47) erg s(-1) shining at z similar to 2-4. We used both proprietary and archival Chandra and XMM-Newton observations. Twenty-one quasars have sufficient quality data to perform a spectroscopic analysis, while for the remaining sources, X-ray properties are derived through hardness-ratio analysis (apart for six sources that result to be undetected). The bulk (similar to 70%) of the detected WISSH quasars exhibit N-H < 5 x 10(22) cm(-2), in agreement with their optical Type 1 AGN classification. All but three quasars show unabsorbed 2-10 keV luminosities L2-10 >= 10(45) erg s(-1). Thanks to their extreme radiative output across the mid-IR-to-X-ray range, WISSH quasars therefore offer the opportunity to significantly extend and validate the existing relations involving L2-10. Specifically, we studied the X-ray luminosity as a function of (i) X-ray-to-optical (X/O) flux ratio; (ii) mid-IR luminosity (LMIR); (iii) L-Bol, and (iv) alpha(OX) versus 2500 angstrom luminosity. We find that the WISSH quasars show (i) unreported very low X/O (<0.1) compared to typical AGN values; (ii) L2-10/L-MIR ratios that are significantly smaller than those derived for AGN with lower luminosity; (iii) a large X-ray bolometric correction, k(Bol,X) approximate to 100-1000; and (iv) steep -2 >= alpha(OX) >= -1.7. These results lead to a scenario in which the X-ray emission of hyper-luminous quasars is relatively weaker compared to lower luminosity AGN. Models predict that such an X-ray weakness can be relevant for the acceleration of powerful high-ionization, emission-line-driven winds, which are commonly detected in the UV spectra of WISSH quasars and can, in turn, perturb the X-ray corona and weaken its emission. Accordingly, hyper-luminous quasars represent the ideal laboratory to study the link between the AGN energy output and wind acceleration. Additionally, WISSH quasars exhibit very large SMBH masses (log[M-BH/M-circle dot] >= 9.5). This enables a more robust modeling of the Gamma-M-BH relation by increasing the statistics at high masses. We derive a flatter dependence than previously found over the broad range 5 less than or similar to log(M-BH = M-circle dot) <= 11. Finally, we estimate that only 300 ks observations of X-IFU on board Athena will offer a detailed view of the properties of absorption features associated with powerful X-ray SMBH winds for a representative sample of WISSH quasars.

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