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X-ray emission from AGN is believed to be produced via Comptonization ofoptical/UV seed photons emitted by the accretion disk, up-scattered by hotelectrons in a corona surrounding the black hole. A critical compactness vs.temperature threshold is predicted above which any increase in the sourceluminosity, for a fixed size, would then generate positron-electron pairsrather than continue heating the coronal plasma. Current observations seem toconfirm that all AGN populate the region below this critical line. Thesemodels, however, have never been probed by observations in the high-luminosityregime, where the critical line is expected to reach low temperatures. To fillthis observational gap, we selected two luminous ($log(L_{bol})>47.5$ erg/s)quasars, 2MASSJ1614346+470420 ($z=1.86$) and B1422+231 ($z=3.62$), and obtainedXMM-Newton and NuSTAR deep observations for them. We performed detailedspectral analysis of their quasi-simultaneous soft and hard X-ray data, inorder to constrain the parameters of their coronae. Using a phenomenologicalcut-off power-law model, with the inclusion of a reflection component, wederived rest-frame values of the high energy cut-off of$E_{cut}=106^{+102}_{-37}$ keV and $E_{cut}=66^{+17}_{-12}$ keV, respectively.Comptonization models consistently give as best-fit parameters electrontemperatures of $\sim45$ keV and $\sim28$ keV, respectively, and opticallythick coronae ($\tau>1$). These low coronal temperatures fall in the limitedregion allowed at these luminosities to avoid runaway pair production.

Nustar Measurement Of Coronal Temperature In Two Luminous, High Redshift Qsos