NuSTAR Measurement of Coronal Temperature in Two Luminous, High-redshift Quasars

X-ray emission from the active galactic nucleus (AGN) is believed to be produced via Comptonization of optical/ultraviolet seed photons emitted by the accretion disk, upscattered by hot electrons in a corona surrounding the black hole. A critical compactness versus temperature threshold is predicted above which any increase in the source luminosity, for a fixed size, would then generate positron-electron pairs rather than continue heating the coronal plasma. Current observations seem to confirm that all AGNs populate the region below this critical line. These models, however, have never been probed by observations in the high-luminosity regime, where the critical line is expected to reach low temperatures. To fill this observational gap, we selected two luminous (log(L-Bol) > 47.5 erg s(-1)) quasars, 2MASSJ1614346+470420 (z = 1.86) and B1422+231 (z = 3.62), and obtained XMM-Newton and NuSTAR deep observations for them. We performed detailed spectral analysis of their quasisimultaneous soft and hard X-ray data, in order to constrain the parameters of their coronae. Using a phenomenological cutoff power-law model, with the inclusion of a reflection component, we derived rest-frame values of the high-energy cutoff of E-cut = 106(-37)(+102) keV and E-cut = 66(-12)(+17) keV, respectively. Comptonization models consistently give as best-fit parameters electron temperatures of similar to 45 keV and similar to 28 keV, respectively, and optically thick coronae (tau > 1). These low coronal temperatures fall in the limited region allowed at these luminosities to avoid runaway pair production.