THE HIGH-ENERGY CUT-OFF IN HIGH-Z QSOS: A FIRST CUT

We propose NuSTAR observations of two among the Xray brightest (F 0.510 > 510^13 ergs s^1 cm^2, L 210 > 10^45 ergs s^1) radioquiet QSOs at z > 1.8,complemented by a joint, short XMMNewton exposure for one of them. Our totaltime request is 250ks with NuSTAR and 30ks with XMMNewton. By looking atrestframe energies up to virgul 200300 keV, our goal is to measure unambiguouslythe presence of spectral cutoffs at restframe energies as high as E c500 keV.This is not possible for targets in the local Universe, where NuSTAR is justsensitive to E c < 250 keV. Our sample is unique and will allow unprecedentedmeasurements of E c in distant AGN with such Xray luminosities. Our programwill prove for the first time whether there is a physical limit to the temperature of AGN. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]