THE HIGH-ENERGY CUT-OFF IN HIGH-Z/HIGH-L QSOS

We propose NuSTAR observations of two bright (FX>5 10^13 cgs, LX>10^46 erg/s)radioquiet QSOs at z>2, plus short XMM exposures for each of them: our totaltime request is 320k NuSTAR and 70ks XMM. By looking at restframe energies upto 300 keV, our goal is to measure unambiguously the presence of spectralcutoffs at restframe energies up to E c500 keV. This is not possible forlocal targets, where NuSTAR is just sensitive to E c<250 keV. The two brightestsources of a sample of four have been successfully observed in Cycle 3. Theseobservations allowed to measure for the first time E c in distant, luminousQSOs. The proposed observations will allow to build a a small but unique sampleof highz, highLX QSOs with accurate Ec measurements, to test current coronal models. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]