091219

We propose joint XMM and NuSTAR observations of 12 high-redshift (z and amp amp amp gt 3) blazars which exhibit brightest and flattest X-ray spectra and host massive ( and amp amp amp gt 1e9 Msun) black holes. Joint Xmm and NuSTAR observations will lead to the most accurate estimation of the beaming factor which will allow us to put stringent constraint on the space density of blazar parent population and in turn the evolution of massive black holes in jetted AGNs within the first two billion years. Using XMM+NuSTAR data we will also probe the role played by the intergalactic medium in the X-ray absorption towards high-redshift blazars [truncated! Please see actual data for full text]