The spectrum ofthe B road Line R egion and the high-energy em ission ofpowerfulblazars

A B ST R A C T High-energyem ission(from theX-raythroughthe -rayband)ofFlatSpectrum Radio Quasars is widely associated with the inverse Com pton (IC) scattering ofam bient photons,produced eitherby theaccretion disk orby theBroad LineRegion,by high- energy electronsin a relativisticjet.In them odelling oftheIC spectrum oneusually adopts a sim ple black body approxim ation for the externalradiationeld,though the realshape is probably m ore com plex.The knowledge ofthe detailed spectrum ofthe externalradiationeld would allow to bettercharacterizethe soft-m edium X- ray IC spectrum ,which is crucialto address severalissues related to the study of thesesources.Herewepresentarststep in thisdirection,calculating theIC spectra expected by considering a realisticspectrum fortheexternalradiation energy density produced by theBLR,ascalculated with thephotoionization codeCLOUDY.W end that,undera wide range ofthe physicalparam eterscharacterizing the BLR clouds, theIC spectrum calculated with theblack-body approxim ation reproducesquitewell theexactspectrum forenergiesabovefew keV.In thesoftenergyband,instead,theIC em ission calculated using theBLR em ission showsa com plex shape,with a m oderate excess with respect to the approxim ate spectrum ,which becom es m ore im portant fordecreasing valuesofthe peak frequency ofthe photoionizing continuum .W e also show that the high-energy spectrum showsa m arked steepening,due to the energy dependenceofthescatteringcrosssection,aboveacharacteristicenergyof10-20GeV, quasiindependenton theLorentzfactorofthejet. K ey w ords: quasars:general{ X-rays:general{ scattering { radiation m echanism s: non-therm al