Probable spectral steepening of UHEγ-rays of nuclear origin and its consequences

Abstract

Recent accelerator data based parameterization of the inclusive cross section (c - s) for π0 production in hadronic collisions and an explicit incorporation of the finiteness of the relevant projectile hadron spectrum suggest a significant steepening in the spectrum (by as much as 0.4 in the spectral index) of the secondary γ-ray towards the end of the spectrum. We emphasize here that this spectral steepening in conjunction with the possibility that in the bright X-ray binaries the maximum energy to which the γ-ray producing progenitor protons may be accelerated is only ~ l0 PeV, may imply an effective efficiency for γ-ray production, e, as reckoned by the PeV arrays, one or two orders smaller than the previous estimates. To explain the genesis of a given PeV photon flux from an X-ray binary, one, therefore, has to. accordingly consider a much higher value of the progenitor proton beam luminosity, L p. This requirement may raise further questions regarding the actual genesis of PeV γ-rays in X-ray binaries, or alternatively, on the veracity of the high values of the PeV photon fluxes reported by earlier experiments.