Australia Telescope National Facility

On photohadronic processes in astrophysical environments
A. Mücke, J.P. Rachen, Ralph Engel, R.J. Protheroe, Todor Stanev, PASA, 16 (2), in press.

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Conclusions

We use our recently developed Monte Carlo code for photohadronic processes (SOPHIA) to compare predictions of astrophysically relevant quantities with the expectations from the widely used ` $\Delta$ -approximation'. We discuss proton inelasticities and $\gamma$ -ray-to-neutrino energy ratios in the framework of astrophysical environments like AGN, GRBs and for the case of CR propagation. SOPHIA uses the full information of the cross section and the final state particle composition and kinematics of the interaction processes as provided by particle physics. We show that the $\Delta$ -approximation is a reasonable approximation only for a restricted set of applications. Several astrophysical cases are discussed:

Photohadronic interactions of high energy nucleons in typical radiation fields of GRBs, which may be responsible for the highest energy $\gamma$ -rays and neutrinos, occur mainly at high CMF energies. Nucleon inelasticities of
$\approx 0.5{-}0.6$ are expected in this case whereas the $\Delta$ -approximation gives only $\approx 0.2$ . The $\gamma$ -ray and neutrino energy outputs are approximately equal.
High-energy $\gamma$ -ray emission from blazar jets may be explainable by photopion production of relativistic protons bathed in a synchrotron or thermal (UV-photons from the accretion disk, IR-photons from the molecular torus) radiation field (``Proton Blazar'' models). For a flat-spectrum seed photon field photohadronic interactions with high CMF energies become important. SOPHIA simulations show that protons may cool much faster than in the $\Delta$ -approximation because of the growth of the inelasticity K_p. The $\gamma$ -ray-to-neutrino energy ratio may approach unity for both, flat and steep seed photon spectra, whereas many models use
${\cal E}_{\gamma}/{\cal E}_{\nu} = 3$ . This would affect estimates of the predicted neutrino flux.
Interactions of ultra-high energy cosmic rays propagating through the microwave background mainly happen in the region of the $\Delta(1232)$ -resonance for energies <10²¹eV. Above that energy the effects of the second resonant and multiparticle regions increase the fractional energy loss and alter the final shape of the cosmic ray spectrum.

Detailed calculations addressing photomeson production in the above mentioned astrophysical environments will be reported in forthcoming publications.

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