A. Mücke, J.P. Rachen, Ralph Engel, R.J. Protheroe, Todor Stanev, PASA, 16 (2), in press.
| Next Section: Cross section and kinematics Title/Abstract Page: On photohadronic processes in Previous Section: On photohadronic processes in | Contents Page: Volume 16, Number 2 |
Introduction
The energy spectrum of cosmic rays extends to energies above 1020eV. Such particles are most likely extragalactic and propagate through many tens of Mpc before reaching the Earth. The main energy loss mechanism for nucleons in this energy range is photomeson production on the cosmic microwave background radiation (CMBR). This process has an energy threshold of 1.08 GeV in the center of momentum frame (CMF) of the interacting particles. It causes the distortion of the proton spectrum above3 x 1019 eV during propagation, known as the Greisen-Zatsepin-Kuzmin cutoff (Greisen 1966, Zatsepin & Kuzmin 1966; see Protheroe & Johnson 1996 for additional references). In environments harbouring dense radiation fields with higher photon energies the particle production threshold is reached at lower proton energies, e.g.
eV for ambient photon energies of 10 eV. If protons are accelerated in such energetic astrophysical objects, photopion production and subsequent pion decay would lead to the emission of energetic 
-rays and neutrinos (see Berezinsky & Gazizov 1993a,b for neutrino production in 
-scattering). This may be observable from jets in active galactic nuclei (AGN) (Mannheim 1993; Protheroe 1997) and gamma-ray bursts (GRB) (Waxmann & Bahcall 1997; Vietri 1998a,b; Böttcher & Dermer 1998; Rachen & Meszaros 1998).
The prominent 
-resonance near the threshold has often been used to construct approximate pion production cross sections (referred to as the `
-approximation' hereafter) and to determine gross features like -ray-to-neutrino energy yields, proton inelasticities, etc. which enter the relevant astrophysical calculations. In this paper we discuss first results using a new Monte-Carlo event generator for photohadronic interactions of relativistic protons in radiation fields of astrophysical origin, which includes all relevant interaction processes and is based on models and data available from particle physics.
Our event generator, SOPHIA (Simulation Of Photo-Hadronic Interactions in Astrophysics), has been extensively tested on all fixed target and collider experiment data available to us (Mücke et al. 1999a). A comparison of the results from SOPHIA with the -approximation emphasizes the importance of including all interaction processes in calculations of expected astrophysical signals. Detailed estimates of such signals will be subject of future work. This paper concentrates on photopion production features that may be important for astrophysical processes which involve hadronic reactions. In Sec. 2 we summarize the physics implemented in SOPHIA and contrast it with the -resonance approximation. Various astrophysical applications of SOPHIA are discussed in Sec. 3 by considering power-law and thermal photon seed spectra. A summary is given in Sec. 4.
| Next Section: Cross section and kinematics Title/Abstract Page: On photohadronic processes in Previous Section: On photohadronic processes in | Contents Page: Volume 16, Number 2 |
© Copyright Astronomical Society of Australia 1997