GZK photons as UHECR above 10 eV

“GZK photons” are produced by extragalactic nucleons through the resonant photoproduction of pions. We present the expected range of the GZK photon fraction of UHECR, assuming a particular UHECR spectrum and primary nucleons, and compare it with the minimal photon fraction predicted by Top-Down models. The Pierre Auger Observatory [1] may prove photon fractions in the Ultra-High Energy Cosmic Rays (UHECR) above 10 eV at the level of 10% (maybe even a few%). Based on Ref. [2], here we will address the physical implications of such detection or limit. In particular, we will discuss if “GZK photons” could be observed at this level or, otherwise, if limits on the parameters on which their flux depends could be obtained from the non observation of photons at this level. We call “GZK photons” those photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so called GZK effect. In Ref. [2] we fitted the assumed UHECR spectrum above 2 × 10 eV solely with primary nucleons and the GZK photons they produce. The GZK photon flux depends on the UHECR spectrum assumed, the slope and maximum energy of the primary nucleon spectrum, the minimum distance to the sources and the intervening radio background and average extragalactic magnetic field. We took a phenomenological approach in choosing the range of the several relevant parameters, namely we took for each of them a range of values mentioned in the literature, without attempting to assign them to particular sources or acceleration mechanisms. We also assumed the existence of a galactic or extragalactic Low Energy Component (LEC) when necessary to fit the assumed UHECR at energies below 10 eV, taking care that it is negligible at energies 3×10 eV and above. We used a numerical code developed in Ref. [3] to compute the flux of GZK photons produced by an homogeneous distribution of sources emitting originally only protons (however the results at the high energies considered is the same for primary neutrons). It uses the kinematic equation approach and calculates the propagation of nucleons, stable leptons and photons using the standard dominant processes. We parametrized the initial proton flux for any source with a power law function, F (E) = f E−α θ(Emax − E). The power law index α and maximum energy Emax were considered free parameters. The amplitude f was fixed by normalizing the final proton flux from all sources to the observed flux of UHECR, which we took to be either the AGASA spectrum or the HiRes spectrum (given that a reliable Auger spectrum in not yet available). We considered the power law index to be in the range 1 ≤ α ≤ 2.7 and Emax between 10 20 eV and 10 eV. For the average extragalactic magnetic field we took the range 10G to 10G, [4] and for the radio background we considered three estimates: one from Clark et al. and two higher ones from Protheroe and Biermann [5]. 1 Talk given at TAUP2005, Sept. 10-14 2005, Zaragoza (Spain) The largest GZK photon fractions in UHECR happen for small values of α, large values of Emax, small minimal distance to the sources (which is compatible with a small frequency of clustering of the events) and small intervening backgrounds. The smallest GZK photon fluxes are obtained with the opposite choices. In the most favorable cases for a large photon flux, GZK photons could dominate the UHECR flux in an energy range above 10 eV. This allowed us [2] to fit the AGASA data, at the expense of assuming that the initial protons could have a hard spectrum ∼ 1/E and be accelerated to energies as high as 10 eV. In this extreme case, the AGASA data can be explained without any new physics, except in what the mechanism of acceleration of the initial protons is concerned. With the HiRes spectrum the GZK photons are always subdominant and can be neglected for the fit. Proceeding in this manner, in Ref. [2] we fitted the AGASA [6] and the HiRes monocular [7] data trying to minimize and to maximize the number of GZK protons produced, to obtain the expected range of the GZK-photon fraction in UHECR. We found (see the pink bands in Fig. 1) that the GZK photon fraction of the total integrated UHECR flux, for the AGASA spectrum is between 5% and 7% above 10 eV and between 30% and 60% above 10 eV, thus Auger should be able to see these photons or place interesting bounds on the flux parameters. Recall that fitting the AGASA data with astrophysical sources requires the extreme choices for the initial spectrum mentioned above. With the HiRes spectrum, instead the predicted GZK photon fraction is between 0.01% and 1% of the UHECR above 10 eV and between 0.001% and 4% above 10 eV, thus these photons may or may not be within the reach of Auger.