Auger Test of the Cen A Model of Highest Energy Cosmic Rays

Abstract

If, as recently proposed by Farrar and Piran, Cen A is the source of cosmic rays detected above the Greisen-Zatsepin-Kuz'min cutoff, neutrons are ≈ 140 more probable than protons to be observed along its line of sight. This is because the proton flux is rendered nearly isotropic by O(µG) intergalactic magnetic fields. With the anticipated aperture of the Southern Auger Observatory, one may expect on the order of 2 neutron events/year above 10 20 eV in the line of sight of Cen A.The energy spectrum of cosmic rays (CRs) is well fitted by power laws with increasing index for energies above 4 × 10 15 eV (the "knee") flattening again above 5 × 10 18 eV (the "ankle"), yielding the overall shape of a leg. Over the last third of the century, ingenious installations with large effective areas and long exposure times-needed to overcome the steep falling flux-have raised the tail of the spectrum up to an energy of 3 × 10 20 eV, with no evidence that the highest energy recorded thus far is Nature's upper limit [1]. The origin of these extraordinarily energetic particles continues to present a major enigma to high energy physics [2].The main problem posed by the detection of CRs of such energy (if nucleons, gammas, and/or nuclei) is energy degradation through inelastic collisions with the universal radiation fields permeating the universe. Therefore, if the CR sources are all at cosmological distances, the observed spectrum must virtually end with the Greisen-Zatsepin-Kuz'min (GZK) cutoff at E ≈ 8 × 10 19 eV [3]. The spectral cutoff is less sharp for nearby sources (within 50 Mpc or so). The arrival directions of the trans-GZK events are distributed widely over the sky, with no plausible counterparts (such as sources in the Galactic Plane or in the Local Supercluster). Furthermore, the data are consistent with an isotropic distribution of sources in sharp constrast to the anisotropic distribution of light within 50 Mpc [4]. The difficulties encountered by conventional acceleration mechanisms in accelerating particles to the highest observed energies have motivated suggestions that the underlying production mechanism could be of non-acceleration nature. Namely, charged and neutral primaries, mainly light mesons (pions) together with a small fraction (3%) of nucleons, might be produced at extremely high energy by decay of supermassive elementary X particles (m X ∼ 10 22 − 10 28 eV) [5]. However, if this were the case, the observed spectrum should be dominated by gamma rays and neutrinos, in contrast to current observation [6]! Alternative explanations involve undiscovered neutral hadrons with masses above a few GeV [7], neutrinos producing nucleons and photons via resonant Z-production with the relic neutrino background [8], or else neutrinos attaining cross sections in the millibarn range above the electroweak scale [9]. A controversial correlation between the arrival direction of CRs above 10 20 eV and high redshift compact radio quasars seems to support these scenarios [10].Over the last few years, it has become evident tha...

Astrophysical origins of ultrahigh energy cosmic rays

“…(37) [251]. For our choices of B and ℓ, and T on = 400 Myr, we find for E = 10 19 eV (E = 10 20 eV) that α = 0.04 (α = 0.07).…”

“…that for 3.4 Mpc the diffusion time of any proton with energy above the photopion production threshold is always less than the GZK-time, and consequently energy losses can be safely neglected. This implies that the density of protons at the present time t of energy E at a distance r from Cen A (which is assumed to be continuously emitting at a constant spectral rate dN p+n 0 /dE dt from time t on until the present) can be obtained by solving the Kolmogorov-diffusive-equation, and is found to be [251] dn(r, t)…”

Ultrahigh energy nuclei in the turbulent Galactic magnetic field

“…In this section we test the hypothesis that all UHECR detected at Earth may be due to one nearby source such as Cen A. Cen A has been suggested to be a source of UHECR since a long time [47,48,49].…”

The Search for Anisotropy in the Arrival Directions of Ultra-High Energy Cosmic Rays Observed by the High Resolution Fly's Eye Detector in Monocular Mode

“…However, these limits are not small enough to exclude the theoretical predictions [1,2,3]. Also, they do not exclude the findings of the AGASA collaboration in terms of the intensity of the dipole effect that they observed or in terms of the energy considered beSource α Galactic 0.005 ± 0.055 Centaurus A −0.005 ± 0.065 M87 −0.010 ± 0.045 Table 1 Estimation of α via the value of <cos θ> for the dipole function.…”