GZK photons as ultra-high-energy cosmic rays
- PDF / 462,725 Bytes
- 22 Pages / 612 x 792 pts (letter) Page_size
- 109 Downloads / 201 Views
UCLEI, PARTICLES, FIELDS, GRAVITATION, AND ASTROPHYSICS
GZK Photons as Ultra-High-Energy Cosmic Rays1 G. B. Gelminia, b, O. E. Kalashevc, *, and D. V. Semikozb, d, ** a
Department of Physics and Astronomy UCLA, Los Angeles, CA 90095-1547, USA b CERN, PH-TH, CH-1211 Genève 23, Switzerland c Institute for Nuclear Research, Russian Academy of Sciences, Moscow, 117312 Russia d APC, College de France 75005, Paris, France *e-mail: [email protected] **e-mail: [email protected] Received October 12, 2007
Abstract—We calculate the flux of “GZK photons,” namely, the flux of ultra-high-energy cosmic rays (UHECRs) consisting of photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so-called GZK effect. We show that for primary nucleons, the GZK-photon fraction of the total UHECR flux is between 10–4 and 10–2 above 1019 eV and up to the order of 0.1 above 1020 eV. The GZK-photon flux depends on the assumed UHECR spectrum, the slope of the nucleon flux at the source, and the distribution of sources and intervening backgrounds. Detection of this photon flux would open the way for UHECR gamma-ray astronomy. Detection of a larger photon flux would imply the emission of photons at the source or new physics. We compare the photon fractions expected for GZK photons and the minimal fractions predicted by top–down models. We find that the photon fraction above 1019 eV is a crucial test for top–down models. PACS numbers: 96.50.sh, 96.50.sb DOI: 10.1134/S106377610806006X 1
1. INTRODUCTION
Cosmic rays with energies beyond the Greisen–Zatsepin–Kuzmin (GZK) cutoff [1] at 4 × 1019 eV present a challenging outstanding puzzle in astroparticle physics and cosmology [2, 3]. Nucleons cannot be confined to our Galaxy for energies above the “ankle,” i.e., above 1018.5 eV. This and the absence of a correlation of arrival directions with the Galactic plane indicate that if nucleons are the primary particles of ultra-high-energy cosmic rays (UHECRs), these nucleons should be of extragalactic origin. But nucleons with energies above 5 × 1019 eV could not reach Earth from a distance beyond 50 to 100 Mpc [4] because they scatter on the cosmic microwave background (CMB) photons with a resonant photoproduction of pions: pγ
∆*
Nπ,
where the pions carry away approximately 20% of the original nucleon energy. The mean free path for this reaction is only 6 Mpc. Photons with comparable energy pairs produce electrons and positrons on the radio background and, likewise, cannot reach Earth from beyond 10 to 40 Mpc [5] (although the photon energy attenuation length is uncertain due to uncertainties in the spectrum of the absorbing radio background). There are only a few known astrophysical sources within those distances that could produce such 1
The text was submitted by the authors in English.
energetic particles, but they are not located along the arrival directions of the observed cosmic rays. Intervening sheets of large-scale intense extragalactic magnetic fields (EGMFs), with intensities of B ~
Data Loading...
