On the origin of ultra high energy cosmic rays: subluminal and superluminal relativistic shocks

被引：28

作者：

Meli, A. ^{[1
,2
,3
]}

Becker, J. K. ^{[2
,4
]}

Quenby, J. J. ^{[5
]}

机构：

[1] Natl Univ Athens, Dept Phys, Panepistimiopolis 15783, Greece

[2] Univ Dortmund, Inst Phys, D-44221 Dortmund, Germany

[3] Univ Erlangen Nurnberg, Erlangen Ctr Astroparticle Phys, D-8520 Erlangen, Germany

[4] Univ Gothenburg, Inst Fys, S-41296 Gothenburg, Sweden

[5] Univ London Imperial Coll Sci Technol & Med, Blackett Lab, High Energy Phys Grp, London SW7 2BZ, England

来源：

ASTRONOMY & ASTROPHYSICS | 2008年 / 492卷 / 02期

关键词：

acceleration of particles; shock waves; waves; magnetic fields; scattering;

D O I：

10.1051/0004-6361:20078681

中图分类号：

P1 [天文学];

学科分类号：

0704 ;

摘要：

Aims. The flux of ultra high energy cosmic rays (UHECRs) at E > 10(18.5) eV is believed to arise in plasma shock environments in extragalactic sources. In this paper, we present a systematic study of cosmic ray (CR) particle acceleration by relativistic shocks, in particular concerning the dependence on bulk Lorentz factor and the angle between the magnetic field and the shock flow. The contribution to the observed diffuse CR spectrum provided by the accelerated particles is discussed. Methods. For the first time, Monte Carlo simulations for super- and subluminal shocks are extended to boost factors up to Gamma = 1000 and systematically compared. The source spectra derived are translated into the expected diffuse proton flux from astrophysical sources by folding the spectra with the spatial distribution of active galactic nuclei (AGN) and gamma ray bursts (GRBs). Results of these predictions are compared with UHECR data. Results. While superluminal shocks are shown to be inefficient at providing acceleration to the highest energies ( E > 10(18.5) eV), subluminal shocks may provide particles up to 10(21) eV, limited only by the Hillas-criterion. In the subluminal case, we find that mildly-relativistic shocks, thought to occur in jets of AGN (Gamma similar to 10-30), yield energy spectra of dN/dE similar to E-2. Highly relativistic shocks expected in GRBs (100 < Gamma < 1000), on the other hand, produce spectra as flat as similar to E-1.0 above 10(9.5) GeV. The model results are compared with the measured flux of CRs at the highest energies and it is shown that, while AGN spectra provide an excellent fit, GRB spectra are too flat to explain the observed flux. The first evidence of a correlation between the CR flux above 5.7 x 10(10) GeV and the distribution of AGN provided by Auger are explained by our model. Although GRBs are excluded as the principle origin of UHECRs, neutrino production is expected in these sources either in mildly or highly relativistic shocks. In particular, superluminal shocks in GRBs may be observable via neutrino and photon fluxes, rather than as protons.

引用

页码：323 / 336

页数：14

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