Modeling Source Spectra and Composition of Ultrahigh Energy Cosmic Rays
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ELEMENTARY PARTICLES AND FIELDS Experiment
Modeling Source Spectra and Composition of Ultrahigh Energy Cosmic Rays V. N. Zirakashvili1)* , E. G. Klepach1), S. I. Rogovaya1), and V. S. Ptuskin1) Received July 11, 2019; revised July 11, 2019; accepted July 11, 2019
Abstract—The inverse problem of cosmic ray transport of ultra-high energy cosmic rays is considered. The source spectrum and composition are derived based on the recent Auger data on energy spectrum, energy dependence of mean logarithm of atomic mass number and its variance. The dependence of results on the extrapolation of observable spectrum beyond energies 1020 eV is investigated. DOI: 10.1134/S1063778819660529
1. INTRODUCTION The origin of ultra-high energy cosmic rays is a key problem of cosmic ray astrophysics. The observed suppression cosmic ray flux at energies above ∼5 × 1019 eV is most probably associated with two effects: the GZK cutoff [1, 2] and acceleration limit in cosmic ray sources [3, 4]. The isotropy of the highest energy cosmic rays are indicative of their extragalactic origin. The list of potential sources which could give the observed cosmic ray flux includes active galactic nuclei, gamma-ray bursts, fast spinning newborn pulsars, interacting galaxies, large-scale structure formation shocks, and some other objects, see reviews [5, 6] and references therein. The present knowledge about the highest energy cosmic rays was mainly acquired from the Pierre Auger Observatory (Auger), Telescope Array experiment (TA), and from the Yakutsk complex EAS array, see [7, 9]. The mass composition of these cosmic rays remains uncertain. The interpretation of HiRes and TA data favors predominantly proton composition at energies 1018 to 5 × 1019 eV, whereas the Auger data indicate that the cosmic ray composition is becoming heavier with energies changing from predominantly proton at 1018 eV to more heavy composition at about 5 × 1019 eV. The mass composition interpretation of the measured quantities depends on the assumed hadronic model of particle interactions which is based on not well determined extrapolation of physics from lower energies. The energy spectrum in extragalactic sources is commonly determined by the trail-and-error method 1)
Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio wave Propagation Russian Academy of Sciences, Troitsk, Moscow, 108840 Russia. * E-mail: [email protected]
when one makes the calculations of the expected at the Earth cosmic ray intensity assuming some shape of the source energy spectrum and the source composition. The calculations follow cosmic ray propagation from the source to the observer, e.g. [10]. The standard assumption is that the source spectrum is a power law on magnetic rigidity up to some maximum rigidity. In our previous works [11, 12] we showed how to inverse the procedure and calculate the source function starting from the observed at the Earth spectrum without ad hoc assumptions about the shape of the source spectrum. The only assumptions done are that different nuclei follow the same sp
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