Ultrahigh energy cosmic rays and neutrinos from light nuclei composition

Abstract

The baryonic mass composition of ultrahigh energy (≳1018 eV ) cosmic rays (UHECRs) at injection accompanied by their interactions on universal photon backgrounds during propagation directly governs the UHECR flux on the Earth. Secondary neutrinos and photons produced in these interactions serve as crucial astrophysical messengers of UHECR sources. A modeling of the latest data obtained by the Pierre Auger Observatory (PAO) suggests a mixed element composition of UHECRs with the subankle spectrum being explained by a different class of sources than the superankle region (>1 018.7 eV ). In this work, we obtain two kinds of fit to the UHECR spectrum—one with a single population of sources comprising of 1H and 2He, over an energy range commencing at ≈1018 eV —another for a mixed composition of representative nuclei 1H, 4He, 14N and 28Si at injection, for which a fit is obtained from above ≈1 018.7 eV . In both cases, we consider the source emissivity evolution to be a simple power-law in redshift. We test the credibility of H +He composition by varying the source properties over a wide range of values and compare the results to that obtained for H +He +N +Si composition, using the Monte Carlo simulation tool CRPropa 3. The secondary electrons and photons are propagated using the cosmic ray transport code DINT. We place limits on the source spectral index, source evolution index and cutoff rigidity of the source population in each case by fitting the UHECR spectrum. Cosmogenic neutrino fluxes can further constrain the abundance fraction and maximum source redshift in case of light nuclei injection model.