Progress towards characterizing ultrahigh energy cosmic ray sources

Abstract

We use a multimessenger approach to constrain realistic mixed composition models of ultrahigh energy cosmic ray sources using the latest cosmic ray, neutrino, and gamma-ray data. We build on the successful Unger-Farrar-Anchordoqui 2015 (UFA15) model which explains the shape of the spectrum and its complex composition evolution via photodisintegration of accelerated nuclei in the photon field surrounding the source. We explore the constraints which can currently be placed on the redshift evolution of sources and the temperature of the photon field surrounding the sources. We show that a good fit is obtained to all data either with a source which accelerates a narrow range of nuclear masses or a Milky Way-like mix of nuclear compositions, but in the latter case the nearest source should be 30-50 Mpc away from the Milky Way in order to fit observations from the Pierre Auger Observatory. We also ask whether the data allow for a subdominant purely protonic component at UHE in addition to the primary UFA15 mixed composition component. We find that such a two-component model can significantly improve the fit to cosmic ray data while being compatible with current multimessenger data.

Features of the Energy Spectrum of Cosmic Rays above 2.5×1018 eV Using the Pierre Auger Observatory

“…Some cautionary comments on the illustrative model considered here are in order. The presence of a subdominant light component at the highest energies is not excluded by our data, see, e.g., [45]. Also, viable source scenarios can be found without resorting to a mixed composition with a rigidity-dependent maximum energy if, for instance, predominately heavy (Si to Fe) nuclei are accelerated and photodisintegrate in the source environment [46] or en route to Earth [47,48].…”

POEMMA: Probe Of Extreme Multi-Messenger Astrophysics

“…The dN/dX max tail depends on the cosmic ray mass composition. The composition above 40 EeV is modeled here with two simple representative scenarios guided by the cosmic ray composition analysis described in [65]. A conservative UHECR composition scenario with 10% protons and 90% nitrogen has a 20% proton fraction in the high X max tail.…”

Current constraints from cosmogenic neutrinos on the fraction of protons in UHECRs

“…In fact, Ref. [16] has shown that, for the specific model discussed there, an additional proton component even improves the fit to the UHECR spectrum and composition.…”

“…In, e.g., Refs. [12,13,14,15,16] the UHECR spectrum and composition (assuming specific hadronic interaction modes) are fitted and corresponding cosmogenic neutrino fluxes are obtained under the assumptions of a continuous distribution of identical sources and rigidity-dependent maximum energies. In most cases these combined fits find as a best fit a relatively hard spectral index (α 1.3), a composition dominated by nuclei with a charge Z ≥ 6 and a relatively low maximum rigidity (R max ≡ E max /Z 7 EV).…”