# ICRC 2021

Jul 12 – 23, 2021
Online
Europe/Berlin timezone

## Combined fit of the energy spectrum and mass composition across the ankle with the data measured at the Pierre Auger Observatory

Jul 13, 2021, 6:00 PM
1h 30m
03

#### 03

Talk CRI | Cosmic Ray Indirect

### Speaker

Eleonora Guido (Università degli Studi di Torino)

### Description

The combined fit of the energy spectrum and mass composition data above $5\cdot10^{18}\:\mathrm{eV}$ suggested the presence of extragalactic sources ejecting ultra-high-energy cosmic rays with relatively low maximum energies, hard spectral indices and mixed chemical compositions, dominated by the contribution of intermediate mass groups. Here we present an extension of the fit to lower energies, to include the feature observed near $5\cdot10^{18}\:\mathrm{eV}$ in the all-particle energy spectrum, the so-called ankle.
We show that it is possible to generate such a change of slope assuming that the flux below the ankle is provided by the superposition of some additional contributions. The simplest extension of this sort consists of introducing a supplemental extragalactic component at low energy, characterised by different physical parameters with respect to the one contributing above the ankle: such a component may originate from a different population of sources or be provided by interactions occurring in the acceleration sites. In this framework we also explore the possibility of including the end of a Galactic contribution at low energies.
The fit suggests that these scenarios provide a reasonable description of the measurements across the ankle, without affecting the results obtained for the above-ankle region.
In order to evaluate our capability to constrain the source models, we finally discuss the impact of the main experimental systematic uncertainties and of the theoretical models choice on the fit results.

### Keywords

UHECR; ultra-high-energy cosmic rays; combined fit; energy spectrum; mass composition; ankle energy region; transition region; Auger; Pierre Auger Observatory; UHECR propagation; extragalactic sources

Subcategory Experimental Results Auger

### Primary authors

Eleonora Guido (Università degli Studi di Torino)