Where do ultra-high-energy cosmic rays come from?

The Pierre Auger Observatory measures and studies several quantities of ultra-high-energy cosmic rays: their energies, their depth of maximum shower development, and their arrival directions. Especially the latter is interesting to look into when searching for the sources of the particles - it is expected that more particles come from the directions of nearby sources than from the rest of the sky. By doing such a comparison to the expected fluxes from nearby starburst galaxies and active galactic nuclei, the Pierre Auger Collaboration was able to show that those powerful galaxies match well with the arrival direction measurements.

That realization naturally raised exciting follow-up questions: Can we describe all data - meaning also the energies and shower maximum depths in addition to the arrival directions - by the models using nearby starburst galaxies or active galactic nuclei? And which galaxies actually fit better?

It turns out that the model based on starburst galaxies can describe all the data simultaneously, and also as a function of the energy! To describe the data best, the contribution of the nearby starburst galaxies is estimated to be around 20% at an energy of 40 EeV, and it rises to around 40% at the highest detected energy. Other interesting quantities are also estimated using the model, like what energies the particles emitted at the sources have, what kind of particles are emitted, and how large the influence of cosmic magnetic fields on the arrival directions is.

The model based on active galactic nuclei, where the flux of cosmic rays was assumed to be proportional to the gamma-ray flux, however, does not adequately describe the data. Only the nearby active galaxy Centaurus A is able to explain the data almost equally well as the starburst galaxy model.

2023 08 constraining ecf2

Figure 1: Arrival directions for the starburst galaxy model (right), and the model based on the nearby active galaxy Centaurus A (left). The directions of the nearby galaxies are indicated by stars. It is visible that the relative contribution of these  nearby sources to the total flux rises with the energy (upper row: lower energy, lower row: higher energy).


2023 08 constraining ecf2

Figure 2: The energy spectrum of ultra-high-energy cosmic rays (black circles) is well described by
both the model based on Centaurus A (left) and the one based on starburst galaxies (right). The
contribution by the nearby catalog is indicated by a dashed line.


Related paper:

Constraining models for the origin of ultra-high-energy cosmic rays with a novel combined analysis of arrival directions, spectrum, and composition data measured at the Pierre Auger Observatory
The Pierre Auger Collaboration, JCAP01(2024)022
[arxiv.org/abs/2305.16693] [doi: 10.1088/1475-7516/2024/01/022]

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