Where do the most energetic particles in the universe come from?

This looks like the most natural question regarding ultra-high energy cosmic rays, but it’s also one of the hardest ones to be answered.

Cosmic rays are charged particles and thus they are deflected by the magnetic fields that exist both in our Galaxy and outside of it. So, even after we detect cosmic rays and measure their arrival direction, the source that accelerated them is not exactly in that direction. Given that the deflection of charged particles is inversely proportional to the energy, i.e. the most energetic cosmic rays are also the least deflected, this makes ultra-high energy cosmic rays (UHECRs) are particularly interesting. Unfortunately, the highest energy cosmic rays are also the rarest, and only a few have been observed so far. Observing the highest energies has another advantage, though: particles at these energies do not travel undisturbed through space. They can interact with the photon backgrounds that permeate the universe and lose their energy, so the sources of the ones we observe on Earth cannot be very far away (for astronomical standards): less than 100 Mpc or so. This limits a lot the region where to search for the sources of UHECRs!

The Pierre Auger Observatory collaboration has studied the arrival direction of cosmic rays since the very beginning of its operation. In 2022, we published the most comprehensive studies on the arrival direction of the highest energy cosmic rays, namely above 32 EeV (1EeV=1018 eV). The most energetic event observed by Auger has an energy of about 160 EeV. After 17 years of operation, we detected more than 2500 events with energy above 32 EeV.

A first test to be performed is to check if there is an excess of these cosmic rays from specific directions in the sky. We found that the largest excess is in the direction of the constellation of the Centaurus, but that it was not statistically significant when taking into account the so-called look elsewhere effect. This means that it is more likely to find something with a very low probability of happening by chance, the higher the number of trials one makes.

So the ultra-high energy cosmic rays are isotropic? They arrive from each direction equally? No! We just have to be more precise and look for correlation between the arrival directions and specific classes of astrophysical objects that we think can accelerate UHECRs. This way, we can reduce the number of trials by only looking around specific positions in the sky. We tested four different catalogs of galaxies: a generic one, mapping the distribution of all nearby extragalactic matter, a catalog of starburst galaxies, which have a higher rate of star formation and two catalogs of active galactic nuclei (AGN), which are galaxies in which the supermassive black hole at their center is actively absorbing matter. We also tested the direction of the closest AGN, Centaurus A (which is also in both the AGN catalogs mentioned before).

We saw that many of these catalogs gave significant correlations with our data. In particular, around the region of Centaurus A there are many cosmic rays, more than we would expect from a random distribution, on an angular scale of around 25°. You can see in Figure 1 which are the regions of the sky that have the largest flux of UHECRs. Given the large angular radius, we cannot however conclude that those events come from Centaurus A: near its position one can also find two very close and bright starburst galaxies. In Figure 2, you can see how these objects are distributed in that region. Indeed, the starburst catalog is the one that gives the higher correlation, with only 1 probability out of 30000 that it happens by chance (even considering the look elsewhere effect). Also the region of Centaurus A alone has an excess with less than 1 probability out of 20000 to happen by chance, and so we cannot at this moment disentangle the different scenarios, and more data is needed to claim to have found the sources of the most energetic particles in the Universe.



Figure 1: the flux of the most energetic cosmic rays observed by Auger, in Galactic Coordinates (the Galactic center is at the center and the Galactic plane is the equatorial line). The white region is the part of the sky not visible to Auger.



Figure 2: the dense region in the Centaurus constellation. Within a 20° radius, one can find Centaurus A, the closest AGN and radiogalaxy, and NGC 4945 and M83, two of the closest and most prominent starburst galaxies. These galaxies are at distances between 10 and 15 million light-years from us.

A similar research was also performed together with other observatories, Telescope Array which measures cosmic rays in the northern hemisphere and IceCube and ANTARES which detect high-energy neutrinos. Neutrinos are particles that interact rarely and thus cross even large amounts of matter undisturbed. Moreover, being neutral, they are not deflected by magnetic fields. Detecting them requires then very large volumes, and IceCube and ANTARES use respectively the Antarctic Ice and the Mediterranean water as targets to reach huge sizes (1km3 for IceCube!). We searched for correlation between the high-energy neutrinos and the UHECRs. Even if past works suggested a possible correlation, with new data and updated analyses we discovered that that was probably a statistical fluctuation and that no correlation can be found in our current data. This is, however, not unexpected: even if neutrinos are produced by cosmic rays interacting inside the source that accelerated them, the neutrinos observed by ANTARES and IceCube are of lower energies than the one of the expected “daughters” of the ultra-high energy cosmic rays observed by Auger and TA, so it is probable that their sources are, at least in part, different. As mentioned in the beginning, UHECRs can come only from relatively nearby sources while neutrinos, on the other hand, can come from farther away and this limits the number of neutrinos that we expect to arrive in spatial coincidence with UHECRs. In any case, with this new result, we were able to set interesting constraints that help in the quest for the sources of ultra-high energy cosmic rays.



Figure 3: the cosmic rays detected by Auger and TA (blue and yellow dot respectively) and the neutrinos detected by ANTARES (red squares) and IceCube (black diamonds and crosses). Tracks and cascades are two different types of neutrino events that the IceCube detector observes. The arrival directions are shown in equatorial coordinates, the projection of Earth’s coordinates in the sky (i.e. the top part of the plot is the celestial north pole and the lower part is the celestial south pole).


Related papers:

Arrival Directions of Cosmic Rays above 32 EeV from Phase One of the Pierre Auger Observatory
The Pierre Auger Collaboration, The Astrophysical Journal 935 (2022)170
[arxiv.org/abs/2206.13492] [doi: 10.3847/1538-4357/ac7d4e]

Search for Spatial Correlations of Neutrinos with Ultra-High-Energy Cosmic Rays
Antares, IceCube, Pierre Auger and TA Collaborations, The Astrophysical Journal 934 (2022)164
[arxiv.org/abs/2201.07313] [doi: 10.3847/1538-4357/ac6def]


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