In a recent publication, the Pierre Auger Observatory has presented measurements of the large-scale cosmic-ray (CR) anisotropies in right ascension, using data collected by the surface detector array over more than 14 years, up to 31st August 2018. The dataset covers the energy range from ~0.03 EeV up to the highest detected energies.
The distribution of arrival directions of the cosmic rays is an important observable, which is expected to provide valuable information to understanding the CR origin. Being charged particles, CRs are significantly deflected by the magnetic fields present in our universe. It is only at the highest energies that the deflections are expected to get small enough so that one may hope to observe localized flux excesses associated with individual CR sources. On the other hand, as the energies decrease and the deflections become large, the propagation eventually becomes diffusive and it is likely that only large-scale patterns, such as a dipolar flux modulation in which the flux from one half of the sky becomes larger than the flux in the opposite hemisphere, may be detectable. However, the small amplitudes of these anisotropies make their observation quite challenging.
Exploiting the fact that, due to the Earth rotation the Observatory covers very uniformly the sky in the different directions along the Equator (measured by the right ascension coordinate), the analysis method aims at the reconstruction of the equatorial component of a dipole distribution. This component is recovered through a Fourier analysis in right ascension that also includes weights for each event so as to account for the main detector-induced systematic effects (such as remaining non-uniformities in the exposure). For the energies at which the trigger efficiency of the array is small (below ~2 EeV), the “East-West” method is employed, that relies on the relative difference between the rates of events arriving from the East and those arriving from the West. This method has the advantage that it is essentially insensitive to most of the systematic effects that pollute the right ascension distribution, and hence provides a very reliable result, but for which one pays the price of a higher statistical uncertainty than with the usual Fourier analysis.
Fig 1. shows the results of the reconstructed dipole amplitude and phase in different energy bins. The 99% CL upper limit are shown for the cases in which the measured amplitude has more than 1% probability to be a fluctuation from an isotropic distribution. The results obtained by the IceCube, IceTop and KASCADE-Grande experiments in the 1-30 PeV range are also displayed.
A trend of increasing amplitudes with increasing energies is observed, with values going from 0.1% at PeV energies, to ~1% at EeV energies and reaching ~10% at 30 EeV. The most significant modulation appears for the inclusive bin above 8 EeV (shown as a grey band), with an equatorial dipole amplitude of 6%, which has a significance of 6σ. The phase of the maximum of this modulation is at 98° ± 9°, indicating an extragalactic origin for these CRs. Below 8 EeV, none of the amplitudes obtained are significant and 99% CL upper bounds at the level of 1 to 3% are set.
Fig.1: Reconstructed equatorial-dipole amplitude (left) and phase (right).
Regarding the phases determined in the different bins, a transition between values lying close to the right ascension of the Galactic center, which is around -94°, towards values in a nearly opposite direction (~100°), is observed to take place around a few EeV. All this suggests that these dipolar anisotropies have a transition from a predominantly Galactic origin to an extragalactic one somewhere in the range between 1 EeV and few EeV.
In Fig. 2, the behavior of the equatorial dipole is shown in a dx - dy plane. In these plots, the right ascension is the polar angle, measured anti-clockwise from the x-axis. The circles shown have a radius equal to the 1σ uncertainties in the dipole components. One can appreciate in this plot how the amplitudes decrease for decreasing energies, and how the phases change as a function of the energy, pointing almost in the opposite direction with respect to the Galactic center for energies above 4 EeV, and not far from it below 1 EeV.
Fig.2: Components of the dipole in the equatorial plane for different energy bins above 0.25 EeV (left panel) and below 1 EeV (right panel).
More data, as well as analyses exploiting the discrimination between the different cosmic-ray mass components that will become feasible with the upgrade of the Pierre Auger Observatory currently being implemented, will be crucial to understand in depth the origin of the cosmic rays at these energies and to learn how their anisotropies are produced.
Related paper:
Cosmic ray anisotropies in right ascension measured by the Pierre Auger Observatory
The Pierre Auger Collaboration, The Astrophysical Journal, Volume 891, 142 (2020)
[arxiv.org/abs/2002.06172] [doi: 10.3847/1538-4357/ab7236]
