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The Pierre Auger Observatory has for the first time caught the shadows of the Moon and the Sun cast by particles with record-breaking energies.
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Cosmic rays aren't rays in the usual sense: they're fast particles bombarding Earth from all directions. Mostly they're protons and atomic nuclei. Their energies vary widely, from modest to fantastically huge. When such a particle heads our way and the Moon or Sun gets in its path, it gets absorbed. Behind the celestial body, a 'shadow' appears — in that region, where particles don't reach, instruments register fewer signals.
The Pierre Auger Observatory sits in the Argentine pampas. Its ground-based detectors, spread over 3,000 square kilometers, catch showers of secondary particles created in the atmosphere when a cosmic ray strikes. In the new study, scientists analyzed more than 10 million such events and for the first time saw the shadows of the Moon and the Sun cast by particles with an average energy of 7×10¹⁷ electronvolts. For comparison, that's like the energy of a flying tennis ball, but packed into a single subatomic particle.
Previous shadow observations were limited to energies up to 10¹⁶ eV. The new data leap almost a hundred times further up the energy scale.
The shadows were detected with a combined statistical significance of about 3σ (sigma). That means the probability of a chance coincidence is less than one percent. The observations confirmed that the observatory's ground-based detector array is looking exactly where it should be, with an angular accuracy of about 0.6 degrees. Such calibration using natural 'beacons' is critical for hunting down the sources of cosmic rays — because we still don't know precisely where the most energetic ones are born.
Interactions of cosmic rays with the Moon and the Sun produce deficits in their arrival-direction distributions relative to an isotropic flux. Such shadows have been observed previously at energies between $$10^{12}\,$$eV and $$10^{16}\,$$eV. We report the first observation of the Moon and Sun shadows at cosmic-ray energies larger than about $$10^{16}\,$$eV (average energy of $$7\times10^{17}\,$$eV), using data collected by the Pierre Auger Observatory. We employ data from three detector arrays covering $$3000\,\text{km}^2$$, $$27\,\text{km}^2$$, and $$2\,\text{km}^2$$, with spacings of $$1500\,\text{m}$$, $$750\,\text{m}$$, and $$433\,\text{m}$$, respectively. The data amount to over 10.6 million events. The Moon and Sun shadows are detected with a combined significance of approximately $$3\sigma$$. These observations confirm the pointing accuracy of the Surface Detector of the Pierre Auger Observatory using celestial bodies. From the combined Sun and Moon shadows, we infer an overall angular resolution of $$(0.59^{+0.15}_{-0.11})^\circ$$.
The energy of a particle at 7×10¹⁷ eV is comparable to the impact of a mosquito landing on your arm, but it's packed not into a whole insect, but into a single proton.