Cosmic-rays, that is, the high energy subatomic particles, which constantly bombard the Earth’s atmosphere, were discovered by Victor Hess in 1912 in a series of very daring, high altitude balloon flights. With the measurements they made, Hess and his team showed that the ionization level of air in the atmosphere increases with altitude, a confirmation that extra-terrestrial high-energy particles, were constantly impacting atmospheric molecules.
Fifty years later, in the Volcano Ranch experiment, led by John Linsley, the first ultra-high energy cosmic ray (UHECR) with energy exceeding 1020 eV, roughly 1 billion times higher than the energy of protons accelerated at the Large Hadron Collider, was discovered. This discovery led to the development of the scientific field dedicated to the search for the origin of UHECRs, the most energetic particles ever produced after the Big Bang.
Extensive Air Showers
When a 1020 eV particle collides with an air molecule in the upper atmosphere, a shower of lower energy particles develops in the atmosphere. By the time the shower reaches ground level, millions of these lower energy, secondary particles have been produced, and the footprint of the shower can extend more than 3 km2 across, that is, roughly 1/3rd of the surface area of State College, PA. By studying the air showers, scientists can measure the properties of the original cosmic ray particles.
The Pierre Auger Observatory
The world’s largest UHECR detector, the Pierre Auger Observatory (hereafter Auger), is located in the Pampa Amarilla, in the Mendoza region of Argentina. The experiment covers an area of 3000 km2, that is, more than 30 times the surface area of Paris. Auger consists primarily of two types of extensive air-shower detectors. Water Cherenkov surface stations, and Fluorescence telescopes. The water Cherenkov stations are large plastic tanks, each containing 10 tonnes of purified water that register the blueish Cherenkov light which gets produced when particles travelling at the speed of light collide with the water molecules inside the tank. There are 1660 such stations, spaced 1.5 km apart, forming the Auger surface array. They have 100% duty cycle. At the four edges of the array, surrounding the surface detectors are 27 fluorescence telescopes. These operate only on dark moonless nights, detecting the fluorescence light produced by the de-excitation of nitrogen molecules as the shower propagates in the atmosphere. The power produced by a single UHECR shower is roughly that produced by a single 60 watt light bulb travelling at the speed of light, so this is a challenging measurement. The combination of the fluorescence technique and surface detectors allows us to see UHECR showers in “hybrid” mode, providing the most accurate measurement of the properties of the air shower.
Where do UHECRs come from?
The origin of UHECRs is a mystery. In what kinds of sources do particles get accelerated to 5×1019 eV? It is this question that scientists working in Auger have been addressing. Although we don’t yet have a definitive answer, there are certain minimal requirements that an astrophysical source must satisfy in order to be a plausible UHECR accelerator. A simple requirement can be stated as follows: the Larmor radius of the particle in the magnetic field of the source, must be larger than the radius of the acceleration site, in order for the particle to be effectively confined in the source. This requirement is what limits the energy to which protons can be accelerated in the Large Hadron Collider; the strength of the magnets, and the radius of the tunnel. This simple argument, known as the Hillas criterion, rules out many known classes of powerful astrophysical sources as possible UHECR acceleration sites: supernova explosions, regular galaxies, including our own Milky Way, white dwarf stars, etc.; the list is long. It only leaves a few known source classes as possible UHECR accelerators, all of which are extragalactic: gamma-ray bursts, active galactic nuclei, neutron starts, and rare, powerful shocks in the intergalactic medium.
Two features of the intergalactic propagation of UHECRs make us hopeful that we can discover the origin of UHECRs, by finding associations between known powerful astrophysical accelerators and UHECR arrival directions. Firstly, cosmic rays with energy exceeding 5×1019 eV have a short propagation horizon. They are so energetic that when they collide with a photon from the Cosmic Microwave Background, the cosmic photons that are left over from the Big Bang, they produce a Pi meson (or pion). This is an interaction that results in significant energy loss (generally between 14-50%) for the cosmic ray. The average distance that a 6×10^19 eV a stripped Hydrogen nucleus (i.e. a proton) can travel, while retaining 0.23 of it’s energy is 100 megaparsecs (Mpc): a small distance in extragalactic terms. An analogous (and often smaller) horizon exists for heavier elements such as Helium, Carbon, Oxygen, Iron, Silicon: other frequently observed cosmic ray species. In summary, independent of the chemical composition, the sources of UHECRs must be powerful astrophysical accelerators within ~100 Mpc.
Secondly UHECRs are expected to experience small deflections, after they escape their sources, in the weak magnetic fields that permeate the intergalactic medium. A 5×1019 eV proton is expected to experience a deflection, θ, of 3 degrees or less, over 100 Mpc of propagation. Therefore, the arrival direction of protons are expected to exhibit a correlation with the positions of their sources, within a few degrees. Deflections scale linearly with charge Z, as θZ(E) = Z x θproton(E), hence heavier nuclei are expected to have more significantly deviated arrival directions.
Searches for associations so far, have not resulted in statistically significant correlations between known extragalactic sources, and observed UHECR arrival directions (see e.g. Oikonomou et al. 13, Oikonomou et al. 15, Abreu et al. 14). The absence of a clear correlation is slightly disappointing, as well as puzzling: we still haven’t solved this mystery! This absence of associations could mean that UHECRs are heavy nuclei (e.g. Carbon nuclei), thus suffering large deflections, and not pointing back to their sources, or that magnetic fields in the direction of (at least some of) the sources of UHECRs are stronger than we previously inferred from independent astrophysical measurements. Other possibilities include an origin of UHECRs in some unknown population of astrophysical sources, or something more exotic, like the decay of supermassive primordial particles left over from the Big Bang. Despite the lack of a clear answer so far, the origin of UHECRs remains a very active research topic within the Auger Collaboration, and beyond. As more data get collected with existing, and future experiments, we will get closer to the answer to this mystery.