Further along the celestial path to the origin of extreme-energy cosmic rays
Ultra-high-energy cosmic rays(opens in new window) (UHECRs) are the most energetic particles ever measured. According to Ioana Maris(opens in new window) from ULB(opens in new window), at about a million times more energetic than the particles produced at CERN’s largest hadron collider, these energies are unattainable by human-made accelerators. And she adds: “It has been calculated that only four particles could light a 60 W bulb for a second.” “UHECRs are messengers from the most violent phenomena in our universe, offering a unique opportunity to explore fundamental particle physics. But despite now knowing that they are extragalactic, we still don’t know exactly where they were produced, or how a galaxy accelerates them,” says Maris, coordinator of the EU-funded GADGET project, set up to probe these questions. As charged particles, thanks to the galactic and extragalactic magnetic fields present in the entire universe, UHECRs don’t travel along a straight path. Consequently, calculating their trajectory relies on detailed knowledge of these magnetic fields. GADGET’s work focused on improving magnetic field modelling, introducing new measurements and understanding the effects of local phenomena, shown to be crucial to modelling accuracy.
Creating a 3D description of the magnetic field
Prior to GADGET, project collaborator Michael Unger(opens in new window) and colleagues had created 3D magnetic field models using two sets of measurements: of synchrotron radiation(opens in new window) and Faraday rotations(opens in new window). Diffuse synchrotron (electromagnetic) radiation is emitted by Milky Way electrons spiralling around magnetic fields at radio wavelengths and speeds proportional to the curved path created by the magnetic field, while the Faraday rotation of light polarisation from distant sources, like pulsars, depends on the strength of the magnetic field. GADGET’s contribution was to introduce insights about the effects of the local universe. “We live in a local void, the so-called ‘Local Bubble’, where the matter around us has a density about 20 times lower than the interstellar medium, probably due to multiple supernova explosions that whipped out the matter, influencing the magnetic fields,” explains Maris. Using dust density maps, Marie Skłodowska-Curie Actions(opens in new window) fellow Vincent Pelgrims(opens in new window) developed a model explaining how these violent supernova explosions influence the magnetic field, enabling him to deduce the shape of the Local Bubble, and so model the local universe magnetic field. Combined with Michael’s work, this offers valuable information about the shape of the galactic magnetic field. “Including GADGET’s findings changed the interpretation of the synchrotron radiation, especially at large latitudes, emphasising the importance of taking the Local Bubble into account when modelling the galactic magnetic fields,” explains Maris.
Backtracking to the point of origin
A key challenge when investigating UHECRs is their rarity, with only one particle per square kilometre per century reaching the Earth – around 10 000 particles daily. While impossible to build an Earth-sized detector, the biggest, the Pierre Auger Observatory in Argentina, has been recording data since 2004, allowing researchers to measure over 100 particles at the highest energies. The team’s collaboration with the Observatory, recently upgraded with extra detectors for increased sensitivity and precision, is holding out the very real prospect of locating the origin of UHECRs, with first results recently presented at a series of conferences. “Backtracking or tracing the origin of the UHECRs would be impossible without our account for the influence of magnetic fields,” notes Maris. “We’ve seen hints that UHECRs could be produced in starburst galaxies with very high new star formation rates, or in the region of Centaurus. I am very excited that we could soon unveil the mystery of these enigmatic high-energy particles.”