Lofar hears cosmic rays hit

The earth is bombarded with high-energy particles from space. Although these do not generally affect us, astronomers are interested in their origin. For the first time scientists have studied the radio waves emitted by these particles, using the Lofar radio telescope in the Netherlands. They published an article about this in the journal Nature on 3 March.

A cosmic ray that enters the atmosphere at almost the speed of light soon collides with atomic nuclei, generating a shower of particles that can collide once again. In an area spanning 3000 square kilometres the Pierre Auger Observatory i n Argentina has a large number of detectors on the ground that can capture the droplets of this shower.

The particle shower also produces radio waves, a flash lasting a few billionths of a second. During the construction of the Lofar radio telescope it seemed like a good idea to see whether it could be used to collect data about high-energy particles. The burning question was whether they originate from within our own Milky Way or beyond.

‘The higher the energy of a particle, the deeper in the atmosphere the shower penetrates’, explains Olaf Scholten, professor of physics at the KVI - Center for Advanced Radiation Technology . He is one of the authors of the Nature article. ‘By measuring the point of greatest activity, you can work out how much energy was contained in a particle that struck.’

The energy falls to earth in a cone shape. The smaller the cone on the ground, the lower in the atmosphere its peak. For the measurements, only the Lofar stations on what is known as the superterp, a circular elevation with a diameter of 300 metres at the heart of the telescope, were used. ‘You look at which stations do and which do not receive a signal. This allows you to determine the size of the cone.’

The information that this provides about the particle shower is then translated into information about its cause. ‘We are able to model the results of strikes by different types of nucleus. We use simulations to discover which particles correspond best with the signal that we measure.’

High-energy particles range from the small (individual protons) to the relatively large (iron atoms). From the Lofar measurements it could be seen that a mixture of light and heavy nuclei struck. ‘Our models show that particles from other galaxies are generally light. Heavy particles are an indicator that they come from our own Milky Way.’ The fact that Lofar found a mixture suggests that the source was within the Milky Way. ‘That could be a very heavy star that works like a particle accelerator.’ That question therefore appears to have been answered.

The cosmic strikes could still provide us with further knowledge, however. The particles measured possess more energy than the protons that the largest accelerator on earth, the LHC in Geneva, can generate. ‘The cosmic rays are like a natural experiment that we can use to gather new information.’ This is currently possible with Lofar, and in a number of years it will also be possible with the Square Kilometre Array (SKA), which will be built in Australia and South Africa.

The experiments began a few years ago already, before Lofar was officially operating. At that time the first author, Stijn Buitink, was working at the University of Groningen. He later moved to Radboud University of Nijmegen and then to the Vrije Universiteit Brussel, which is where Scholten also works alongside his work at the University of Groningen. ‘The project started here in Groningen, but Nijmegen and the VUB went on to make an important contribution too.’

A total of 118 particle showers were detected over 150 observation days. This may not seem like much, says Scholten, but the Pierre Auger Observatory studies about ten per year. ‘But they look at particles with even more energy, which are much rarer.’ Funny to think that a few FM radio antennae in a nature reserve in Drenthe can be used for high-energy physics.

Reference: A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations. Stijn Buitink et al. Nature 3 maart doi:10.1038/nature16976