A storm in the upper atmosphere lasts a few millionths of a second. A cosmic ray, a proton or a helium nucleus, strikes air molecules, sending a swarm of particles downward. Most of them stay together, but occasionally one particle gains so much energy that it surges ahead. The cascade then splits, producing two humps in the longitudinal profile, like two echoes from a single strike.
Until now, such double showers have only been seen in simulations. Real detectors could not distinguish them from ordinary events. The SKA-Low radio telescope, with its dense antenna array and wide frequency band, changes the game. Its sensitivity allows it to capture the radio signal from two separated shower maxima. This signal exhibits characteristic dips: when waves from the two sources combine, they cancel each other at specific frequencies.
The authors built a simple model, treating each hump as a point source of radio waves. This proved sufficient to estimate the depths of both maxima from the positions of the dips. They then applied a brute-force method: they varied the parameters (depths and particle numbers), and for each set they quickly synthesized an artificial double shower and compared it with a reference. In this way, they recovered the full set of parameters with an uncertainty of 15 to 66 grams per square centimeter.
This is the first time the radio fingerprint of double showers has been used to reconstruct their longitudinal profile. Previously, such events were merely a theoretical curiosity. Now they could become a tool: the separation between humps is sensitive to details of particle interactions that physicists describe with different models. By comparing predictions with observations, one can test these models and obtain an independent estimate of the mass composition of cosmic rays.
The next step is to add realistic noise and the full antenna array configuration, which is an order of magnitude denser than in the current simulation. If the spectral dips survive under real conditions, double showers will no longer be ghosts of simulations. They will become another window into the physics of ultra-high energies.
The frequency of the interference notch in the spectrum of a double shower is related to the distance between the two sub-showers: the farther apart the maxima, the lower the frequency of the first notch. For a typical event at $10^{15}$ eV, the notch falls within the SKA-Low band around 100 MHz.
The idea of using interference of signals from separated sources echoes the principle of radio interferometry described in Carl Sagan's novel 'Contact', where the protagonist analyzes a signal coming from different directions to pick out a weak source against the noise.