Backtracking radio signals for the Xₘax measurement of extensive air showers: A new approach

Precise measurements of the composition of cosmic rays in the energy range of 10¹⁷ - 10¹⁸\,eV could provide crucial insights into the long-standing questions about the origin and acceleration of these particles. Ground-based experiments typically rely on determining the position of the extensive air shower maximum (Xₘax) to identify the type of cosmic ray particle. One effective method for determining Xₘax is by analyzing the radio emission produced by these air showers. This approach offers several advantages, including continuous operation and a higher duty cycle compared to fluorescence telescopes, which are limited by weather conditions and lunar phases. However, conventional radio-based methods often involve computationally intensive Monte Carlo simulations, or rely on pre-calculated parameterizations derived from simulations. In this contribution, we present a new method which is highly efficient and has the potential to reconstruct Xₘax with very minimal input from simulations. This method reconstructs the radio emission profile of air showers by backtracking the radio signals recorded by a ground-based antenna array, considering that the signal received by each antenna travels perpendicular to the radio wavefront. By analyzing simulated proton and iron showers in the 10¹⁷-10¹⁸\,eV range, this study reveals a strong correlation between the radio emission profiles in the 20–80 MHz frequency band, and the longitudinal profile of the air shower.