Reconstructing air showers with LOFAR using event specific GDAS atmospheres

Abstract

Estimating the depth of shower maximum $\mathrm{X_{max}}$ with high precision is of great interest for the study of primary particle composition. One of the systematic uncertainties in reconstructing $\mathrm{X_{max}}$ from the radio emission of air showers is the limited knowledge of the atmospheric parameters like humidity, pressure, temperature and the index-of refraction. Using the Global Data Assimilation System (GDAS), a global atmospheric model, we have implemented time-dependent realistic atmospheric profiles in the air shower simulation codes CORSIKA and the radio plug-in CoREAS. This program is now available within CORSIKA and flexible to be adapted for different air shower experiments. We have analyzed the LOFAR cosmic ray data with dedicated simulations for each detected air shower with event specific GDAS atmospheres and investigated the effects of pressure and humidity on the reconstructed $\mathrm{X_{max}}$. This study shows that for bulk of the events, where the ground pressure is close to US standard atmosphere values, there is a small systematic shift in $\mathrm{X_{max}}$ that is less than 2 $\mathrm{g/cm^{2}}$ and for very low pressure values the shift is up to 15 $\mathrm{g/cm^{2}}$