We show, for the first time, radio measurements of the depth of shower
maximum (Xmax) of air showers induced by cosmic rays that are
compared to measurements of the established fluorescence method at the same
location. Using measurements at the Pierre Auger Observatory we show full
compatibility between our radio and the previously published fluorescence data
set, and between a subset of air showers observed simultaneously with both
radio and fluorescence techniques, a measurement setup unique to the Pierre
Auger Observatory. Furthermore, we show radio Xmax resolution as a
function of energy and demonstrate the ability to make competitive
high-resolution Xmax measurements with even a sparse radio array.
With this, we show that the radio technique is capable of cosmic-ray mass
composition studies, both at Auger and at other experiments.Comment: Submitted to Phys. Rev. Let