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The azimuthal asymmetry in the risetime of signals in Auger surface detectorstations is a source of information on shower development. The azimuthalasymmetry is due to a combination of the longitudinal evolution of the showerand geometrical effects related to the angles of incidence of the particlesinto the detectors. The magnitude of the effect depends upon the zenith angleand state of development of the shower and thus provides a novel observable,$(\sec \theta)_\mathrm{max}$, sensitive to the mass composition of cosmic raysabove $3 \times 10^{18}$ eV. By comparing measurements with predictions fromshower simulations, we find for both of our adopted models of hadronic physics(QGSJETII-04 and EPOS-LHC) an indication that the mean cosmic-ray massincreases slowly with energy, as has been inferred from other studies. However,the mass estimates are dependent on the shower model and on the range ofdistance from the shower core selected. Thus the method has uncovered furtherdeficiencies in our understanding of shower modelling that must be resolvedbefore the mass composition can be inferred from $(\sec \theta)_\mathrm{max}$.

Azimuthal asymmetry in the risetime of the surface detector signals of the Pierre Auger Observatory