Cosmic-rays are highly energetic particles originating from outer space. Ultrahigh energy cosmic-rays (UHECRs) are defined as those above 10^18 eV. The Pierre Auger Observatory is a hybrid detector comprising a surface array of over 1660 water-Cherenkov detectors and 27 nitrogen-fluorescence detectors, the data from which can be studied separately or combined in hybrid mode. Data-taking began
in 2004, with construction of the array completed in 2008.
The mass-composition of UHECR, in particular the flux of photons, is currently unknown. UHECR photons are expected from the interaction of protons with energies greater than ∼ 3×10^19 eV with the cosmic microwave background.
Previous limits on the fraction of UHECR photons from surface array data are of the order of a few % above 10^19 eV.
Surface array data have been used to update and improve a mass-sensitive shower-timing parameter, (Δ), derived from the signal risetimes of individual detectors. A complete overhaul of this method has been performed, providing a
more robust parameter sensitive to the mass-composition of UHECR. The change of (Δ) with energy has been investigated and a correlation between (Δ) and another mass-sensitive parameter - the depth of maximum, Xmax, has been found.
A study of the mass-sensitivity of (Δ) has been made by comparison to photon and hadronic simulations. From this comparison it is found that the composition
of UHECRs, on average, tends towards heavier primaries with increasing energy.
Ten events have been identified as potential photon-initiated air showers. Conservative
integral upper limits to the flux of UHE photons have been computed at 8.7 × 10^−3, 4.7 × 10^−3, 2.8 × 10^−3 and 2.3 × 10^−3 km^−2 sr^−1 yr^−1 above 10, 20, 30 and 40EeV respectively. Integral upper limits to the fraction of photons have also been found at 1.5%, 3.2%, 4.9% and 9.1% above these energies. These new limits improve upon previous works and exclude all ‘top-down’ models for photonproduction except the Z-burst model, which is strongly disfavoured. These limits do not yet probe the GZK region.
The arrival directions of these photon-candidates have been compared to the positions of nearby AGN from the VCV and Swift-BAT catalogues. No obvious sources have been found for these events and none of the photon-candidate arrival
directions lie close to Cen A.
The differences between those events above 5 × 10^19 eV that correlate with AGN from the VCV catalogue, or originate from within 18◦ angular separation of Centaurus A, to those that do not, have been studied using the (Δ) parameter.
No significant differences in their average (Δ) have been found, nor is a systematic change in (Δ) observed as a function of angular distance from Cen A.
Finally, the azimuthal asymmetry on the risetimes has been reviewed as a potential mass-sensitive parameter in relation to previous works. A possible alternative
method for determining the dependence of the asymmetry on the energy has been explored with limited success. This new method is in the early stages of development and further study is required
