7. Chemical Composition 8. Anisotropy of Arrival Directions 5 Fig. 7. The average X max as a function of the energy for HiRes stereo [26] and Pierre Auger Observatory hybrid data [25] compared to proton and iron predictions using different hadronic interaction models (solid curves) and different mod- els of UHECR origin [11]. There are still no results above the GZK energy. The knowledge of the mass composition of ultra high energy cosmic rays is extremely important for the study of cosmic rays. For example the hardening of the cosmic ray energy spectrum at the ankle can be explained either by the transition of galactic heavy nuclei to extragalac- tic protons or by a distortion of the extragalactic proton spectrum due to energy losses (see e.g. [24]). More- over, a measurement of the cosmic ray composition is essential to understand the nature of the flux suppres- $^{sion observed above 4}$× 10 19 eV. Due to the low flux of cosmic rays at these energies the composition can not be measured directly but has to be inferred from observations of the extensive air showers. The atmospheric depth X$_{max}$ at which the longitudinal development of the extensive air shower reaches its max- imum is a measure of how fast the energy of the primary particle is transferred to electromagnetic sub-showers. The average of the shower maximum < X$_{max}$ > and particularly its change per decade of energy are sensitive to the particle composition. Another important feature are shower-to-shower fluctuations, which are expected to decrease with the number of nucleons in primary nucleus and to increase with the nuclear interaction length. The fluorescence detector of the Pierre Auger Obser- vatory can be used to measure with good resolution the shower longitudinal profile and the depth at which the shower reaches its maximum size [25]. Proton showers penetrate deeper into the atmosphere and have wider X$_{max}$ distributions than heavy nuclei. The atmospheric depth X$_{max}$ where an air shower reaches its maximum size is measured shower-by-shower with a resolution of 20 g/cm 2 at the Pierre Auger Observatory. The change from protons to heavier composition is observed also in the measured fluctuations of X$_{max}$ . The comparison with results from the previous ex- periment HiRes [26] shows agreement within their un- certainties at the highest energies. However, the re- sults published by HiRes experiment focused only on the < X$_{max}$ > as a function of energy, but did not include a study of the energy evolution of the X$_{max}$ -fluctuations and could not reach far above 10 19 eV because of limited statistics. The mass composition interpretation of the measured quantities depends on the assumed hadronic model. There is a problem that uncertainties on the pre- dictions of hadronic models are unknown, because each model is based on an extrapolation of the physics from lower energies. Searches for anisotropies have been done in sky re- gions, where some candidates for UHECRs sources can be located. An attractive candidate is the Galactic Cen- tre region, since it houses some objects that are candi- dates for powerful accelerators. Neutrons could be pro- duced due to interactions of accelerated particles with ambient matter. The distance of the Galactic center from the Earth is 8.5 kpc and a significant fraction of neutrons with energies higher than 10 18 eV will survive their path to the Earth. As neutral particles the neu- trons will point back to their site of origin, which in this case is around the position of the Galactic Centre. The exposure of the position of the Galactic Centre at the Pierre Auger Observatory is higher than that of previous experiments. However, the results of the Pierre Auger Observatory do not show significant excesses for a point- like source or overdensities for larger angular windows around the Galactic Centre [27]. One of the most important results found in the study of arrival directions was presented by the Pierre Auger The largest excess of events as compared to isotropic expectations is observed from a region of the sky close to the location of the radio source Cen A (galactic coor- dinates l=50.5 ◦ , b=19.4 ◦ ). The excess of events in cir- cular windows around Cen A with the smallest isotropic chance probability corresponds to a radius of 18 ◦ , which contains 12 events where 2.7 are expected on average if the flux were isotropic. By contrast, the region around the Virgo cluster is densely populated with galaxies but does not have an excess of events above isotropic expec- tations. In particular, a circle of radius 20 ◦ centred at the location of M87 (galactic coordinates l=76.2 ◦ , b=74.5 ◦ ) does not contain any of 58 events. However, this is a region of relatively low exposure for the Pierre Auger Observatory and only 1.2 event is expected on average with the current statistics if the flux were isotropic. Since then 31 additional events were measured and eight of them have arrival directions within the pre- scribed area of the sky (i.e. around the positions of se- lected AGN). This update of the analysis of the correla- tion with AGN, with nearly twice the previous exposure was done, neither strengthens nor does contradict the earlier result [31]. The anisotropy of UHECRs arrival directions at the highest energies is still observed. Collaboration in 2007 [28, 29]. Using data collected between 1 January, 2004 and 31 August, 2007, the Pierre Auger Observatory has reported the evidence of anisotropy in the arrival directions of cosmic rays with energies exceeding 6 × 10 19 eV. The arrival directions were correlated with the positions of nearby objects from the 12th edition of the catalog of quasars and active galactic nuclei by Veron-Cetty and Veron [30]. In orig- inal analysis 20 from 27 measured events were found closer than 3 . 1 ◦ from positions of AGN lying within 75 megaparsecs (see Fig. 8.). This degree of correlation led to the rejection of the hypothesis of an isotropic distri- bution of these cosmic rays with at least a 99% con- fidence level from a prescribed a priori test. The ob- served correlation is compatible with the hypothesis that the highest-energy particles originate from nearby extra- galactic sources whose flux has not been substantially reduced by interactions with the cosmic background ra- diation. AGN or objects having a similar spatial distri- bution could be possible sources.