Recent results from the Pierre Auger Observatory are presented, focusing on a measurement of the cosmic-ray energy spectrum above 10^18 eV, cosmic-ray composition, and the anisotropy in the cosmic ray arrival directions. The flux of cosmic rays can be well described by a broken power-law, with a flattening of the spectrum above 4x10^18 eV and a softening of the spectrum beginning at about 3x10^19 eV. The flux suppression at highest energies is consistent with the predictions of Greisen, and Zatsepin and Kuzmin. Longitudinal development of cosmic-ray air showers provides information on the mass of the primary particle. When compared to model predictions, our measurements of the mean and spread of the longitudinal position of the shower maximum are indicating a composition transition from light to heavier with increasing energy. For highest energies in our data-set we observe evidence for a correlation between the cosmic-ray arrival directions and the nearby extragalactic objects.
The Pierre Auger Observatory [1] was designed to measure properties of the extensive air showers produced by cosmic rays with ultra-high energies above 10 18 eV. Since the occurrence of these rare events is of the order of magnitude of 1 per km 2 per century, the Observatory has a large aperture in order to gather a statistically significant sample. The Observatory is featuring complementary detection techniques to lessen some of the systematic uncertainties associated with deducing properties of cosmic rays from air shower observables.
The Observatory is located in the vicinity of the small city Malargüe in Mendoza Province, Argentina, and began collecting data in 2004. The construction of the fundamental design was completed by the end of 2008. Until October 2010 the Observatory has collected around 20 000 km 2 sr yr in exposure, which is significantly more than past cosmic-ray observatories combined. The Observatory is built around two types of detectors. Detectors on the ground sample air-shower particles as they arrive at the Earth’s surface, while fluorescence detectors are measuring the light emitted when air-shower particles excite nitrogen molecules in the atmosphere.
The surface array [2] consists of 1600 fully autonomous surface detector (SD) stations, each being a light-tight tank filled with 12 t of ultra-purified water observed by 3 photomultiplier tubes detecting the Cherenkov light produced as charged particles are traversing the water. The signals from the photomultipliers are read out with flash analog-to-digital converters with 40 MHz sampling and stamped by the GPS time, allowing for detailed study of the arrival-time profile of shower particles. The tanks are placed on a triangular grid with a 1.5 km spacing. The whole array covers an area of 3000 km 2 . The surface array operates with close to a 100% duty cycle, and the acceptance for events with energy above 3×10 18 eV is nearly 100% [3].
The fluorescence detectors (FD) [4] are placed in 4 buildings, each hosting 6 telescopes overlooking the surface array. Each telescope is equipped with 11 m 2 segmented mirror, focusing the fluorescence light entering through a 2.2 m diaphragm onto a camera made of 440 photomultiplier-tube pixels. The photomultiplier signals are sampled with 10 MHz, delivering a time profile of the shower as it develops through the atmosphere. The FD can be operated only in darkness (night) with clear sky conditions (low aerosol and cloud coverage), and has a duty cycle of approximately 10 to 15%. In contrast to the SD, the acceptance of FD depends strongly on the energy of the primary particle [5], and has an useful range extending down to around 10 18 eV.
The two conceptually different detector systems provide complementary information about the particular air shower. The SD measures the lateral distribution and time structure of shower particles arriving at the ground, while the FD measures the longitudinal development of the shower through the atmosphere. Only a relatively small subset of showers is observed simultaneously by the SD and FD. These “hybrid” events are providing an invaluable calibration tool (see Fig. 1 -left). Particularly, the FD is performing a roughly colorimetric measurement of the shower energy since the amount of emitted fluorescence light is proportional to the deposited energy. On the other hand, the SD is extracting the shower energy through analysis of particle densities at the ground. These rely strongly on predictions of hadronic interaction models. Furthermore, we have to use model predictions describing physics at energies far beyond those accessible to current accelerator experiments, where the models have actually been tuned. Hybrid events therefore offer much more reliable estimate for a model-independent energy scale of the SD array. This is the crucial point in the successful design of the Pierre Auger Observatory since the SD has a much greater data sample than the FD due to the greater live time and coverage.
Ultra-high energy cosmic-ray energy spectrum is one of the key parts in understanding the their origin and acceleration processes. These energies are up to eight orders of magnitude higher (or more than one order of magnitude higher in the center-of-mass energy) than those available from human-made accelerators like the LHC. The S 38 • surface-detector energy estimator vs. the energy measured by the fluorescence detector for a sample of 795 high-quality hybrid events used to calibrate the surfacedetector energy estimator. Right: Combined energy spectrum from hybrid and surface-detector events [9]. The flux is multiplied by E 3 to straighten the otherwise steeply falling spectrum. The spectrum is compared to the HiRes results [10] (open circles). The results of the two experiments are consistent within the systematic uncertainties (two-sided arrow).
As noted above, the most reliable measurement of the primary energy is done by the observation of the fluorescence emis
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