2.5 Anisotropy
3.3.4 Additional Detection Techniques
Measurements by Gorham et al. [71] gave the indication of the possibility to measure the molecular bremsstrahlung radiation in the microwave regime, from collisions of free electrons in the post-shower. The results of different test facilities have been published, for example in [72–74].
Chapter 4
The Pierre-Auger-Observatory
The Pierre Auger Observatory is located in the Pampa Amarilla, in the province Mendoza and north of the town Malarg¨ue and is currently the largest detector for ultra-high energy cosmic rays (UHECR). Fig. 4.1a shows the po- sition in a map of South America, while Fig. 4.1b gives a schematic overview of the array. The basic design as a hybrid detector consists of the Fluores- cence Detector (FD) and of the Surface Detector (SD), which are described in more detail in sections 4.1 and 4.2. The aim is to study cosmic rays, via the detection of air showers, at the highest energies E > 1017eV. When both
detectors observe the same showers, this delivers the capability of studying systematics effects, as well as an improvement of the shower parameters, as for example arrival direction, energy or composition by the combination of measured data.
In 2004 the data taking was started with the first several hundreds of de- ployed detectors, while in 2008 the array was completed. Several low energy extension were deployed, as the SD-Infill, a region with a lower spacing of the SD-Detectors, AMIGA, a muon counter burrowed under the SD-Detector or the HEAT extension of the FD, a standard FD telescope which is tilted by 30◦ to a higher field of view. Another enhancement is the Auger Engineer- ing Radio Array (AERA), an original test facility for the radio detection of cosmic rays, which is today running stable as one detection component. A more detailed description follows in section 4.3.
Today the international collaboration consists of ∼ 500 scientists from more than 60 institutions spread around the world.
Section 4.4 will give a short introduction of the Auger software analysis framework, called Offline.
CHAPTER 4. THE PIERRE-AUGER-OBSERVATORY
(a) Map of South America (b) Map of the Pierre-Auger-Observatory
Figure 4.1: (a) shows the location of the Pierre-Auger-Observatory near Malarg¨ue, Mendoza in Argentina, from [75].(b) gives an overview of the Pierre Auger Observatory. Each dot indicates the position of a water-Cherenkov de- tector, the fluorescence telescopes and their field of view are represented by the solid blue lines, for the HEAT extension orange lines. Additionally the positions of AERA as well as the atmospheric monitoring facilities Balloon Launching Site BLS (‘balloon’), Central Laser Facility CLF and eXtreme Laser Facility XLF are indicated, from [76].
4.1
Fluorescence Detector
As already described, the frequency range of the fluorescent light is in a wavelength range of 300-400 nm. Thus, the telescopes are optimized for this range. The incoming light is first filtered by a MUG-6-filter which is trans- parent up to 410 nm in the ultraviolet. The camera is constructed following a Schmidt camera scheme, but with some modifications. To enlarge the field of view, a correction ring is located behind the filter, which directs light, which would pass on the edge of the camera, back on this. A 13 m2 large mirror
of several segments reflects the incident photons to the camera. The cam- era is composed of 440 hexagonal photomultiplier tubes (PMTs), which are arranged in a honeycomb structure with 20 x 22 PMTs. The entire camera has a size of approximately 90 x 90 cm, while a PMT has the width of about 4.3 cm. The field of view of a camera is 30◦ × 30◦. The three telescopes of
the HEAT extension can be inclined by 30◦, which leads to a vertical viewing angle of 30◦ - 60◦. The measured signals are digitized with a sampling rate of
4.1. FLUORESCENCE DETECTOR
10 MHz. A trigger decision is formed afterwards by a hierarchical structure of signal patterns and sent to the Central Data Acquisition System (CDAS), where even single SD detectors, corresponding to the FD event, are read out.
(a) Schematic View of a FD camera (b) Picture of a FD camera
Figure 4.2: Schematic view [77] (a) and a picture [78] (b) of a fluorescence telescope of the Pierre Auger Observatory.
Six of these telescopes are housed in each of the four buildings in a kind of semi-circle. These are located, as can be seen in Figure 4.1b, at the edges of the observatory: Los Leones in the South, Los Morados in the East, Loma Amarilla in the North and in the West Coihueco. The three telescopes of the HEAT extension are located at Coihueco.
Using the pointing direction of the pixels and the time information the shower axis can be determined. In addition the SD data can be used to restrict the geometry. This results in a resolution of the arrival direction below 1◦. As described in section 3.3.2 the Gaisser-Hillas function is fitted to describe the longitudinal shower profile, which leads to an accuracy of ∼ 20% in the energy estimation, including systematic uncertainties and a resolution of ∼ 20 g/cm2
in the position of the shower maximum Xmax [79].
For energies above ∼ 1018 eV the detection of the FD gets fully efficient.
Due to the limited observation time (dark, cloud less night) the uptime is about only ∼ 15%. To ensure high quality data, the atmospheric conditions, especially the aerosols, are monitored during each FD shift with the XLF and CLF laser facilites [80] inside the array. An additional LIDAR system is located at each FD building [81].
CHAPTER 4. THE PIERRE-AUGER-OBSERVATORY