The reconstruction of extensive air showers (arrival direction, core position and energy estimation) by the surface detector of the Pierre Auger Observatory is discussed together with the corresponding accuracy. We determine the angular reconstruction accuracy as a function of the station multiplicity by using two different aproaches. We discuss statistical and systematic uncertainties in the determination of the signal at 1000 m from the core, S(1000), which is used to estimate the primary energy.
The Pierre Auger Observatory has been taking data on the highest energy cosmic rays for the equivalent of over a year of full detector aperture. Comments are presented on results published so far.
Highlights from the Pierre Auger Observatory are presented. In particular there is a detailed discussion of of the cosmic ray energy spectrum from 0.3 EeV to 100 EeV and of the mass composition above 1 EeV.
Several analyses of the data collected by other experiments have found an excess of cosmic rays in correlation with subclasses of BL Lacs. Data from the Pierre Auger Observatory do not support previously reported excesses. The number of events correlated with BL Lac positions is compatible with that expected for an isotropic flux.
Earth--skimming UHE tau neutrinos have a chance to be detected by the Fluorescence Detector (FD) of Pierre Auger Observatory if their astrophysical flux is large enough. A detailed evaluation of the expected number of events is here performed for a wide class of neutrino flux models.
Fluorescence detectors of the Pierre Auger Observatory have been operating in a stable manner since January 2004. After a brief review of the physical characteristics of the detectors, the associated atmospheric monitoring, the calibration infrastructure and the detector aperture, we will describe the steps required for the reconstruction of fluorescence event data, with emphasis on the shower profile parameters and primary energy.
The Pierre Auger Observatory, a hybrid detector for the study of ultra-high energy cosmic rays (UHECRs), is now approaching completion. After describing Auger present status and performance, with an emphasis on the advantages provided by the combination of two different detection techniques, this contribution presents a brief panorama of the first scientific results achieved and of their impact on our knowledge of the UHECRs' origin and composition.
The Pierre Auger Observatory has the capability of detecting neutrino-induced extensive air showers by searching for very inclined showers with a significant electromagnetic component. In order to study the detector response of the surface array of the Pierre Auger Observatory, Monte Carlo simulations of up-going and down-going neutrino showers for all flavors were performed. A set of relevant observables were determined to discriminate these showers from the background of very inclined hadronic showers and the identification efficiency was studied. The acceptance and the expected event rates,…
Data taken at the Pierre Auger Observatory are used to search for air showers initiated by ultra-high energy (UHE) photons. Results of searches are reported from hybrid observations where events are measured with both fluorescence and array detectors. Additionally, a more stringent test of the photon fluxes predicted with energies above 10^19 eV is made using a larger data set measured using only the surface detectors of the observatory.
Major uncertainties in the fluorescence energy measurement come from the precision of various atmospheric transmission, air Cerenkov subtraction, light multiple-scattering and cloud corrections. The Auger program for atmospheric monitoring, designed to measure these corrections and to minimize these uncertainties, is summarized in this paper.
If clustering of the arrival directions of ultra high energy cosmic rays is discovered, this would provide important information about their origin, composition, and the galactic and extragalactic magnetic fields. We present here the analysis of the autocorrelation function of the data from the Pierre Auger Observatory as a function of the angular scale and the energy threshold. We compare our results with the signals found by previous experiments.
Many models for ultra-high energy cosmic rays postulate exotic scenarios to explain the sources or the nature of these particles. A characteristic feature of these models is the prediction of a significant flux of photons at ultra-high energy. The Pierre Auger Observatory offers a great potential to search for such photons. We present shower observables with sensitivity to photons and the search strategy employed. An upper limit to photon primaries is derived from first Auger data. Prospects for constraining theoretical source models are discussed.
We present the status and the recent measurements from the Pierre Auger Observatory. The energy spectrum will be described and its steepening discussed. The mass composition is addressed with the measurements of the variation of the depth of shower maximum with energy. We also report on upper limits in the primary photon fraction. And finally, searches for anisotropies of cosmic rays arrival directions are reported.
While the completion of the Pierre Auger Observatory (or simply ``Auger'') is still underway, the 5165 km^2.sr.yr integrated acceptance accumulated since the January 1st, 2004 is now significantly larger than what was gathered by the previous experiments dedicated to the detection of ultra-high-energy cosmic rays (UHECRs). We report on the development status of Auger and present some results related to the cosmic-ray energy spectrum, composition and anisotropies, and the photon fraction at ultra-high energy. We briefly discuss the importance of the ankle region to understand the overall phenom…
The rate of events measured with the surface detector of the Pierre Auger Observatory is found to be modulated by the weather conditions. This effect is due to the increasing amount of matter traversed by the shower as the ground pressure increases and to the inverse proportionality of the Moliere radius to the air density near ground. Air-shower simulations with different realistic profiles of the atmosphere support this interpretation of the observed effects.
We present a method to measure the relative spectral response of the Pierre Auger Observatory Fluorescence Detector. The calibration was done at wavelengths of 320, 337, 355, 380 and 405 nm using an end-to-end technique in which the response of all detector components are combined in a single measurement. A xenon flasher and notch-filters were used as the light source for the calibration device. The overall uncertainty is 5%.
The building block of the surface detector of the Pierre Auger Observatory is a water Cherenkov tank. The response to shower particles is simulated using a dedicated program based on GEANT4. To check the simulation chain, we compare the simulated signals produced by cosmic muons at various zenith angles with experimental data from a special Cherenkov detector equipped with a muon hodoscope. The signals from muon-decay electrons and the evolution of the charge with water level are also studied.
The Pierre Auger Observatory can detect with high efficiency the air showers induced by ultra-high energy cosmic rays incident at large (> 60 deg.) zenith angles. We describe here the specific characteristics of inclined and horizontal showers, as well as the characteristics of their signal in the surface detector. We point out their relevance both to extend the potential of the detector, and in the context of the detection of high-energy cosmic neutrinos.
The focus of this article is on recent results on ultra-high energy cosmic rays obtained with the Pierre Auger Observatory. The world's largest instrument of this type and its performance are described. The observations presented here include the energy spectrum, the primary particle composition, limits on the fluxes of photons and neutrinos and a discussion of the anisotropic distribution of the arrival directions of the most energetic particles. Finally, plans for the construction of a Northern Auger Observatory in Colorado, USA, are discussed.
With the Pierre Auger Observatory we have the capability of detecting ultra-high energy neutrinos by searching for very inclined showers with a significant electromagnetic component. In this work we discuss the discrimination power of the instrument for earth skimming tau neutrinos with ultra-high energies. Based on the data collected since January 2004 an upper limit to the diffuse flux of neutrinos atEeV energies is presented and systematic uncertainties are discussed.
The Pierre Auger Observatory detects ultra-high energy cosmic rays by implementing two complementary air-shower techniques. The combination of a large ground array and fluorescence detectors, known as the "hybrid" concept, means that a rich variety of measurements can be made on a single shower, providing much improved information over what is possible with either detector alone. In this paper the hybrid reconstruction approach and its performance are described.
The Pierre Auger Observatory was designed for a high statistics, full sky study of cosmic rays at the highest energies. Energy, direction and composition measurements are intended to illuminate the mysteries of the most energetic particles in nature. The Auger Observatory utilizes a surface array together with air fluorescence telescopes which together provide a powerful instrument for air shower reconstruction. The southern part of the Auger Observatory, now under construction in the Province of Mendoza, Argentina, is well over half finished. Active detectors have been recording events for on…
Taking into account the Pierre Auger Observatory limits on the photon fraction among the highest energy cosmic rays, we show that the models based on the decay of super-heavy dark matter in the halo of our Galaxy are essentially excluded from being the sources of UHECRs unless their contribution becomes significant only above 100 EeV. Some top-down models based on topological defects are however compatible with the current data and may be best constrained in the future by the high-energy neutrino flux limit.
The southern site of the Pierre Auger Observatory is under the construction near Malargue in Argentina and now more than 60% of the detectors are completed. The observatory has been collecting data for over 1 year and the cumulative exposure is already similar to that of the largest forerunner experiments. The hybrid technique provides model-independent energy measurements from the Fluorescence Detector to calibrate the Surface Detector. Based on this technique, the first estimation of the energy spectrum above 3 EeV has been presented and is discussed in this paper.
The water-Cherenkov tanks of the Pierre Auger Observatory can detect particles at all zenith angles and are therefore well-suited for the study of inclined and horizontal air showers (60 - 90 degrees). Such showers are characterised by a dominance of the muonic component at ground, and by a very elongated and asymmetrical footprint which can even exhibit a lobular structure due to the bending action of the geomagnetic field. Dedicated algorithms for the selection and reconstruction of such events, as well as the corresponding acceptance calculation, have been set up on basis of muon maps obtai…
The Southern part of the Pierre Auger Observatory is nearing completion, and has been in stable operation since January 2004 while it has grown in size. The large sample of data collected so far has led to a significant improvement in the measurement of the energy spectrum of UHE cosmic rays over that previously reported by the Pierre Auger Observatory, both in statistics and in systematic uncertainties. We summarize two measurements of the energy spectrum, one based on the high-statistics surface detector data, and the other of the hybrid data, where the precision of the fluorescence measurem…
We consider the hypothesis that luminous infrared galaxies (LIRGs) are sources of the UHECRs. By associating the AGASA triplet with the Arp 299 galaxy we obtain reasonable values for Galactic and extragalactic magnetic fields. We predict what the southern sky, to be seen by the Auger experiment, should look like, so that the LIRG hypothesis could be verified soon.
Measurements of air showers are accumulating at an increasing rate while construction proceeds at the Pierre Auger Observatory. Although the southern site is only half complete, the cumulative exposure is already similar to those achieved by the largest forerunner experiments. A measurement of the cosmic ray energy spectrum in the southern sky is reported here. The methods are simple and robust, exploiting the combination of fluorescence detector (FD) and surface detector (SD). The methods do not rely on detailed numerical simulation or any assumption about the chemical composition.
We discuss the statistical and systematic uncertainties in the event reconstruction (core location, and determination of S(1000), i.e., the signal at a distance of 1000 m from the shower core) by the Pierre Auger surface detector for showers with zenith angle less than 60 degrees. The method is based on a maximum likelihood method where the reference lateral distribution function is obtained through the experimental data. We also discuss S(1000) as primary energy estimator.
The Pierre Auger Observatory aims to determine the nature and origin of the ultra-high energy cosmic rays (UHECR). The Auger hybrid detector combines fluorescence observations of extended air showers, initiated in the atmosphere by these most energetic particles, with measures of the shower front at the ground level by its large array of Cherenkov water tanks. This allows to improve considerably the precision on reconstructed primary cosmic ray parameters, and to make important cross-calibrations between two techniques at these energies, unreachable with accelerator experiments. The Southern A…
The Pierre Auger Observatory detects ultra-high energy cosmic rays by implementing two complementary air-shower techniques. The combination of a large ground array and fluorescence detectors, known as the hybrid concept, means that a rich variety of measurements can be made on a single shower, providing much improved information over what is possible with either detector alone. In this paper I describe the hybrid reconstruction approach and the latest hybrid measurements.
Three months of data taken by the Engineering Array Surface Detector of the Pierre Auger Observatory have been analyzed with the aim of setting a procedure for the study of showers at E_0 ~ 10^18 eV. We present the results, concerning: (i) the selection procedure and the check of its efficiency; (ii) the event rate and its stability; (iii) the characteristics of such events (i.e., angular direction, energy estimator S(1000), rise time vs core distance, correlation with pressure) showing their consistency with the expectations. The performances of the whole Surface Detector of the Auger Observa…
The Surface Detector of the Pierre Auger Observatory will consist of 1600 water Cherenkov tanks sampling ground particles of air showers produced by energetic cosmic rays. The arrival times are obtained from GPS and power is provided by solar panels. The construction of the array is nearly completed and a large number of detectors has been operational for more than three years. In this paper the performance of different components of the detectors are discussed. The accuracy of the signal measurement and the trigger stability are presented. The performance of the solar power system and other h…
We present a study about the possibility to detect neutrino induced extensive air showers at the Pierre Auger Observatory. The Monte Carlo simulations performed take into account the details of the neutrino propagation inside the Earth, the air as well as the surrounding mountains which are modelled by a digital elevation map. Details on the sensitivity with respect to the incoming direction as well as the aperture and the total observable event rates are calculated on the basis of various assumptions of the incoming neutrino flux.
We review in these notes the status of the construction of the Pierre Auger Observatory and present the first Physics results, based on the data collected during the first year and a half of operation. These results are preliminary, once the work to understand the systematics of the detectors are still underway. We discuss the cosmic ray spectrum above 3 EeV, based on the measurement done using the Surface Detector and the Fluorescence Detector, both, components of the observatory. We discuss, as well, the search for anisotropy near the Galactic Center and the limit on the photon fraction at t…
The goal of the Pierre Auger Observatory is to determine the still unknown nature and origin of ultra-high energy cosmic rays. The study of these elusive particles probes astrophysical sites of particle acceleration as well as fundamental interactions at energies unattainable in accelerator facilities. Auger combines two observational techniques, a large surface array and air fluorescence detectors, to observe the extended air showers generated in the atmosphere by cosmic rays. This hybrid observation yields an excellent resolution and allows for important cross-checks. The Auger South site, l…
We fit the recently published Pierre Auger ultra-high energy cosmic ray spectrum assuming that either nucleons or nuclei are emitted at the sources. We consider the simplified cases of pure proton, or pure oxygen, or pure iron injection. We perform an exhaustive scan in the source evolution factor, the spectral index, the maximum energy of the source spectrum Z E_{max}, and the minimum distance to the sources. We show that the Pierre Auger spectrum agrees with any of the source compositions we assumed. For iron, in particular, there are two distinct solutions with high and low E_{max} (e.g. 6.…
During the past ten years, gamma-ray bursts (GRB) have been extensively studied in the keV-MeV energy range but the high energy emission still remain mysterious. Ground based observatories have the possibility to investigate energy range around one GeV using the "single particle technique". The aim of the present study is to investigate the capability of the Pierre Auger Observatory to detect the high energy emission of GRBs with such a technique. According to the detector response to photon showers around one GeV, and making reasonable assumptions about the high energy emission of GRBs, we sh…
A key feature of the Pierre Auger Observatory is its hybrid design, in which ultra high energy cosmic rays are detected simultaneously by fluorescence telescopes and a ground array. The two techniques see air showers in complementary ways, providing important cross-checks and measurement redundancy. Much of the hybrid capability stems from the accurate geometrical reconstruction it achieves, with accuracy better than either the ground array detectors or a single telescope could achieve independently. We have studied the geometrical and longitudinal profile reconstructions of hybrid events. We …
The Pierre Auger Observatory, with an array of currently more than 1200 Cherenkov detectors filled with 12 m$^3$ of water, can detect the putative high energy emission of a GRB (photons down to a few hundreds of MeV) by the so-called ``single particle technique'', through a coherent increase in the average background particle rates over the whole array, due to secondary particles in the photon-induced showers. We present a search for bursts on data collected since September 2005, as well as a search for excesses in coincidence with bursts observed by satellites.
The Southern Pierre Auger Observatory is now nearing completion and the accumulating data sample has already allowed to extract scientific results of relevance for ultra-high energy cosmic-ray (UHECR) physics. This lecture focuses on the description and outcomes of the studies on the UHECR arrival directions (and in particular the anisotropy searches around the Galactic Center) and composition (with the extraction of a limit on the flux of UHE photons and its implications). Perspectives concerning the study of inclined showers and the potential of detection of UHE neutrinos are also briefly di…
One of the most fascinating puzzles in particle astrophysics today is that of the origin and nature of the highest energy cosmic rays. The Pierre Auger Observatory (PAO), currently under construction in Province of Mendoza, Argentina, and with another site planned in the Northern hemisphere, is a major international effort to make precise, high statistics studies of the highest energy cosmic rays. It is the first experiment designed to work in a hybrid mode incorporating both a ground-based array of 1600 particle detectors spread over 3000 km$^2$ with fluorescence telescopes placed on the boun…
A wide range of atmospheric monitoring instruments is employed at the Pierre Auger Observatory: two laser facilities, elastic lidar stations, aerosol phase function monitors, a horizontal attenuation monitor, star monitors, weather stations, and balloon soundings. We describe the impact of analyzed atmospheric data on the accuracy of shower reconstructions, and in particular study the effect of the data on the shower energy and the depth of shower maximum (Xmax). These effects have been studied using the subset of golden hybrid events - events observed with high quality in the fluorescence and…
We determine the instantaneous aperture and integrated exposure of the surface detector of the Pierre Auger Observatory, taking into account the trigger efficiency as a function of the energy, arrival direction (with zenith angle lower than 60 degrees) and nature of the primary cosmic-ray. We make use of the so-called Lateral Trigger Probability function (or LTP) associated with an extensive air shower, which summarizes all the relevant information about the physics of the shower, the water tank Cherenkov detector, and the triggers.
The Pierre Auger Observatory allows the measurement of both longitudinal profiles and lateral particle distributions of high-energy showers. The former trace the overall shower development, mainly of the electromagnetic component close to the core where the latter reflect the particle densities in the tail of the shower far away from the core and are sensitive to both the muonic and electromagnetic components. Combining the two complementary measurements, predictions of air shower simulations are tested. In particular the muon component of the tank signals, which is sensitive to hadronic inter…
The Engineering Array of the Auger Observatory has been running successfully since 2001 and inclined showers have been recorded from the start. We have analysed the events with zenith angle $>$ 70$^0$ recorded between May and November 2002. The different algorithms developed to analyze these showers are also discussed. An preliminary discussion of a reconstructed event having 20 detectors hit is presented. Inclined showers will be detected by the full Auger Observatory and they will allow significant enhancement of the array aperture. High energy events will be seen as spectacular events with …
Based on observations of the depth of shower maximum performed with the hybrid detector of the Auger Observatory, an upper limit on the cosmic-ray photon fraction of 26% (at 95% confidence level) is derived for primary energies above 10^19 eV. Additional observables recorded with the surface detector array, available for a sub-set of the data sample, support the conclusion that a photon origin of the observed events is not favoured.
The Pierre Auger Observatory is designed to study cosmic rays with energies greater than 10^{19} eV. Two sites are envisaged for the observatory, one in each hemisphere, for complete sky coverage. The southern site of the Auger Observatory, now approaching completion in Mendoza, Argentina, features an array of 1600 water-Cherenkov surface detector stations covering 3000 km^2, together with 24 fluorescence telescopes to record the air shower cascades produced by these particles. The two complementary detector techniques together with the large collecting area form a powerful instrument for thes…
The Pierre Auger Observatory has been collecting data in a stable manner since January 2004. We present here a study of the cosmic ray composition using events recorded in hybrid mode during the first years of data taking. These are air showers observed by the fluorescence detector as well as the surface detector, so the depth of shower maximum, Xmax, is measured directly. The cosmic ray composition is studied in different energy ranges by comparing the observed average Xmax with predictions from air shower simulations for different nuclei. The change of <Xmax> with energy (elongation rate) is…
The aperture of the Fluorescence Detector (FD) of the Pierre Auger Observatory is evaluated from simulated events using different detector configurations: mono, stereo, 3-FD and 4-FD. The trigger efficiency has been modeled using shower profiles with ground impacts in the field of view of a single telescope and studying the trigger response (at the different levels) by that telescope and by its neighbours. In addition, analysis cuts imposed by event reconstruction have been applied. The hybrid aperture is then derived for the Auger final extension. Taking into account the actual Surface Detect…
Muons decaying in the water volume of a Cherenkov detector of the Pierre Auger Observatory provide a useful calibration point at low energy. Using the digitized waveform continuously recorded by the electronics of each tank, we have devised a simple method to extract the charge spectrum of the Michel electrons, whose typical signal is about 1/8 of a crossing vertical muon. This procedure, moreover, allows continuous monitoring of the detector operation and of its water level. We have checked the procedure with high statistics on a test tank at the Observatory base and applied with success on t…
The Central Laser Facility is located near the middle of the Pierre Auger Observatory in Argentina. It features a UV laser and optics that direct a beam of calibrated pulsed light into the sky. Light scattered from this beam produces tracks in the Auger optical detectors which normally record nitrogen fluorescence tracks from cosmic ray air showers. The Central Laser Facility provides a "test beam" to investigate properties of the atmosphere and the fluorescence detectors. The laser can send light via optical fiber simultaneously to the nearest surface detector tank for hybrid timing analyses.…
Ultra high energy neutrinos are produced by the interaction of hadronic cosmic rays with the cosmic radiation background. More exotic scenarios like topological defects or new hadrons predict even larger fluxes. In particular, Earth-skimming tau neutrinos could be detected by the Fluorescence Detector (FD) of Pierre Auger Observatory. A detailed evaluation of the expected number of events has been performed for a wide class of neutrino flux models. An updated computation of the neutrino-nucleon cross section and of the tau energy losses has been carried out. For the most optimistic theoretical…
We analyze the active-galaxy correlation reported in 2007 by the Pierre Auger Collaboration. The signal diminishes if the correlation-function approach (counting all "source-event" pairs and not only "nearest neighbours") is used, suggesting that the correlation may reveal individual sources and not their population. We analyze available data on physical conditions in these individual correlated sources and conclude that acceleration of protons to the observed energies is hardly possible in any of these galaxies, while heavier nuclei would be deflected by the Galactic magnetic field thus spoil…
The Pierre Auger Observatory has been designed to study the highest-energy cosmic rays in nature (E > 10^{18.5} eV). The determination of their arrival direction, energy and composition is performed by the analysis of the atmospheric showers they produce. The Auger Surface Array will consist of 1600 water Cerenkov detectors placed in an equilateral triangular grid of 1.5 km spacing. The aim of this paper is to show that the addition of a "small" area of surface detectors at half or less the above mentioned spacing would allow a dramatic increase of the physical scope of this Observatory, reach…
The scale and scope of the physics studied at the Auger Observatory offer significant opportunities for original outreach work. Education, outreach, and public relations of the Auger collaboration are coordinated in a task of its own whose goals are to encourage and support a wide range of efforts that link schools and the public with the Auger scientists and the science of cosmic rays, particle physics, and associated technologies. This report focuses on the impact of the collaboration in Mendoza Province, Argentina, as: the Auger Visitor Center in Malargue that has hosted over 29,000 visitor…
The Pierre Auger Southern Observatory in Argentina has begun taking data as it is being developed up to a final enclosed area of 3000 square kilometres. A key aspect of the project is to provide information on the origin of the highest energy cosmic rays through understanding the arrival directions of those particles. To avoid claims of a spurious anisotropy detection because trials have not been properly accounted, the Auger Collaboration has agreed 'a priori' to the analysis prescription presented here. It specifies the following: 1. The accumulation time for the (future) data set to be anal…
At the southern site of the Pierre Auger Observatory, which is close to completion, an exposure that significantly exceeds the largest forerunner experiments has already been accumulated. We report a measurement of the cosmic ray energy spectrum based on the high statistics collected by the surface detector. The methods developed to determine the spectrum from reconstructed observables are described. The energy calibration of the observables, which exploits the correlation of surface detector data with fluorescence measurements in hybrid events, is presented in detail. The methods are simple a…
The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau-neutrinos $ν_τ$ that interact in the Earth's crust. Tau leptons from $ν_τ$ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 is used to place an upper limit on the diffuse flux of $ν_τ$ at EeV energies. Assuming an $E_ν^{-2}$ differential energy spectrum the limit set at 90 % C.L. is $E_ν^{2} \mathrm{d}N_{ν_τ}/\mathrm{d}E_ν < 1.3 \times 10^{…
The Pierre Auger Cosmic-Ray Observatory uses the earth's atmosphere as a calorimeter to measure extensive air-showers created by particles of astrophysical origin. Some of these particles carry joules of energy. At these extreme energies, test beams are not available in the conventional sense. Yet understanding the energy response of the observatory is important. For example, the propagation distance of the highest energy cosmic-rays through the cosmic microwave background radiation (CMBR) is predicted to be strong function of energy. This paper will discuss recently reported results from the …
The Pierre Auger Observatory in Malargue, Argentina, is designed to study the origin of ultrahigh energy cosmic rays with energies above 10^18 eV. The energy calibration of the detector is based on a system of four air fluorescence detectors. To obtain reliable calorimetric information from the fluorescence stations, the atmospheric conditions at the experiment's site need to be monitored continuously during operation. One of the components of the observatory's atmospheric monitoring system is a set of four elastic backscatter lidar stations, one station at each of the fluorescence detector si…
