Results from the Pierre Auger Observatory

The Pierre Auger Observatory is the largest observatory of high-energy cosmic rays. It is located in Argentina and has been taking data since January 2004. Extensive air showers initiated by cosmic rays are measured by the hybrid detector, which combines the sampling of particle density at ground by...

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Datum:	2012
1. Verfasser:	Šmída, R.
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Sprache:	English
Veröffentlicht:	Головна астрономічна обсерваторія НАН України 2012
Schriftenreihe:	Advances in Astronomy and Space Physics
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spelling	irk-123456789-1191762017-06-05T03:03:07Z Results from the Pierre Auger Observatory Šmída, R. The Pierre Auger Observatory is the largest observatory of high-energy cosmic rays. It is located in Argentina and has been taking data since January 2004. Extensive air showers initiated by cosmic rays are measured by the hybrid detector, which combines the sampling of particle density at ground by water-Cherenkov tanks and the measurement of atmospheric fluorescence light by telescopes. New detection techniques, like radio and microwave measurement, are also being tested. Results regarding the energy spectrum, mass composition and arrival directions of cosmic rays are presented here. 2012 Article Results from the Pierre Auger Observatory / R. Šmída // Advances in Astronomy and Space Physics. — 2012. — Т. 2., вип. 1. — С. 67-72. — Бібліогр.: 22 назв. — англ. 2227-1481 http://dspace.nbuv.gov.ua/handle/123456789/119176 en Advances in Astronomy and Space Physics Головна астрономічна обсерваторія НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
description	The Pierre Auger Observatory is the largest observatory of high-energy cosmic rays. It is located in Argentina and has been taking data since January 2004. Extensive air showers initiated by cosmic rays are measured by the hybrid detector, which combines the sampling of particle density at ground by water-Cherenkov tanks and the measurement of atmospheric fluorescence light by telescopes. New detection techniques, like radio and microwave measurement, are also being tested. Results regarding the energy spectrum, mass composition and arrival directions of cosmic rays are presented here.
format	Article
author	Šmída, R.
spellingShingle	Šmída, R. Results from the Pierre Auger Observatory Advances in Astronomy and Space Physics
author_facet	Šmída, R.
author_sort	Šmída, R.
title	Results from the Pierre Auger Observatory
title_short	Results from the Pierre Auger Observatory
title_full	Results from the Pierre Auger Observatory
title_fullStr	Results from the Pierre Auger Observatory
title_full_unstemmed	Results from the Pierre Auger Observatory
title_sort	results from the pierre auger observatory
publisher	Головна астрономічна обсерваторія НАН України
publishDate	2012
url	http://dspace.nbuv.gov.ua/handle/123456789/119176
citation_txt	Results from the Pierre Auger Observatory / R. Šmída // Advances in Astronomy and Space Physics. — 2012. — Т. 2., вип. 1. — С. 67-72. — Бібліогр.: 22 назв. — англ.
series	Advances in Astronomy and Space Physics
work_keys_str_mv	AT smidar resultsfromthepierreaugerobservatory
first_indexed	2025-07-08T15:22:26Z
last_indexed	2025-07-08T15:22:26Z
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fulltext	Results from the Pierre Auger Observatory R. �mída1∗for the Pierre Auger Collaboration2 Advances in Astronomy and Space Physics, 2, 67-72 (2012) © R. �mída, 2012 1Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2Observatorio Pierre Auger, Av. San Martín Norte 304, 5613 Malargüe, Argentina The Pierre Auger Observatory is the largest observatory of high-energy cosmic rays. It is located in Argentina and has been taking data since January 2004. Extensive air showers initiated by cosmic rays are measured by the hybrid detector, which combines the sampling of particle density at ground by water-Cherenkov tanks and the measurement of atmospheric �uorescence light by telescopes. New detection techniques, like radio and microwave measurement, are also being tested. Results regarding the energy spectrum, mass composition and arrival directions of cosmic rays are presented here. Key words: astroparticle physics, cosmic rays introduction The goal of the Pierre Auger Observatory [10] is the measurement of ultra-high energy cosmic rays (UHECRs), i.e. cosmic rays with energies above 1018 eV. The origin and properties of UHECRs are not fully understood yet. These particles have been observed for more than half a century and special attention has been paid to the most energetic ones. Unlike optical or radio photons, the cosmic rays are charged nuclei and are de�ected by magnetic �elds during their propagation through interstellar and intergalactic space. Our knowledge of the galac- tic and extragalactic magnetic �elds is limited. Since the de�ections caused by the magnetic �elds are in- versely proportional to the particle energy, we might take advantage of using the most energetic measured cosmic rays for backtracking to a source location. The investigation of UHECRs is di�cult and re- quires a large e�ort. First of all, the �ux1 of UHE- CRs is very low. It falls steeply with energy, decreas- ing by almost three orders of magnitude per decade of energy. Only one particle per km2 per year arrives at the Earth above an energy of 1018 eV and the �ux reduces to less than one particle per km2 per century at energies above 1020 eV. It is clear that a huge de- tector is necessary for the successful measurement of UHECRs. Another di�culty in the study of UHECRs is their interaction in the atmosphere. The direct de- tection of primary particles of cosmic rays is possi- ble only at altitudes higher than ∼20 km. Balloon or space borne experiments are too limited in their col- lection area to measure the low �ux of UHECRs and cannot be used. In the 1930s French physicist Pierre Auger [16] found that the primary cosmic ray particle interacts with the atmosphere and many secondary particles are produced in the �rst and subsequent interactions. Such a cascade of secondary particles is called an extensive air shower (EAS). An exten- sive air shower develops in the atmosphere while the energies of the secondary particles are su�cient to produce new particles. The number of secondary particles in the shower maximum can be as high as a few billion, but only a small fraction of the secondary particles arrive at ground level. Two methods have been established for the mea- surement of EAS. The �rst one samples the particle density at the ground with an array of detectors mea- suring in coincidence. The other method is a mea- surement of ultraviolet (UV) nitrogen �uorescence light emitted along the track of EAS. The properties of the primary cosmic ray particle are reconstructed from the measured data. A detector using both, a surface and a �uorescence detector, is called a hy- brid detector. Hybrid detection gives more accurate results in comparison with the results obtained by individual detection techniques. More details about the measurement and prop- erties of UHECRs can be found in review articles [17, 21] and references therein. pierre auger observatory More than 1600 water-Cherenkov surface detec- tors cover a �at semi-desert area of more than ∗radomir.smida@kit.edu 1The �ux is de�ned as the number of cosmic rays arriving on a unit area from unit solid angle per unit of time. 67 Advances in Astronomy and Space Physics R. �mída for the Pierre Auger Collaboration 3000 km2 near the city of Malargüe, Argentina (69.3◦W, 35.3◦ S, 1400m a.s.l.) on a triangular grid with 1.5 km spacing. Three large photomultipliers measure Cherenkov photons radiated by relativis- tic particles passing through puri�ed water in each detector. Each detector is equipped with necessary electronics, battery, solar panel, GPS and radio com- munication antenna (see Fig. 1). Fig. 1: Water-Cherenkov detector with its components. A secondary particle passing through the detector is in- dicated together with emitted Cherenkov photons. The advantages of the surface detector are its au- tonomous operation, uptime of almost 100% and its well de�ned aperture [6]. On the other hand the en- ergy reconstruction depends on hadronic interaction models, which can only be validated up to energies available in man-made accelerators. Any usage of hadronic interaction models is necessarily based on theoretical extrapolations from lower to higher ener- gies. However, using the measurements of the �uo- rescence detector, the absolute energy scale can be estimated almost entirely based on measured data. The �uorescence detector consists of four sites, each with six �uorescence telescopes, located at the boundary of the surface detector array. The �uo- rescence telescopes view the atmosphere above the array during clear, almost moonless, nights [7]. The uptime of the �uorescence detector is about 13%. Each �uorescence telescope consists of a spherical segmented mirror, a camera with a matrix of 440 photomultipliers and an aperture with a UV band- pass �lter and corrector ring (see Fig. 2). One of the main advantages of the �uorescence detector is the calorimetric measurement of cosmic ray energies. The charged secondary particles of an extensive air shower excite atmospheric molecular ni- trogen, which then emits photons isotropically in the UV range (i.e. into several spectral bands between 300 and 420 nm). Because the emitted intensity is proportional to the energy deposited by the shower along its path, the energy reconstruction is indepen- dent of hadronic interaction models, except for only a few percent correction for the invisible component due to muons and neutrinos. Another advantage is the observation of the longitudinal pro�le, i.e. the number of secondary particles of EAS as a function of atmospheric depth. The position of the shower max- imum is sensitive to the type of the primary particle. Therefore the chemical composition of UHECRs can be studied with the �uorescence detector. Knowl- edge of the chemical composition is crucial for the study of cosmic ray acceleration and propagation. Fig. 2: Drawing of a �uorescence telescope. Light passes through the aperture and is re�ected by the mirror to the camera. Because the �uorescence emission and also light scattering and attenuation depends on the atmo- spheric conditions between the shower and the tele- scope, a large array of atmospheric monitors is oper- ated at the Pierre Auger Observatory [5]. The data are also used to prevent cloud-obscured data from distorting estimates of the shower energies, shower maxima, and the detector aperture. Moreover, the sensitivity of the �uorescence detector is regularly monitored using di�erent light sources. cosmic ray energy spectrum For the calculation of the cosmic ray �ux, a knowledge of the detector exposure together with the precise energy reconstruction is necessary. The surface detector has a well de�ned aperture above energies of 3 × 1018 eV and, moreover, its exposure does not depend on weather conditions. The energy estimator of the surface detector is calibrated by the energy given by the �uorescence detector for a sub- set of EAS simultaneously detected by both detec- tors [2, 8, 12]. The correlation between the cosmic ray energy measured by the �uorescence detector and the energy estimator of the surface detector is shown in Fig. 3. The combined energy spectrum calculated from the surface and hybrid data is shown in Fig. 4. While 68 Advances in Astronomy and Space Physics R. �mída for the Pierre Auger Collaboration the �uorescence detector provides data below an en- ergy of 3×1018 eV, the surface detector has su�cient statistics even for energies above a few 1019 eV be- cause of its higher uptime. Both spectra show agree- ment for the intermediate energy range. lg(E FD /eV) 18.5 19 19.5 /V E M ) 3 8 lg (S 1 1.5 2 2.5 3 Fig. 3: Correlation between the energy estimator of the surface detector (S38) and the energy measured by the �uorescence detector (EFD). The �ux of UHECRs follows a power law, with two changes of the spectral index. The �attening of the spectrum takes place at an energy of 4× 1018 eV and this kink is called the ankle. The ankle might indicate the transition from galactic to extragalac- tic cosmic rays. A similar feature in the cosmic ray spectrum could also result from the propagation of protons from extragalactic sources, placing the tran- sition from galactic to extragalactic cosmic rays at a much lower energy. Various models predict mea- surable di�erences not only for the energy spectrum, but also for the chemical composition (see e.g. [17]). A signi�cant suppression of the �ux of UHECRs is observed above 4 × 1019 eV. The suppression is similar to the prediction of the Greisen-Zatsepin- Kuzmin (GZK) mechanism [20, 22], but it could also be related to a change of the injection spectrum in the sources. The GZK mechanism is an interac- tion of cosmic ray protons above the GZK energy of ∼ 4 × 1019 eV with the photons of the cosmic microwave background. The energy loss of the in- teracting cosmic ray continues until the particle en- ergy falls below the GZK energy. As a result, cos- mic rays above the GZK energy cannot travel over distances larger than ∼ 100Mpc without signi�cant energy losses2. This mechanism excludes far distant sources from making a signi�cant contribution into the UHECR �ux above the GZK energy. The shape of the cosmic ray spectrum di�ers for proposed models of the origin of UHECRs. It is af- fected not only by the properties of the sources but also by propagation processes. Energy [eV] 1810 1910 2010 ] 2 e V -1 s r -1 y r -2 J (E ) [k m 3 E 3710 3810 (E/eV) 10 log 18 18.5 19 19.5 20 20.5 (E)=22%sysσ HiRes Auger power laws power laws + smooth function Fig. 4: Scaled energy spectrum derived by the Auger experiment and compared with the data from another detector. The systematic uncertainty of the energy scale of 22% is indicated by arrows. cosmic ray composition The development of an extensive air shower de- pends on the type of the primary particle. The lon- gitudinal pro�le measured by the �uorescence tele- scopes and also the lateral distribution of secondary particles sampled by the surface detector provide in- formation about the properties of the primary parti- cle. It has been found, that the contribution of neu- tral particles, which could be interesting for �nding the UHECRs sources, to the �ux of UHECRs is very low. The upper limits on the �uxes of known sta- ble neutral particles, photons [3] and τ -neutrinos [4], are so low, that several exotic models of the origin of UHECRs (e. g. the decay of superheavy particles) have been excluded. The �ux of UHECRs is mainly composed of atomic nuclei. The most precise measurement of the chemical composition is obtained by the observation of the depth of the shower maximum as well as its �uctuations [9]. The EAS produced by lighter pri- maries (e. g. protons) propagate deeper into the at- mosphere than heavier nuclei (e. g. iron nuclei) and show larger �uctuations. The identi�cation of the primary particle is not possible for a single event, because of the random nature of EAS. The average depth of the shower maximum is found to change with energy as do shower-to-shower �uctuations. Protons or other light nuclei would be preferable for the study of UHECR sources, because they would be less a�ected by the magnetic �elds 2Ultra-high energy nuclei lose energy in photodisintegration on all photon �elds. 69 Advances in Astronomy and Space Physics R. �mída for the Pierre Auger Collaboration during their propagation from the sources. Unfor- tunately, the results of both methods suggest heav- ier composition at the highest studied energies (see Fig. 5). It is worth mentioning, that the composi- tion above the energy of 4× 1019 eV is not clear yet because of a lack of statistics. Fig. 5: Average depth of shower maximum (top) and shower-to-shower �uctuations (bottom) as a function of energy together with air shower simulations using di�er- ent hadronic interaction models. cosmic ray arrival directions The uncertainty of reconstructed arrival direc- tions of UHECR by the Pierre Auger Observatory is less than 1.5◦ for events triggering 4 surface detector stations and better than 1.0◦ for events with six and more stations (i.e. for higher energies). The de�ec- tion of cosmic rays propagating in known magnetic �elds is comparable with the angular resolution of the detector even for protons at the highest energies. The de�ection is more than one order of magnitude higher for iron nuclei. As mentioned above, the sources of UHECRs above energies of 4 × 1019 eV must lie closer than ∼ 100Mpc. The number of possible astronom- ical sources (candidate sites are active galactic nuclei (AGN), radiogalaxies, clusters of galaxies, etc.) within the GZK horizon is limited. These close-by objects are clustered and we might expect anisotropic arrivals of UHECRs even for a mixed hadronic composition. An anisotropy has been observed by the Pierre Auger Observatory for events above ∼ 5.5× 1019 eV. The sky positions of measured arrival directions are preferably closer than ∼ 3◦ from the positions of AGN with distances less than 75Mpc [1]. The latest results show that the fraction of the highest energetic cosmic rays correlating with nearby AGN directions is 38%, while 21% is expected for isotropic �ux, see Fig. 6. The list of events has been published in [13]. The largest excess has been measured around the position of our closest radiogalaxy Centaurus A. Fig. 6: The arrival directions of 69 events measured by the Pierre Auger Observatory above 5.5× 1019 eV (small dark dots) together with the positions of nearby AGN (dimmer areas, where darker shade indicates larger rel- ative exposure). Galactic coordinates are used in the map. No other excess has been found in the data col- lected by the Pierre Auger Observatory. The direc- tion towards the Galactic Centre shows no excess of arrival directions [11], nor is there any positive sig- nal found in large-scale anisotropy studies [14]. The upper limit for the latter result is shown in Fig. 7. enhancements of the observatory The study of cosmic rays with energies between 1017 and 5×1018 eV are of special interest. The tran- sition from a galactic to an extragalactic �ux might occur in this energy range. A discrimination between astrophysical models requires a precise measurement of the spectrum as well as the chemical composition. Two extensions have been built in the Auger experi- ment. High Elevation Auger Telescopes (HEAT) are three �uorescence telescopes which can be tilted 45◦ above the horizon. They extend the lower energy threshold to well below 1017 eV. A shower measured by a HEAT telescope and one standard �uorescence telescope is shown in Fig. 8. This is a low energy shower which could be reconstructed due to the ad- ditional measurement at higher elevation. There is 70 Advances in Astronomy and Space Physics R. �mída for the Pierre Auger Collaboration also an extension of the surface detector close to the HEAT site which is going to be combined with un- derground muon counters. This in�ll array lowers the energy threshold of the surface detector down to ∼ 5× 1017 eV. Energy [eV] 1410 1510 1610 1710 1810 1910 2010 E qu at or ia l d ip ol e d -410 -310 -210 -110 1 A S C-G XGal EAS-TOP KASCADE Gra nd e AGASA Auger Gal Fig. 7: Upper limits on a dipolar-type anisotropy given by di�erent experiments compared with calculated pre- dictions (for more detail see [14]). Fig. 8: Trace of a low energetic event measured by two cameras. The upper one belongs to the elevated HEAT telescope, the lower one is a standard �uorescence tele- scope. Colours indicate the time evolution of the mea- sured signal. Other activities are connected with the radio de- tection of EAS. Radio detection might play an im- portant role in the future, because a low-cost radio detector could have 100% uptime and sensitivity to the chemical composition. Radio emission from EAS in the frequency range of 30�80MHz is detected by a prototype radio telescope array (AERA � Auger Engineering Radio Array) [18]. A simulated radio signal is shown in Fig. 9. Predicted microwave emis- sion [19] is going to be investigated by parabolic an- tennas equipped with a matrix of receivers measur- ing in C band (3.4�4.2GHz) and Ku band (10.95� 14.50GHz). -15000 -10000 -5000 0 5000 10000 15000 1900 1950 2000 2050 2100 2150 2200 2250 2300 v o lt a g e [µ V ] time [ns] channel east channel north Fig. 9: Time traces of a simulated radio signal in two channels of the AERA detector [15]. conclusions The Pierre Auger Observatory and its results have been described. The observatory provides valu- able data for the study of ultra-high energy cosmic rays. Major achievements of the observatory are the precise measurement of the cosmic ray �ux and chemical composition, as well as the detailed study of arrival directions. In addition, the observatory has become the basis for testing several new detection methods. references [1] Abraham J., Abreu P., Aglietta M. et al. 2007, Science, 318, 938 [2] Abraham J., Abreu P., Aglietta M. et al. 2008, Phys. Rev. Lett., 101, 061101 [3] Abraham J., Abreu P., Aglietta M. et al. 2009, Astropar- ticle Phys., 31, 399 [4] Abraham J., Abreu P., Aglietta M. et al. 2009, Phys. Rev. D, 79, 102001 [5] Abraham J., Abreu P., Aglietta M. et al. 2010, Astropar- ticle Phys., 33, 108 [6] Abraham J., Abreu P., Aglietta M. et al. 2010, Nuclear Instruments and Methods in Physics Research A, 613, 29 [7] Abraham J., Abreu P., Aglietta M. et al. 2010, Nuclear Instruments and Methods in Physics Research A, 620, 227 [8] Abraham J., Abreu P., Aglietta M. et al. 2010, Phys. Lett. B, 685, 239 [9] Abraham J., Abreu P., Aglietta M. et al. 2010, Phys. Rev. Lett., 104, 091101 71 Advances in Astronomy and Space Physics R. �mída for the Pierre Auger Collaboration [10] Abraham J., Aglietta M., Aguirre I. C. et al. 2004, Nu- clear Instruments and Methods in Physics Research A, 523, 50 [11] Abraham J., Aglietta, M., Aguirre C. et al. 2007, As- troparticle Phys., 27, 244 [12] Abreu P., Aglietta M., Ahn E. J. et al. 2008, Astropar- ticle Phys., 34, 368 [13] Abreu P., Aglietta M., Ahn E. J. et al. 2010, Astropar- ticle Phys., 34, 314 [14] Abreu P., Aglietta M., Ahn E. J. et al. 2011, Astropar- ticle Phys., 34, 627 [15] Abreu P., Aglietta M., Ahn E. J. et al. 2011, Nuclear Instruments and Methods in Physics Research A, 635, 92 [16] Auger P., Ehrenfest P., Maze R., Daudin J. & Fréon R.A. 1939, Rev. Mod. Phys., 11, 288 [17] Blümer J., Engel R. & Hörandel J. R. 2009, Progress in Particle and Nuclear Physics, 63, 293 [18] Dallier R. 2011, Nuclear Instruments and Methods in Physics Research A, 630, 218 [19] Gorham P.W., Lehtinen N.G., Varner G. S. et al. 2008, Phys. Rev. D, 78, 032007 [20] Greisen K. 1966, Phys. Rev. Lett., 16, 748 [21] Nagano M. & Watson A. 2000, Rev. Mod. Phys., 72, 689 [22] Zatsepin G.T. & Kuzmin V.A. 1966, JETP Lett., 4, 78 72

Results from the Pierre Auger Observatory

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