The Pierre Auger Observatory in 2007

The Pierre Auger Observatory in 2007 Henryk Wilczyński 1 for the Pierre Auger Collaboration 2 1 Institute of Nuclear Physics PAN, Kraków, Poland 2 Pierre Auger Observatory, Malargüe, Mendoza, Argentina Abstract The current status of the Pierre Auger Observatory construction is presented. Advantages of using the hybrid technique of air shower detection are discussed. Early results of Auger data analysis are presented concerning composition, energy spectrum and arrival directions of ultra-high energy cosmic rays. With the current data, if models of hadronic interactions are correct, the cosmic ray composition does not appear to be proton-dominated at the highest energies. The ankle at 4.5 EeV and a steepening at about 40 EeV are observed in the energy spectrum. The steepening is in the energy range expected for the Greisen-Zatsepin-Kuzmin effect. No anisotropies of arrival directions reported by previous experiments are found in the Auger data. 1 Introduction The energy spectrum of cosmic rays extends to beyond 100 EeV, i.e. 10 20 ev. Ultra-high energy cosmic rays (UHECR), those with energies above 1 EeV (10 18 ev), are especially interesting. It was pointed out [1] that if cosmic rays are protons, at energies above about 50 EeV they should lose energy in interactions with photons of the cosmic microwave background radiation, mainly through production of pions. A series of such interactions reduces the proton energy during propagation, so that the cosmic ray flux above the threshold energy should be suppressed this is the Greisen-Zatsepin-Kuzmin (GZK) effect. Thus the range of protons with energies above 100 EeV is limited to a few tens megaparsecs. Also nuclei suffer energy losses via photo disintegration on similar distance scales to protons. In consequence, a cosmic ray arriving to the Earth with energy 100 EeV must come from a source located within some 50 Mpc. Known intergalactic magnetic fields within this distance can deflect the ultra-high energy particles by only a few degrees, so that the arrival direction of an ultra-high energy cosmic ray should point well to its source. However, despite intensive search efforts, the sources of UHECR remain unclear. The only way to detect the UHECRs is an indirect one, through recording extensive air showers initiated by incoming cosmic ray particles. Two methods are used for shower detection: one of them is an array of particle detectors located on the ground, which sample the shower front, so that the lateral distribution of particles in the shower can be derived. The other method is to record air fluorescence light induced by charged particles of a shower, using appropriate optical telescopes. With this method the longitudinal profile of shower development is determined. With both methods one can reconstruct the shower and determine 1

Figure 1: The construction status of the Pierre Auger Observatory in October 2007. The dots represent the surface detector stations. Out of 1600 planned stations, 1438 take data (in the shaded area). All 24 telescopes of the fluorescence detector are operational. The lines mark field of view limits of individual telescopes. its energy and direction, but each technique uses different observables, so that systematic effects are different. The two largest cosmic ray detectors in the ultra-high energy range have been: AGASA [2], a ground array of detectors and HiRes [3], using the fluorescence technique of shower detection. The energy spectra reported by these experiments show relatively large differences, especially in the highest energy range [4]. The AGASA collaboration found no indication of the GZK cutoff expected in the spectrum, while the HiRes spectrum is consistent with the cutoff. Therefore, the existence of the GZK effect in the spectrum is unclear. 2 The Auger Observatory The aim of the Pierre Auger Observatory [6] is to provide the large-statistics, high-accuracy data which are necessary to determine the composition, energy spectra and arrival direction distribution of ultra-high energy cosmic rays. The Auger Observatory is a hybrid detector, making use of the two detection techniques simultaneously: a surface array of particle detectors and a fluorescence detector. It is planned that observatory sites on both hemispheres Figure 2: The principle of hybrid detection of air showers. Triggered pixels in the fluorescence detector define the shower-detector plane. The position of the shower axis within this plane is determined from arrival times of signals in the pixels. Timing information from the surface detector greatly improves the accuracy of the fit compared to monocular fluorescence detector reconstruction. (The diagram adapted from the Fly s Eye group). 2

Figure 3: The accuracy of hybrid reconstruction compared to monocular FD reconstruction of the laser beam position (left) and angle (right) [5]. will be built: in Argentina (now under construction) and in USA (in planning stage). The large area of the observatory compensates for the small flux of UHECR. The array of surface detectors (SD) at the southern site in the province of Mendoza in Argentina will consist of 1600 water Cherenkov detectors, each containing 12 m 3 of purified water. Cherenkov light induced by charged particles of a shower in water is recorded by photomultipliers. The detectors are deployed on a hexagonal grid with 1.5 km spacing between individual detector stations (see Figure 1). The fluorescence detector (FD) consists of four ensembles of six telescopes overlooking the surface detector. The Schmidt optics telescopes each have the field of view 30 30. The segmented mirror of a telescope with dimensions 3.6 3.6 m 2 focuses the light on the camera consisting of 440 pixels, each with 1.5 viewing angle. The hybrid detection technique (see Figure 2) enables a considerable improvement in accuracy of shower reconstruction. As a part of the calibration and monitoring system, a pulsed laser is used to shoot a steerable light beam into the air, while an optical fiber directs a portion of the laser beam into a nearby surface detector station. The scattered light from the laser beam is recorded by the fluorescence detector, so that a hybrid detection of air showers is emulated. In Figure 3 the reconstruction accuracy of the laser beam position and direction in the hybrid mode is compared to the monocular FD reconstruction. This shows the superior accuracy of the hybrid detection method. Similarly, the hybrid mode is much more accurate than the SD-only reconstruction of cosmic ray events. The construction of the southern site of the Pierre Auger Observatory in Argentina nears completion. All 24 fluorescence telescopes are completed and 90% of the planned 1600 surface array stations are operational. It is expected that the southern site will be finished in early 2008. The surface array works continuously, while the fluorescence detector can work only during clear moonless nights, i.e. 10-15% of the time. However, these 10% of events recorded in the hybrid mode are sufficient for cross-calibration of the surface and fluorescence detectors, as will be demonstrated below. 3

3 Results Figure 4: The average depth of shower maximum as a function of energy compared to predictions from CORSIKA shower simulations using different hadronic interaction models. The numbers of events are shown for each bin. Although the Observatory is still under construction, it has been collecting useful data since January 2004. Each detector station is completely autonomous and becomes part of the active array when logically included into the data acquisition system. Thus the completed part of the array simply works as a smaller array. In this paper we show some results from data accumulated over the period January 2004 February 2007. 3.1 Composition Primary cosmic ray particles cannot be observed nor identified directly, so one has to resort to indirect methods of inferring the primary particle type based on properties of air showers. Showers initiated by heavy nuclei generally develop earlier in the atmosphere, (i.e. at smaller atmospheric depths) than those initiated by protons. The depth at which the shower maximum occurs, X max, is larger for proton showers. On the other hand, X max increases with the primary energy. The rate of this increase is determined using shower simulation codes, like CORSIKA [7] or AIRES [8] with models of hadronic interactions, such as QGSJET [9], Sibyll [10] or EPOS [11]. Properties of hadronic interactions must be extrapolated over several orders of magnitude from accelerator energies, at which they have been tested. This introduces considerable uncertainties in shower modelling. Figure 4 shows the X max versus energy dependence predicted by several models along with the Auger data based on 4105 hybrid events [12]. It is readily seen that the data cannot be fitted well by a single slope. At lower energies, below 2 EeV, the rate of X max increase with energy is larger than generally predicted by the models and gets smaller at higher energies. This means that the cosmic ray composition gets lighter with increasing energy at lower energies, and this tendency is reversed at higher energies. Therefore, if the models are correct, cosmic rays at all energies appear to have a mixed composition. The Auger data do not contradict those from earlier experiments, but have smaller uncertainties so that the above conclusion can be made provided, of course, that the hadronic models are correct. No photon-initiated showers have been identified in the Auger data sample. One can set therefore upper limits to the photon fraction among cosmic rays [13, 14]. The upper limits constrain the top-down models [15] of cosmic ray origin. In these models, the observed cosmic rays are decay products of the hypothetical exotic heavy particles. The Auger result 4

strongly disfavours most top-down models of cosmic ray origin [16]. 3.2 Energy spectrum Figure 5: The calibration of the S 38 parameter versus energy from the fluorescence detector, using 387 hybrid events. The hybrid concept of shower detection helps considerably to reduce uncertainties of shower energy determination. In the surface detector, the parameter used as the energy measure is S(1000), the particle signal at 1000m from the shower axis. The constant intensity cut method is used to normalize it to the mean zenith angle of 38 [17]. The resulting S 38 parameter is calibrated against energy using hybrid events, in which the energy was measured by the fluorescence detector. This calibration, based on 387 hybrid events, is shown in Fig. 5. The random uncertainty in the energy scale is 18%. The energy spectrum is derived using well-contained SD events those whose axes fall inside active hexagons, i.e. are surrounded by six live stations. In this way the aperture of the array can be reliably determined. The energy scale is determined by the near-calorimetric FD energy measurement, with the systematic uncertainty estimated at 22%, dominated by the uncertainty in the fluorescence yield. For zenith angles less than 60, the detection efficiency in the surface array saturates (at 100%) above 3 EeV. We note therefore that above this energy the SD detector area and efficiency are independent of energy. Hence, in the hybrid system one avoids both the energy scale uncertainties due to shower modeling Figure 6: The cosmic ray energy spectrum determined from the surface array using events at zenith angles smaller than 60. The number of events used is shown for each bin. 5

Figure 7: The combined energy spectrum derived as a weighted average of spectra from surface detector events with inclinations less than 60, from inclined events and from hybrid events. Residuals of the spectrum relative to AE 2.6 spectrum are shown to demonstrate details of the spectrum shape. and mass composition uncertainties (characteristic for all SD-only detectors) as well as large uncertainties in aperture determination (characteristic for FD-only detectors). The total accumulated exposure until February 2007 is 5165 km 2 sr yr, which is about 3 times that of AGASA. The resulting spectrum from events at zenith angles less than 60 is shown in Figure 6. Between 4.5 EeV and 36 EeV it can be well fitted by a power law with index -2.62±0.06. At higher energies, this extrapolated spectrum would yield 132±9 events above 40 EeV and 30±2.5 above 100 EeV, while the observed numbers of events are 51 and 2 respectively. It is evident therefore that the spectrum steepens above 40 EeV, i.e. in the energy range in which the GZK effect is expected. Independently, the spectrun can also be determined from inclined SD events (θ > 60 ) and from hybrid events, each having different systematics. The three spectra agree well and can be combined into one. The shape of the combined spectrum is better seen in Fig. 7 in which residuals from the E 2.6 spectrum are shown. The ankle at log E = 18.65 and the steepening at log E = 19.55 are clearly seen. 3.3 Distribution of arrival directions The arrival directions of UHECR have been extensively studied with the Auger data to verify earlier claims of anisotropies. Among these was the excess from the galactic center, reported previously by AGASA [18] and the correlation with BL Lacs reported by HiRes [19]. No such anisotropies are seen in the Auger data [20]. No clustering nor large-scale anisotropies like a dipole have been found, either [21]. A correlation of arrival directions with positions of nearby active galaxies is observed [22]. Further studies are in progress. 4 Summary The construction of the southern Pierre Auger Observatory is about 90% completed now and is expected to be finished in early 2008. Already at this early stage, Auger has collected more events above 1 EeV than were gathered in all previous experiments. Use of the hybrid detection system of air showers resulted in unprecedented precision of the data. Early results of Auger data analysis indicate, if the current models of hadronic interactions are correct, a mixed composition of cosmic rays at highest energies. This is a rather unexpected result and it will be checked on larger statistics. Correctness of hadronic models at highest energies is very important for data interpretation. 6

In the energy spectrum, the ankle at 4.5 EeV is clearly seen as well as a steepening at about 40 EeV. In wiev of the composition result, it remains to be seen whether this steepening is the long-sought GZK effect. In the arrival direction distribution, none of earlier claims of anisotropies have been confirmed. No statistically significant signs of galactic center excess, clustering or correlation with BL Lacs is seen. As the data statistics quickly increases, more results are expected soon. Acknowledgements. I would like to thank all my Auger collaborators for their efforts. This research was supported by the respective funding agencies of the institutions participating in the Pierre Auger Project. In Poland, this effort was supported by the Ministry of Science and Higher Education (grants N202 090 31/0623 and PAP/218/2006). References [1] K.Greisen, Phys. Rev. Lett. 16 (1966) 748; G.T.Zatsepin, V.A.Kuzmin, Pisma Zh. Eksp. Teor. Fiz. 4 (1966) 144. [2] N.Hayashida et al., J. Phys. G 21 (1994) 1101. [3] T.Abu-Zayyad et al., Nucl. Instr. Meth. A 450 (2000) 748. [4] S.Yoshida, Proc. 29 th ICRC (Pune 2007) vol.10, p.297. [5] M.Mostafá et al., Pierre Auger Collaboration, Proc. 29th ICRC (Pune 2005), vol.7, p.369. [6] J.Abraham et al., Pierre Auger Collaboration, Nucl. Instr. Meth. A 523 (2004) 50. [7] D.Heck et al., Forschungszentrum Karlsruhe Report FZKA 6019 (1998). [8] S.Sciutto, www.unlp.edu.ar/auger/aires. [9] S.Ostapchenko, Phys. Rev. D 74 (2006) 014026. [10] R.S.Fletcher et al., Phys. Rev. D 50 (1994) 5710; R.Engel et al., Proc. 26th ICRC (Salt Lake City 1999), vol. 1, p.415. [11] T.Pierog, K.Werner, astro-ph/0611311; Proc. 30 th ICRC (Merida 2007). [12] M.Unger et al., Pierre Auger Collaboration, Proc. 30 th ICRC (Merida 2007); arxiv:0706.1495. [13] J.Abraham et al., Pierre Auger Collaboration, Astropart. Phys. 27 (2007) 155. [14] M.Healy et al., Pierre Auger Collaboration, Proc. 30 th ICRC (Merida 2007), arxiv:0710.0025. [15] P.Bhattacharjee, G.Sigl, Phys. Rep. 327 (2000) 109. [16] D.Semikoz et al., Pierre Auger Collaboration, Proc. 30 th ICRC (Merida 2007); arxiv:0706.2960. [17] Papers by the Pierre Auger Collaboration, Proc. 30 th ICRC (Merida 2007): M.Roth et al., arxiv:0706.2096; L.Perrone et al., arxiv:0706.2643; P.Facal San Luis et al., arxiv:0706.4322; T.Yamamoto et al., arxiv:0707.2638. [18] N.Hayashida et al., Astropart. Phys. 10 (1999) 303. [19] C.B.Finley et al., Proc. 29th ICRC (Pune 2005), vol. 7, p.339. [20] J.Abraham et al., Pierre Auger Collaboration, Astropart. Phys. 27 (2007) 244. [21] Papers by the Pierre Auger Collaboration, Proc. 30 th ICRC (Merida 2007): E.Santos et al., arxiv:0706.2669; E.Armengaud et al., arxiv:0706.2640; D.Harari et al., arxiv:0706.1715; S.Mollerach et al., arxiv:0706.1749. [22] Pierre Auger Collaboration, Science, to be published 9 Nov. 2007. 7