132 Acta Polytechnica CTU Proceedings 1(1): 132–138, 2014 132 doi: 10.14311/APP.2014.01.0132 Auger Highlights Antonella Castellina 1 for the Pierre Auger Collaboration 2 1Istituto Nazionale di Astrofisica - OATo and INFN Torino 2Observatorio Pierre Auger, Av. San Mart́ın Norte 304, 5613 Malargüe, Argentina (Full author list : http://www.auger.org/archive/authors 2012 12.html) Corresponding author: castellina@to.infn.it Abstract The Pierre Auger Observatory has been designed to investigate the origin and nature of the ultra high energy cosmic rays using a hybrid detection technique. A review of selected results is presented, with the emphasis given to the measurement of energy spectrum, mass composition and arrival directions. Keywords: ultra high energy cosmic rays - sources - energy spectrum - primary composition. 1 Introduction Understanding the sources, nature and propagation properties of the cosmic rays at ultra high energies (E > 1018 eV) is one of the key questions in astroparti- cle physics. From the experimental point of view, their study can be performed indirectly, by exploiting the Ex- tensive Air Showers they produce by interacting with the nuclei in the Earth atmosphere. Among the different features characterizing the spectral shape, the region between ' 1018−1019 eV is thought to host the transition from galactic to extragalactic cosmic rays. Different models explain it as due to e+/e− pair production of protons with the photons of the cosmic microwave background (CMB) [1], or more tradition- ally to the intersection of a steep galactic component and the onset of a flatter extragalactic one [2]. At even higher energies, above ' 4 1019 eV, a cut-off in the cosmic ray flux is expected, due to photo-pion production of extragalactic protons in the CMB (the ”GZK cut-off” [3]) , although the same feature could arise when reaching the limits in the maximum energy of the sources. However, the all particle spectrum cannot provide a dis- crimination among the different hypotheses, and the de- termination of the primary composition is mandatory to reach any reliable conclusion. The analysis of the arrival directions and their anisotropy can give further insight into the sources and provide information about the magnetic fields which the ultra high energy (UHE) cosmic rays experience during their travel to Earth. The Pierre Auger Observatory has been specifically de- signed to investigate the origin and the nature of ultra high energy cosmic rays. It is located in Malargüe, Ar- gentina, and consists of a surface array (SD) of 1660 wa- ter Cherenkov stations on an area of ' 3000 km2, over- looked by 27 air fluorescence telescopes (FD) grouped in four sites [4]. Thanks to the possibility of combin- ing the information from the surface array, measuring the lateral distributions of secondary particles at the ground, and the fluorescence telescopes, observing the longitudinal profile, the reconstruction capabilities are enhanced with respect to the individual detector com- ponents. 2 The Energy Spectrum The energy spectrum above 2.5 1018 eV has been de- termined using the data from the SD [5], considering only events up to 60◦. The exposure is obtained by in- tegrating the number of active stations over time; the overall acceptance uncertainty is ' 3% [6]. The energy calibration is derived directly from data, using a subset of high quality hybrid events, i.e. events reconstructed by both the FD and the SD [7]. Despite the low duty cycle of the FD, the energy spectrum could be extended to 1018 eV using hybrid events, thanks to the good en- ergy resolution and low threshold, thus investigating the transition region in detail [8]. The total systematic uncertainty in the energy scale is about 22%, the main contribution coming from the uncertainty in the fluo- rescence yield (14%) and in the reconstruction of the longitudinal profile (10%). The SD and hybrid spectra can be combined using a maximum likelihood method, since both have the same systematic uncertainties in the energy scale. The nor- malization uncertainties are on the contrary indepen- dent and have been used as additional constraints in the 132 http://dx.doi.org/10.14311/APP.2014.01.0132 Auger Highlights procedure. The resulting spectrum is shown in Fig.1; a fit with three power laws is shown by the dashed lines, while the solid line indicates the result of a fit with two power laws and a smooth function. The ankle feature is present at an energy of 1018.62 eV; the cutoff is clearly seen with a significance of 20 σ. Different astrophysi- cal models can be compared to our data; however, the energy spectrum can be described by both a heavy or proton composition at the highest energies and the in- formation must be complemented by independent mea- surements of the primary composition. Figure 1: The combined Auger energy spectrum. Only statistical uncertainties are shown. The system- atic uncertainty on the energy scale is 22%. A comparison of the Auger results with data from HiRes, Telescope Array and Yakutsk has been recently performed [9]. The various fluxes can be rescaled as- suming that any difference among them be due solely to energy scale and not to aperture calculations or energy resolution. The differences found are entirely consistent with the systematic energy uncertainties quoted by the experiments. 3 The Nature of the Primaries The most direct information about composition can be obtained by measuring the longitudinal development of showers in hybrid events, thus determining the mean depth of maximum development, Xmax, and its fluc- tuation, RMS(Xmax). For each event, Xmax depends on the depth of first interaction of the primary in the atmosphere and on the subsequent development of the shower; for this reason, the interpretation of the results in terms of composition is complicated by the uncer- tainties in the hadronic interaction models used in sim- ulations. About six years of hybrid data have been analyzed ap- plying FD quality cuts and ensuring that no bias with respect to the cosmic ray composition is introduced in the data sample [10]. Having been corrected for the detector resolution, the Xmax and its fluctuations are detector independent and can be directly compared to the predictions of different models, as shown in Fig.2. Both observables show a change for E > 5 1018 eV towards an increasingly heavy composition in compari- son to the model predictions. The average resolution of Xmax ' 20 g/cm2 in the considered energy range. Figure 2: < Xmax > and RMS(Xmax) as a function of energy, compared with the predictions of air shower simulations using different hadronic interaction models. In general, different values for RMS(Xmax) are al- lowed for different combinations of elements [11]. The fluctuations predicted by the considered hadronic inter- action models and shown in Fig.2 are evaluated only for pure compositions . A different conclusion, leading to a light composition up to the highest energies, has been drawn from the data of the HiRes and Telescope Array. However, a di- rect comparison of their results with the Auger ones is not possible, as the detector biases are included in their simulation. Furthermore, their dataset is smaller that that of Auger. A lenghty discussion about this compar- ison can be found in [12] (and refs. therein). Starting from an extension of the Heitler model of ex- 133 Antonella Castellina tensive air showers [13], a method for interpreting the results of Xmax and RMS(Xmax) in terms of mass com- position has been developed. As discussed in [14], Xmax is only function of the mean logarithmic mass < ln A >, and as such it carries information on the average com- position. On the contrary, both the shower-to-shower fluctuations and the dispersion in the mass distribution contribute to RMS(Xmax). This information can be used to extract the < lnA > and its variance from the observables and to build the plots shown in Fig.3, where the size of the data points increases with increasing en- ergy. The energy evolution of the composition is common to all models; reduced systematic uncertainties will allow in the future to test or even exclude some of them. Different consequences from the astrophysical point of view can be derived from this comparison. Extragalactic sources of protons seem to be disfavored by our composition result, within the uncertainties on the hadronic interaction models used to interpret the data. In a propagation scenario, nuclei from nearby sources could produce small mass dispersion at Earth, as propagation would not be able to degrade mass and energy. If on the other hand the proton component is depleted by the reach of a rigidity dependent end of the injection spectrum, and if sources are uniformly distributed, hard injection spectra with low energy cut- off, together with local sources, could explain the data [15, 16]. Figure 3: The Pierre Auger data in the (< lnA > ,σ2lnA) plane for different hadronic interaction models. Grey contours limit the allowed region, the systematic uncertainties are shown by the black lines. 4 Primary Photons and Neutrinos UHE primary photons and neutrinos can provide in- valuable information about the astrophysics of cosmic rays. Their detection would be a direct proof of the GZK cutoff; limits on exotic models [17] and tests for new physics [18] could be obtained from a positive or negative result on their detection. In both cases, their search is based on the characteristic features of the showers they produce in comparison to the hadronic ones. Primary photons produce late developing showers, a characteristic further enhanced by the LPM effect [19]. The deeper Xmax, observable by the FD, is associated to a more dispersed distribution of the arrival time of the particles at ground level. At a given distance from the shower axis, the arrival time of the first particles is delayed with respect to a planar shower front and the radius of curvature is thus expected to decrease for pho- ton induced showers. These observables can be recorded by means of the SD. The upper limits derived from both the SD and the hy- brid data collected by Auger are shown in Fig.4 and dis- cussed in [20, 21]. Astrophysical scenarios are favoured with respect to top-down models. Figure 4: Upper limits on the integral photon flux. Different expectations are shown for comparison. Primary neutrinos can produce showers character- ized by a significant electromagnetic component; the huge hadronic background can be eliminated by look- ing at large zenith angles. ντ can interact by charged current in the Earth crust, producing Earth-skimming upward-going showers; neutrinos of any flavour can in- teract in the atmosphere by neutral or charged current giving rise to downward-going events. Assuming a dif- fuse neutrino flux ' k E−2, 90% CL limits on their flux are obtained, as shown in Fig.5. Neutrinos from point sources were also searched for, over a broad declination range (north of ' −65◦ and 134 Auger Highlights south of ' 55◦), for Eν > 1017 eV. For a differential neutrino flux ' kPS E−2, 90% CL limits of ' 5 · 10−7 and 2.5 · 10−6 GeV cm−2 s−1 have been obtained for up-going and down-going events respectively [22]. Figure 5: Differential and integrated upper limits on the single flavour E−2 ν flux (thin black lines: downward-going ν, thick red line: Earth-skimming ν). Different models are shown for comparison and dis- cussed in [23] 5 Anisotropies The spatial distribution of the arrival directions of UHE cosmic rays as a function of energy is a key observable to provide information about their sources and nature, complementary to those of energy spectrum and com- position. Particles of UHE are most probably extra- galactic, and if the observed cutoff in the spectrum can be attributed to the GZK propagation effect we could expect their sources to be confined in our courtyard, within about 100 Mpc. In 2007 [24] the Auger Collaboration reported the ob- servation of a correlation between the arrival directions of the highest energy cosmic rays and the positions of nearby AGN from the Véron-Cetty-Véron catalogue [25]. The result came from an analysis of indipendent data with a priori parameters determined from an ex- ploratory scan; this allowed to avoid the use of penalty factors which would be needed in a posteriori analyses. The most recent update of this search is shown in Fig.6 [26]: the fraction of correlating cosmic rays is (33±5)% (28 events correlating out of a total of 84). The proba- bility of this correlation to occur by chance if the true distribution of arrival directions is isotropic stays below 1%. The independent averages of 10 consecutive events are also shown (black dots). A recent comparison of our result with the Telescope Array and the Yakutsk ones showed that the correlat- ing fractions are compatible [27]. More data are nec- essary to show whether this correlation is statistically significant or not. Figure 6: The correlating fraction as a function of the total number of time-ordered events. Different con- fidence levels are shown, together with the isotropy value p=0.21 and the current estimate of all data, pdata = 0.33 ± 0.05. Another possible scenario is that the anisotropy is dominated by cosmic rays originating from the vicin- ity of Centaurus A, the nearest active galaxy with an estimated distance of about 3.8 Mpc, since 19 events out of 7.6 expected have arrival directions within 24◦ of CenA. A Kolmogorov-Smirnov test shows that the chance probability for this to happen is 4%. Direction- ally aligned events, or ”multiplets”, can be expected from the same source after deflection in the magnetic fields, showing a correlation between the arrival direc- tion and the inverse of energy. The largest multiplet found was one 12-plet, but also in this case the prob- ability for it to come from an isotropic distribution is ' 6% [28]. Potential sources of galactic cosmic rays have been looked for by performing a blind search for neutron primaries [29]. In fact, due to the relativistic time di- latation the UHE neutron mean decay length is (9.2 × E/EeV ) kpc; above 2 EeV, neutron emitters can be searched for in the whole Galaxy. Auger can detect neutron showers by a simple search for an excess of proton-like showers from a specific direction in the sky. No candidates have been found, bringing to a median flux upper limit of 0.0114 n km−2 yr−1 above 1 EeV. The absence of a neutron flux from the Galaxy, which could be expected in the hypothesis of sources steadily emitting protons and neutrons with similar luminosity, could be a hint that the sources at EeV energy could be e.g. extragalactic, transient or weak but densely dis- tributed. The large scale distribution of the arrival directions of cosmic rays is another fundamental tool in the search for their origin. The results from a study performed using data from the SD array are shown in Fig.7 [30]. No significant anisotropies are observed, resulting in the most stringent bounds on the first harmonic amplitude above 2.5 1017 eV. 135 Antonella Castellina Figure 7: Equatorial dipole component (top) and phase of the first harmonic (bottom) as a function of energy. The obtained limits already exclude some of the galactic models (labeled in Fig.7 with A and S, indi- cating antisymmetric and symmetric galactic magnetic fields), according to which the cosmic rays at these en- ergies are galactic and can escape by diffusion and drift motion. In the model labeled Gal, cosmic rays are as- sumed to be galactic at all energies, and the anisotropy is due to purely diffusive motion caused by the turbu- lent component of the galacic magnetic field. In extra- galactic models, the transition is put at the second knee and the cosmic rays large scale distribution is influenced by the relative motion of the observer with respect to the frame of the source. Assuming that the frame in which the cosmic ray distribution at these energies is isotropic is coincident with the cosmic microwave back- ground rest frame, a small anisotropy (the extragalactic Compton-Getting effect, labeled C-G Xgal) is expected. Interestingly, the phase of the first harmonic shows a smooth transition between a common phase of ' 270◦ below 1 EeV and ' 100◦ above 5 EeV. A consistency of the phase in ordered energy intervals can indeed be expected in presence of a real underlying anisotropy, standing out of the background more prominently than the amplitude. However, no confidence level can for the moment be assigned to this result, being an ”a posteri- ori” observation. The study of the large scale anisotropy has been per- formed for the first time with Auger data as a function of both the right ascension and the declination and ex- pressed in terms of dipole and quadrupole amplitudes [31]. No significant deviations from isotropy are detected. Under the hypothesis that any anisotropy is dominated by these moments, the 99% CL upper limits can be derived, as shown in Fig.8. Figure 8: 99% CL upper limits on the dipole and quadrupole momenta as a function of energy. As an example of the power of the measurement to discriminate among different astrophysical models, the experimental limits are compared in the figure with the expectations from a toy model, in which the sources of protons and iron are stationary and uniformly dis- tributed in the galactic disk. Being the expected ampli- tudes for protons largely above the allowed upper limits, we can exclude this scenario for the light component of EeV primary cosmic rays. 6 Future Developments The Pierre Auger Observatory has reached a cumu- lative exposure of more than 26000 km2sr yr. New information about the characteristics of the primary cosmic rays have been derived, opening at the same time more questions and pointing to the need of an 136 Auger Highlights extension of the life time of Auger. The measurement of composition of the primary par- ticles from the EeV region up to the highest energies has emerged as the key for hitting the hottest scientific questions: a) understand the origin of flux suppression discussed in Sect.2, if due to the reach of the maxi- mum energy at injection or to the GZK effect (a clear signature of which would be the observation of a flux of primary photons and neutrinos); b) perform com- position driven anisotropy searches; c) determine the energy at which the transition from galactic to extra- galactic sources of cosmic rays takes place. Acknowledgement The author wishes to thank the organizers for the warm hospitality in Mondello and the stimulating and inter- esting discussions during the Workshop. References [1] V. Berezinski, A. Gazizov, S. Grig- orieva: 2006, Phys.Rev.D 74, 043005. doi:10.1103/PhysRevD.74.043005 [2] A.M. 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