The amplitude and phase of the cosmic ray anisotropy are well established
experimentally between 10^{11} eV and 10^{14} eV. The study of their evolution
into the energy region 10^{14}-10^{16} eV can provide a significant tool for
the understanding of the steepening ("knee") of the primary spectrum. In this
letter we extend the EAS-TOP measurement performed at E_0 around 10^{14} eV, to
higher energies by using the full data set (8 years of data taking). Results
derived at about 10^{14} and 4x10^{14} eV are compared and discussed. Hints of
increasing amplitude and change of phase above 10^{14} eV are reported. The
significance of the observation for the understanding of cosmic ray propagation
is discussed.Comment: 4 pages, 3 figures, accepted for publication on ApJ Letter

A. Castellina

A. Chiavassa

A. Stamerra

Abdo

Aglietta

Alekseenko

Amenomori

Andreev

Antoni

B. Alessandro

B. D'Ettorre Piazzoli

Berezhko

Berezinsky

C. Morello

C. Vigorito

F. Arneodo

Farley

Fenton

G. C. Trinchero

G. Di Sciascio

G. Mannocchi

G. Navarra

Gherardy

Heck

Jones

Kifune

L. Bergamasco

M. Aglietta

M. Bertaina

M. Iacovacci

O. Saavedra

Over

P. Antonioli

P. Galeotti

P. L. Ghia

P. Vallania

Peters

R. Bonino

S. Valchierotti

S. Vernetto

V. V. Alekseenko

W. Fulgione

Zatsepin

English

arXiv

The amplitude and phase of the cosmic ray anisotropy are well established experimentally between 10(11) eV and 10(14) eV. The study of their evolution into the energy region 10(14) − 10(16) eV can provide a significant tool for the understanding of the steepening (“knee”) of the primary spectrum. In this letter we extend the EAS-TOP measurement performed at E0 ≈ 10(14) eV, to higher energies by using the full data set (8 years of data taking). Results derived at about 10(14) and 4 · 10(14) eV are compared and discussed. Hints of increasing amplitude and change of phase above 10(14) eV are reported. The significance of the observation for the understanding of cosmic ray propagation is discussed

D'ETTORRE PIAZZOLI, BENEDETTO

IACOVACCI, MICHELE

Archivio della ricerca - Università degli studi di Napoli Federico II

Evolution of the cosmic-ray anisotropy above 10(14) eV.

Crossref

EVOLUTION OF THE COSMIC-RAY ANISOTROPY ABOVE 10
                    14

The amplitude and phase of the cosmic-ray anisotropy are well established experimentally between 1011 eV and 1014 eV. The study of their evolution in the energy region 1014-1016 eV can provide a significant tool for the understanding of the steepening ("knee") of the primary spectrum. In this Letter, we extend the EAS-TOP measurement performed at E 0 ≈ 1014 eV to higher energies by using the full data set (eight years of data taking). Results derived at about 1014 and 4 × 1014 eV are compared and discussed. Hints of increasing amplitude and change of phase above 1014 eV are reported. The significance of the observation for the understanding of cosmic-ray propagation is discussed

Aglietta M.

Alekseenko V. V.

Alessandro B.

Antonioli P.

Arneodo F.

Bergamasco L.

Bertaina M.

Bonino R.

Castellina A.

Chiavassa A.

D'Ettorre Piazzoli B.

Di Sciascio G.

Fulgione W.

Galeotti P.

Ghia P. L.

Iacovacci M.

Mannocchi G.

Morello C.

Navarra G.

Saavedra O.

Stamerra A.

Trinchero G. C.

Valchierotti S.

Vallania P.

Vernetto S.

Vigorito C.

Archivio della Ricerca - Università degli Studi di Siena

Evolution of the cosmic-ray anisotropy above 1014 eV

Bergamasco, Laura Maria

Bertaina, Mario Edoardo

Bonino, Raffaella

Chiavassa, Andrea

Galeotti, Piero

Navarra, Gianni Maria

Saavedra, Oscar

Vigorito, Carlo Francesco

Institutional Research Information System University of Turin

The Astrophysical Journal, 692:L130–L133, 2009 February 20 doi:10.1088/0004-637X/692/2/L130c© 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.EVOLUTION OF THE COSMIC-RAY ANISOTROPY ABOVE 1014 eVM. Aglietta1,2, V. V. Alekseenko3, B. Alessandro2, P. Antonioli4, F. Arneodo5, L. Bergamasco2,6, M. Bertaina2,6,R. Bonino1,2, A. Castellina1,2, A. Chiavassa2,6, B. D’Ettorre Piazzoli7, G. Di Sciascio7,8, W. Fulgione1,2, P. Galeotti2,6,P. L. Ghia1,5,9, M. Iacovacci7, G. Mannocchi1,2, C. Morello1,2, G. Navarra2,6, O. Saavedra2,6, A. Stamerra6,10,G. C. Trinchero1,2, S. Valchierotti2,6, P. Vallania1,2, S. Vernetto1,2, C. Vigorito2,6, andThe EAS-TOP Collaboration1 Istituto di Fisica dello Spazio Interplanetario, INAF, Torino, Italy2 Istituto Nazionale di Fisica Nucleare, Torino, Italy3 Institute for Nuclear Research, AS Russia, Baksan Neutrino Observatory, Russia4 Istituto Nazionale di Fisica Nucleare, Bologna, Italy5 Laboratori Nazionali del Gran Sasso, INFN, Assergi (AQ), Italy6 Dipartimento di Fisica Generale dell’Universita`, Torino, Italy7 Dipartimento di Scienze Fisiche dell’Universita`, INFN, Napoli, ItalyReceived 2008 December 2; accepted 2009 January 14; published 2009 February 6ABSTRACTThe amplitude and phase of the cosmic-ray anisotropy are well established experimentally between 1011 eV and1014 eV. The study of their evolution in the energy region 1014–1016 eV can provide a significant tool for the under-standing of the steepening (“knee”) of the primary spectrum. In this Letter, we extend the EAS-TOP measurementperformed at E0 ≈ 1014 eV to higher energies by using the full data set (eight years of data taking). Results derivedat about 1014 and 4 × 1014 eV are compared and discussed. Hints of increasing amplitude and change of phaseabove 1014 eV are reported. The significance of the observation for the understanding of cosmic-ray propagation isdiscussed.Key words: cosmic rays – diffusion1. INTRODUCTIONThe steepening (“knee”) observed at E0 ≈ 3 × 1015 eVrepresents a main feature of the energy spectrum of cosmicrays and its characterization is therefore a main tool for theunderstanding of the galactic radiation. Composition studieshave shown that it is related to the steepening of the spectraof the lightest primaries (protons, helium, CNO: Aglietta et al.2004; Antoni et al. 2005).Such an effect can be due, on the one hand, to energy limitsof the acceleration process at the source, namely diffusive shockacceleration in supernova remnants, generally considered tobe the sources of galactic cosmic rays. The maximum energyof the accelerated protons is, indeed, calculated to occur inthe 1015 eV energy region (Berezhko et al. 1996; Berezhko& Volk 2007), but could reach up to about 1017 eV (Ptuskin& Zirakashvili 2003). On the other hand, this feature hasbeen possibly explained in terms of a change in the cosmic-ray propagation properties inside the Galaxy (Peters 1960;Zatsepin et al. 1962). Galactic propagation is described throughdiffusion models whose parameters have been obtained throughcomposition studies (mainly from the ratio of secondary toprimary nuclei) at energies well below 1012 eV (see, e.g., Joneset al. 2001; Strong et al. 2007). The diffusion coefficient, D,is found to increase with magnetic rigidity (D ∝ R0.6, orD ∝ R0.3 for models including reacceleration). However, noconfirmation, and no information, has till now been obtained athigher energies, where the main observable is represented bythe large-scale anisotropy in the cosmic rays’ arrival directionsthat is known to be strictly related to the diffusion coefficient8 Presently at Istituto Nazionale di Fisica Nucleare, Roma Tor Vergata, Italy.9 Presently at Institut de Physique Nucle´aire, CNRS, Orsay, France.10 Presently at Dipartimento di Fisica dell’Universita` and INFN, Pisa, Italy.(see, e.g., Berezinsky et al. 1990). The study of the evolution ofthe anisotropy in the “knee” energy region can therefore providea significant test of the diffusion models and a valuable insightinto the discrimination between the two possible explanationsof the spectral steepening.At E0 ≈ 1014 eV, the EAS-TOP11 results (Aglietta et al.1996) demonstrated that the main features of the anisotropy(i.e., of cosmic-ray propagation) are similar to those measuredat lower energies (1011–1014 eV), both with respect to amplitude((3–6) × 10−4) and phase ((0–4) hr local sidereal time (LST))(Gombosi et al. 1975; Fenton et al. 1975; Nagashima et al. 1989;Alekseenko et al. 1981; Andreev et al. 1987; Ambrosio et al.2003; Munakata et al. 1997; Amenomori et al. 2005; Guillianet al. 2007; Abdo et al. 2008). At higher energies the limitedstatistics do not allow any firm conclusion to be drawn (Kifuneet al. 1984; Gherardy et al. 1983; Antoni et al. 2004; Amenomoriet al. 2006; Over et al. 2007).In this Letter, we present the EAS-TOP measurement basedon the full data set and we extend the analysis to about 4 ×1014 eV.2. THE EXPERIMENT AND THE ANALYSISThe EAS-TOP Extensive Air Shower array was located atCampo Imperatore (2005 m a.s.l., latitude 42◦27′N, longitude13◦34′E, INFN Gran Sasso National Laboratory). The electro-magnetic detector used for the present analysis (Aglietta et al.1993) consisted of 35 modules of scintillator counters, 10 m2each, distributed over an area of about 105 m2. The trigger wasprovided by the coincidence of any four neighboring modules(threshold np ≈ 0.3 minimum ionizing particles per module),the event rate being f ≈ 25 Hz. The data under discussion have11 The Extensive Air Shower array on TOP of the Gran Sasso undergroundlaboratories.L130No. 2, 2009 COSMIC-RAY ANISOTROPY FROM EAS-TOP L131Table 1Characteristics of the Two Classes of Events Used in the AnalysisClass Nmodulesa E0 (eV)b NEWcI 4 1.1 × 1014 1.5 × 109II 12 3.7 × 1014 1.7 × 108Notes.a Number of triggered modules; b primary energy; and c number of collectedevents in the east + west sectors.been collected between 1992 January and 1999 December for atotal of 1431 full days of operation.To select different primary energies, a cut is applied to theevents based on the number of triggered modules (see Table 1).The average primary energies are evaluated for primary protonsand the QGSJET01 hadron interaction model (Kalmykov et al.1997) in CORSIKA (Heck et al. 1998).For the analysis of the anisotropy, we adopt a method basedon the counting rate differences between eastward and westwarddirections, which allows us to remove counting rate variationsof atmospheric origin. The events used in the analysis (see Ta-ble 1) are those with the azimuth angle φ inside ±45◦ aroundthe east and west directions, and the zenith angle θ < 40◦. Thedifference between the number of counts measured from the eastsector, CE(t), and from the west one, CW (t), at time t in a fixedinterval (Δt = 20 minutes), is related to the first derivative of theintensity I (t) as dIdt D(t) =CE (t)−CW (t)δt, where δt is the aver-age hour angle between the vertical and each of the two sectors(1.7 hr in our case). The harmonic analysis is performedon the differences D(t); the amplitudes and phases of thevariation of I (t) are obtained through the integration of thecorresponding terms of the Fourier series (Aglietta et al.2007).3. RESULTSThe harmonic analysis has been performed in solar, sidereal,and antisidereal time.12 We describe in Section 3.1 the results ofthe analysis, while in Section 3.2 we show the related countingrate curves.3.1. The Harmonic AnalysisFor the two different primary energies, the reconstructedamplitudes and phases of the first and second harmonics areshown in Table 2 together with the corresponding Rayleighimitation probabilities (P).Concerning the first harmonic:1. At 1.1 × 1014 eV, from the analysis in solar time, theobtained amplitude and phase(AIsol = (2.8 ± 0.8) × 10−4,φIsol = (6.0 ± 1.1) hr, P Isol = 0.2%)are in excellentagreement with the expected ones from the Compton–Getting effect (Compton & Getting 1935) due to therevolution of the Earth around the Sun: at our latitudeAsol,CG = 3.0 × 10−4, φsol,CG = 6.0 hr.With respect to the sidereal time analysis, the measuredamplitude and phase(AIsid = (2.6 ± 0.8) × 10−4, φIsid =(0.4±1.2) hr LST), with imitation probability P Isid = 0.5%,confirm the previous EAS-TOP result (Aglietta et al. 1996).12 The antisidereal time is a fictitious timescale symmetrical to the siderealone with respect to the solar time and that reflects seasonal influences (Farley& Storey 1954).Figure 1. Bimonthly solar vectors at 1.1 × 1014 eV. Capital and lower-caserefer respectively to the expected and theoretical points (A, a = January +February; B, b = March + April; C, c = May + June; D, d = July + August;E, e = September + October; F, f = November + December). The typical vectorstatistical uncertainty is shown.Bimonthly vectors representing the first harmonic areshown in Figure 1 (dots), together with the expected ones(stars) from the measured solar and sidereal amplitudes.The expected counterclockwise rotation of the vector isclearly visible, showing that, at any time, the compositionof the two vectors is observed, and that the expected andmeasured individual values are fully compatible within thestatistical uncertainties.2. At 3.7 × 1014 eV the amplitude and phase of the mea-sured first harmonic in solar time are still consistent withthe expected ones for the solar Compton–Getting effect,although, due to the reduced statistics, the chance imitationprobability is rather high.Concerning the analysis in sidereal time, we obtainAIsid = (6.4 ± 2.5) × 10−4, φIsid = (13.6 ± 1.5) hr LST,with an imitation probability of about 3.8%. This indicates,therefore, a change of phase (from 0.4 to 13.6 hr) and anincrease of amplitude (by a factor of 2.5) with respect tothe first harmonic measured at 1.1 × 1014 eV.Concerning the second harmonic: most significant(P IIsid =1.6%)is the amplitude observed in sidereal time in the lower-energy class of events (comparable with the first harmonic one:AIIsid = (2.3 ± 0.8) × 10−4, φIIsid = (6.3 ± 0.7) hr LST; see alsoAlekseenko et al. 1981).Both at 1.1 × 1014 eV and 3.7 × 1014 eV, no signifi-cant amplitude is observed in antisidereal time, showing thatno additional correction is required due to residual seasonaleffects.L132 AGLIETTA ET AL. Vol. 692Table 2Results of the Analysis of the First (Amplitude AI, Phase φI, and Rayleigh Imitation Probability PI) and Second Harmonic (AII, φII, PII) in Solar (Columns 2–4),Sidereal (Columns 5–7), and Antisidereal Time (Columns 8–10)E0 (eV) AIsol104 φIsol (hr) P Isol (%) AIsid104 φIsid (hr) P Isid (%) AIasid104 φIasid (hr) P Iasid (%)1.1 × 1014 2.8 ± 0.8 6.0 ± 1.1 0.2 2.6 ± 0.8 0.4 ± 1.2 0.5 1.2 ± 0.8 23.9 ± 2.8 32.53.7 × 1014 3.2 ± 2.5 6.0 ± 3.4 44.1 6.4 ± 2.5 13.6 ± 1.5 3.8 3.4 ± 2.5 22.3 ± 3.2 39.7Aiisol104 φiisol (hr) P iisol (%) Aiisid104 φiisid (hr) P iisid (%) Aiiasid104 φiiasid (hr) P iiasid (%)1.1 × 1014 1.4 ± 0.8 7.0 ± 1.2 21.6 2.3 ± 0.8 6.3 ± 0.7 1.6 0.6 ± 0.8 . . . 75.53.7 × 1014 1.7 ± 2.5 . . . 79.4 1.5 ± 2.5 . . . 83.5 1.2 ± 2.5 . . . 89.1Note. Phases are not defined when amplitudes are smaller than their uncertainties.Figure 2. Thick black lines: counting rate curves in solar (a), sidereal (b), andantisidereal (c) time at 1.1 × 1014 eV. The statistical uncertainty for each bin isgiven in the first one. The curves resulting from the first harmonic analysis arealso shown (light black lines); for the sidereal time curve, the combination ofthe first and second harmonics (dotted black line) is additionally superimposed.3.2. The Counting Rate CurvesBesides the harmonic analysis, it is interesting to visualizethe variations of the cosmic-ray intensity versus time, I (t), asreconstructed by the integration of the east–west differences,D(t). They are shown in Figures 2 and 3 for the classes ofevents at 1.1 × 1014 eV and 3.7 × 1014 eV, respectively (a, b,and c for solar, sidereal, and antisidereal timescales).As already shown by the harmonic analysis, at both energiesthe curves in solar time are dominated by the Compton–Gettingeffect due to the motion of the Earth, and no modulation isvisible in the antisidereal timescale.A main difference is observed in the sidereal time curves:while the shape of the curve at 1.1 × 1014 eV is in remarkableFigure 3. Thick black lines: counting rate curves in solar (a), sidereal (b),and antisidereal (c) time at 3.7 × 1014 eV. The curves resulting from the firstharmonic analysis are also shown (light black lines).agreement with the EAS and muon measurements reported atand below 1014 eV, the curve related to the highest energy classof events is characterized by a broad excess around 13–16 hrLST.4. CONCLUSIONSHigh-stability data obtained from long-time observations(eight years) from the EAS-TOP array confirm the amplitudeand phase of the cosmic-ray anisotropy already reported at1014 eV: AIsid = (2.6 ± 0.8) × 10−4, φIsid = (0.4 ± 1.2) hrLST, with the Rayleigh imitation probability P Isid = 0.5%. Theresult is supported by the observation of the Compton–Gettingeffect due to the revolution of the Earth around the Sun, and bythe absence of antisidereal effects. It confirms the homogeneityof the anisotropy data over the energy range 1011–1014 eV.No. 2, 2009 COSMIC-RAY ANISOTROPY FROM EAS-TOP L133At higher energies (around 4 × 1014 eV) the observedanisotropy shows a larger amplitude, AIsid = (6.4 ± 2.5)×10−4,and a different phase, φIsid = (13.6 ± 1.5) hr LST, with animitation probability of 3.8%. The statistical significance is stilllimited, but the measurement has the highest sensitivity withrespect to previous experiments at these energies, and it is notin contradiction with any of them.The dependence of the anisotropy amplitude over primaryenergy(A ∝ Eδ0)deduced from the present two measurementscan be represented by a value of δ = 0.74 ± 0.41. Therefore, atleast in the energy range (1 − 4) × 1014 eV, such dependence iscompatible with that of the diffusion coefficient as derived bycomposition measurements at lower energies.On the other hand, the sharp increase of the anisotropyabove 1014 eV may be indicative of a sharp evolution of thepropagation properties, and therefore of the diffusion coefficientjust approaching the steepening of the primary spectrum. Thisopens the problems of obtaining an improved theoretical andexperimental description of the whole evolution of the diffusionprocesses versus primary energy, and understanding how suchevolution could affect the energy spectra at the “knee.” Fromthe experimental point of view, the extension of the anisotropymeasurements with high sensitivity to and above 1015 eV willbe of crucial significance.V.V.A. is grateful to the INFN Gran Sasso National Labora-tory for financial support through FAI funds. P.L.G. acknowl-edges the financial support by the European Community 7thFramework Program through the Marie Curie Grant PIEF-GA-2008-220240.REFERENCESAbdo, A. A., et al. 2008, arXiv:0806.2293Aglietta, M., et al. 1993, Nucl. Instrum. Methods Phys. Res. A., 336, 310Aglietta, M., et al. 1996, ApJ, 470, 501Aglietta, M., et al. 2004, Astropart. Phys., 21, 583Aglietta, M., et al. 2007, in Proc. 30th ICRC, ed. R. Caballero, et al., 4, 51Alekseenko, V. V., et al. 1981, in Proc. 17th ICRC, ed. 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Evolution of the Cosmic-Ray anisotropy above 10(14) eV

EVOLUTION OF THE COSMIC-RAY ANISOTROPY ABOVE 1014eV

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Evolution of the cosmic ray anisotropy above 10^{14} eV

Evolution of the cosmic ray anisotropy above 10^{14} eV

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Archivio della ricerca - Università degli studi di Napoli Federico II

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Institutional Research Information System University of Turin

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