Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition . Knurenko 1 and A. Sabourov 2 S. P 1 s.p.knurenko@ikfia.ysn.ru, 2 tema@ikfia.ysn.ru Yu. G. Shafer Institute of cosmophysical research and aeronomy Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 1
1. Introduction The Yakutsk EAS array is designed for detection of cosmic rays (CR) with energy 10 15 − 10 19 eV. It provides measurements of main components of extensive air showers (EAS): electrons, muons and Cherenkov light emission. Recently, experiments on radio-emission detection have been re-initiated. The array itself consists of several instruments, combined in a single system (see next two slides): the main array, small Cherenkov array, Cherenkov tracking detector based on camera-obscura, large muon detector, weather-station and a λ = 532 nm lidar to measure atmosphere parameters. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 2
1.1 The schematics of Yakutsk EAS array network Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 3
1.2 The Yakutsk EAS array layout Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 4
2. Energy spectrum The readings of light-integrating Cherenkov detectors can be analyzed separately from other detectors. The energy of CR particle initiating air shower is determined in almost model independent way involving Cherenkov light measurements. The depth shower maximum x max is derived from observations of Cherenkov light lateral distribution. Knowing the x max for protons and iron nuclei from simulations (QGSJET01), the mean logarithmic mass can be derived from measured x max : � ln A � = x max − x p � ln A Fe � max · max − x p x Fe max The experimental data set ( ∼ 75000 events at E 0 > 10 17 eV) is considered within two, dip and ankle scenarios. In both cases low energy part of CR spectrum J ( E ) is produced in supernova remnants (SNRs). The galactic part of spectrum J g ( E ) and composition are calculated within kinetic nonlinear theory (Berezhko and Völk (2007)). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 5
2.1 The dip-scenario: The second CR component J eg ( E ) is produced by extragalactic sources assuming that they produce CR s /E − 2 . 7 at E > 10 18 eV and taking into account spectrum J eg the modification of this spectrum due to the propagation effects in the intergalactic space (Aloisio et al (2007)). On the next slide: The dip-scenario compared to with several experiments. The dashed line represents the Galactic component. The dash-dotted line represents the assumed extragalactic component. It is seen, that experimental CR in a satisfactory way is consistent with theoretically expected spectrum within the dip scenario. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 6
2.1 The dip-scenario: Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 7
2.2 The ankle scenario: In the ankle scenario the extragalactic source spectrum is assumed to be much harder J eg s /E − 2 : J eg ( E ) becomes dominant above 10 19 eV and therefore the observed CR spectrum needs the third component — reacceleration mechanism (spiral shocks in the galactic wind, pulsar vicinity (Völk & Zirakishvili (2004), Bell (1992), Berezhko (1994))). Instead of J g Z ( E ) for every element with nuclear charge number Z we use J ′ g Z ( E ) which coincides with J g Z ( E ) at E < E max1 and at E > E max2 : � − γ � � � E − E � � J ′ g Z ( E ) = J g E Z Z ( E ) · · exp max1 E Z E Z max1 max2 where E Z max1 — minimal energy of particles involved into reacceleration process and E Z max2 — maximal particle energy achieved in reacceleration. On the next slide: The ankle scenario compared to experimental data. Dashed line — galactic component (including SNRs and reaccelerated CRs). Dash-dotted line — extragalactic component (corresponding to J eg s /E − 2 ). Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 8
2.2 The ankle scenario: Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 9
2.3 Mean CR atomic number Due to dependence of maximal (cutoff) energy of CR produced in SNRs E max ≃ 3 · Z · 10 15 eV on the atomic charge number Z CR composition becomes progressively heavier as the energy increases from E ∼ 10 15 eV to E ≃ 10 16 eV, where iron nuclei become dominant. At higher energies within the dip scenario the contribution of extragalactic CR becomes essential, therefore � ln A � goes down with the increase of the energy towards the value � ln A � ≃ 1 . 5 . Completely different CR composition is expected at E > 10 16 eV within the ankle scenario is expected at E > 10 16 within the ankle scenario. Due to reacceleration heavy CR with dominant CR iron nuclei extend from ∼ 10 15 eV to about ∼ 10 19 eV (see next slide). In this case the transition towards lighter extragalactic component occurs at E ∼ 10 19 eV. Next slide: Mean logarithm of the CR nucleus atomic number as a function of energy calculated within the dip and ankle scenario are represented by solid and dashed lines respectively. Experimental data obtained at ATIC-2, JACEE, KASCADE, HiRes, PAO and Yakutsk EAS experiments. It follows that the Yakutsk EAS array data better agree with the ankle scenario. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 10
2.3 Mean CR atomic number Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 11
3. Longitudinal distribution It is known fact that the depth of shower maximum ( x max ) and fluctuations in EAS development are sensitive to atomic number of primary particle and for this reason they are used to estimate the CR mass composition (Efimov et al (1987)), Dyakonov et al (1989), Knurenko et al (2005)). Here we present the data on longitudinal EAS development reconstructed from Cherenkov emission data. These data were obtained after modernization of the Yakutsk array when the accuracy of main EAS characteristics increased as compared to previous series of observations. It is important to consider not only mean EAS parameters, e.g x max , muon content ρ µ /ρ ch but also their fluctuations in given energy intervals. In order to minimize the latter, it is also a good idea to analyze them at fixed energies. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 12
3.1 Technical aspects Determination of x max in individual shower is based on methods developed at the Yakutsk array and utilize the measurements of EAS Cherenkov light emission at different core distances. the x max is determined by parameter p = lg Q 200 /Q 550 (a relation of Cherenkov light fluxes at 200 and 550 m from the core); involving the reconstruction of EAS development cascade curve, using Cherenkov light lateral distribution function and a reverse solving (Knurenko et al (2001); based on half-width and half-height of Cherenkov light pulse recorded at 200 m from the core; fourth method includes recording of Cherenkov track with several detectors based on camera-obscura located at 300 − 500 m from the array center (Petrov et al (2008)). Following slides: examples demonstrating these techniques for x max estimation. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 13
3.1.1 Estimation by parameter p = lg Q 200 /Q 550 10 10 10 9 Q ( R ), photon/m 2 10 8 10 7 E 0 = 1.3 × 10 19 eV; θ = 25 ° ; x max = 738 g/cm 2 10 6 100 1000 core distance , m Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 14
3.1.2 Estimation by the shape of Cherenkov pulse Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 15
3.1.3 Estimation with Cherenkov tracking detector Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 16
3.1 Technical aspects The accuracy of x max determination in individual showers was estimated in simulation of EAS characteristics measurements at the array involving Monte-Carlo methods and amounted to 30 − 45 g/cm 2 , 35 − 55 g/cm 2 , 15 − 25 g/cm 2 , 35 − 55 g/cm 2 accordingly for first, second, third and fourth methods. Total error of x max estimation included errors associated with core location, atmospheric transparency during observational period, hardware fluctuations and mathematical methods used to calculate main parameters. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 17
3.2 Mean depth of maximal shower development 1000 900 800 x max , g/cm 2 700 600 500 400 300 10 6 10 7 10 8 10 9 Q ( R ), photon/m 2 A cloud of points in x max distribution for showers with energy above 10 17 eV. These data were obtained using all four methods and reflect an alteration of x max towards lower atmosphere depths with growth of energy. Cosmic ray studies at the Yakutsk EAS array: energy spectrum and mass composition – p. 18
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