1. Ultra High Energy Cosmic Rays : 40 years Retrospective of Continuous Observations at the Yakutsk Array. Part 1: Cosmic Rays Spectrum in the Energy Range 10 15 – 10 18 eV and its Interpretation I. S. Petrov, S.P. Knurenko, Z.E. Petrov, I.Ye. Sleptsov Yu. G. Shafer Institute of Cosmophysical Research and Aeronomy of SB RAS, Yakutsk, Russia E-mail: igor.petrov@ikfia.sbras.ruigor.petrov@ikfia.sbras.ru

2. Contents Part 1: Cosmic Rays Spectrum in the Energy Range 10 15 – 10 18 eV and its Interpretation  History of Yakutsk array  Yakutsk array  Cosmic ray spectrum  Spectrum interpretation Part 2: Mass Composition of Cosmic Rays at Ultra High Energies  Methods for estimating mass composition  Mass composition of Cosmic rays data interpretation (recent results)

3. History of Yakutsk array  First phase (1969 - 1974): The array was built on area of 3.5 km 2 with 13 scintillation and Cherenkov detectors, spacing between detectors were from 500 to 1000 m. The array was triggered by air showers event with energy E ≥ 10 17 eV. Photo1: Head of the Yakutsk EAS DD Krasilnikov & John Linsley at the 4th European Symposium on Łódź, 1974.

4.  Second phase (1974 – 1994): The area was increased to 18 km 2 with 43 stations with scintillation detectors, 36 Cherenkov detectors and 3 muon underground stations with triggering energy  1 GeV with detecting area of 16 m 2. Detectors on the periphery were spaced 1km. Detectors in the center of the array spaced 100, 250 and 500 m. Prof. Watson, Yakutsk, 1984 Prof. Cronin to visit Yakutsk array, 1986 In total, at the end of the second phase the arrays consisted o 110 scintillation detectors o 36 Cherenkov detectors o 24 muon detectors o 43 channels for registration of air shower events

5.  Third Phase (1994 - 2004): o Increased total number of stations on the periphery up to 60. o Muon underground detectors – 7, total area ~300m 2. o Cherenkov detectors – 50 o 550 independent channels for registration of air shower events o Area of the array 13 km 2 Prof. Nagano, Yakutsk array ( summer, 1994)

6.  Fourth Phase (2004 - ): Mostly added new measurements within 500 m circle. Which are o 3 tracking Cherenkov detectors 250, 300 & 500 m from center o 8 ground scintillation detectors that measures pulse shape o 4 underground scintillation detectors for registering muons E  0,1 & 1 GeV o 35 channels for direct measuring of Cherenkov light by differential detectors o 12 antennas to register air showers radio emission o Transparency of the atmosphere monitoring, aerosols and electric field near ground level during clear sky and thunderstorms. o Continued modernization of the array in order to improve precision of the measurements of all air shower components, including precision of determination of air shower arrival angle.

7. Contents Part 1: Cosmic Rays Spectrum in the Energy Range 10 15 – 10 18 eV and its Interpretation  History of Yakutsk array  Yakutsk array  Cosmic ray spectrum  Spectrum interpretation Part 2: Mass Composition of Cosmic Rays at Ultra High Energies  Methods for estimating mass composition  Mass composition of Cosmic rays data interpretation (recent results)

8. Yakutsk array Air shower 10 16 – 10 20 eV 61,7° N, 129,4° E 110 m above sea level

9. Yakutsk Small Cherenkov array

10. Yakutsk array Cherenkov detectors

11. Contents Part 1: Cosmic Rays Spectrum in the Energy Range 10 15 – 10 18 eV and its Interpretation  History of Yakutsk array  Yakutsk array  Cosmic ray spectrum  Spectrum interpretation Part 2: Mass Composition of Cosmic Rays at Ultra High Energies  Methods for estimating mass composition  Mass composition of Cosmic rays data interpretation (recent results)

12. Measurements of Cherenkov light emission at Yakutsk array

13. Cherenkov light lateral distribution function. Q(150) and Q(400) classification parameters. EAS energy estimation Cherenkov light lateral distribution (photon/m 2 ) Classification parameters. 1 – Q (150), 2 – Q (400)

14. Algorithm of the energy balancing method:

15. Energy balance Table 1. Primary experimental data n/n lgQ(100) lgQ(200) lg  (300) lg F lgNs(0) lgN  (0) X мах lgk 1 6,01 - 11,42 5,80 4,91 560 205 3,94 2 6,17 - 11, 6,26 5,25 590 188 3,92 4 6,73 - 12,11 6,59 5,49 610 185 3,90 5 6,91 6,42 12,32 6,76 5,62 634 182 3,89 6 7,06 6,58 12,42 6,95 5,76 670 186 3,86 7 7,18 6,70 12,58 7,11 5,88 634 188 3,89 8 7,31 6,82 12,68 7,23 5,97 670 190 3,86 9 7,48 6,97 12,82 7,38 6,08 670 192 3,86 10 7,56 7,09 12,90 7,42 6,16 634 195 3,89 11 7,69 7,19 13,00 7,63 6,26 655 198 3,87 12 7,82 7,28 13,10 7,79 6,38 674 201 3,86

16. Empirical estimate of the energy of EAS at Yakutsk array Table 2. The energy transferred to the different components of the EAS. n/n lgEei lg Eel lg Em lg Ehi lg(Emi + Ev) lgE0 1 15,687 14,506 14,840 14,465 14,721 15,823 2 15,830 14,612 14,951 14,608 14,832 15,961 3 16,064 14,876 15,175 14,842 15,056 16,195 4 16,345 15,199 15,410 15,123 15,291 16,471 5 16,540 15,362 15,506 15,318 15,387 16,650 6 16,797 15,726 15,783 15,575 15,664 16,916 7 16,874 15,851 15,899 15,652 15,780 17,002 8 17,014 16,001 16,015 15,792 15,896 17,139 9 17,116 16,122 16,081 15,894 15,962 17,238 10 17,208 16,269 16,173 15,986 16,054 17,334 11 17,297 16,435 16,306 16,075 16,187 17,436 12 17,374 16,538 16,358 16,152 16,239 17,512 13 17,480 16,646 16,410 16,000 16,291 17,600 14 17,570 16,758 16,456 16,348 16,337 17,700

17. Events selection. Triggering system. The effective area of events selection. Effective area of EAS registration as a function of the classification parameter Q(150) Configuration of master combinations used at the small Cherenkov array

18. Simulation and measurement of the spectrum of CR by Yakutsk Small Cherenkov array. The simulation accounts: threshold of the detector fluctuations of background illumination Follows criterion: Time difference between signal from 3 stations that make up equilateral triangle must be ≤2,5 µs. Configuration of master combinations used at the small Cherenkov array

19. Air shower characteristics measurement precision reached at the Small Cherenkov Yakutsk array Е 0, PeV  (R), m  N s  (Q(100)) phot./m 2  (Q(200) phot./m 2  (Q(400) phot./m 2  (  s (300)) 1/m 2  (  s (600)) 1/m 2     2 9,7 0,15 0,17 1,3 10 7,2 0,11 0,15 1,0 100 15,5 0,27 0,15 0,25 5,7 200 34,6 0,32 0,20 0,22 0,25 5,4 1000 26,7 0,35 - - 0,20 0,17 0,19 3,3 lg(Eo), eV15-15,315,3-15,615,6-15,916,3-16,616,6-16,917,3-17,6 б, m1210,79,48,26,75,1 б Roапп, m21,938,6613,7516,347,5811,35 б Sапп 0,1420,1320,1250,1110,0980,092 б Neапп /Nср0,1520,1150,1290,1140,1280,096 б Xmапп, г/cm 2 70,869,769,567,86563,3

20. Conclusion To estimate the precision of the measured characteristics of air showers complete simulation of model QGSJET was performed, which takes into account the instrumental and methodological errors and fluctuations in the development of air showers. The simulations was performed for the winter atmosphere near Yakutsk. For optical measurements we used real transparency of the atmosphere, which directly measured at the array. According to the space between stations, each triangle effectively selects showers in its energy range, but on threshold of each interval the efficiency of selection of events is disrupted and this affects the calculation of the spectrum intensity. This is so-called transition effect. Here is the transition effect is the transition from one type of trigger system which selects air showers with lower energy to another that selects air showers with higher energy. As shown by simulations, the impact of the transition effect in this case was reduced from 30% to 17% due to corrections to the collecting area of EAS events, which increased to high-energy showers and therefore overstated by the same amount the intensity of the cosmic ray spectrum.

21. Energy spectrum of cosmic rays obtained by the small Cherenkov setup at the Yakutsk array

22. CR spectra obtained at various compact EAS arrays:

23. Summary spectrum according to Yakutsk, Tunka, KASKADE Grande, Gamma and Tibet 3:

24. Contents Part 1: Cosmic Rays Spectrum in the Energy Range 10 15 – 10 18 eV and its Interpretation  History of Yakutsk array  Yakutsk array  Cosmic ray spectrum  Spectrum interpretation Part 2: Mass Composition of Cosmic Rays at Ultra High Energies  Methods for estimating mass composition  Mass composition of Cosmic rays data interpretation (recent results)

25. Possible sources forming part of the spectrum around knee Contribution of CR from different SNR types and contribution of nearby individual sources to the all particle spectrum in the present model together with selective experimental data of small Cherenkov array to Yakutsk [1]. Results from [2]. 10 15 – 10 16 eV – the shape of the spectrum due to the luminosity of nearby sources, like SNR in our galaxy ( Erlykin, Sveshnikova et al (2013) ): HB21, HB9, Cygnus Loop, Vela JR. 9, SN Iib, SN Ia, CC_SNR& Vela Jr [1] S.P. Knurenko, A. Sabuorov. // Proc. 33rd ICRC, Rio De Janeiro, 2013. [2] L.G. Sveshnikova, E.E. Korosteleva, L.A. Kuzmichev et al. Nearby sources in the transition region between Galactic and extragalactic cosmic rays. // arxiv.org:1303.1713 arxiv.org:1303.1713

26. Nearby sources in the transition region between Galactic and Extragalactic CR Following the idea that SNR are the sources of CR [3], K. Kotera [4] performed calculations for the three propagation model and Galactic, Metagalactic component of CR in the Universe. The purpose of these calculations was the interpretation of experimental spectrum in the energy range 10 15 – 10 18 eV, obtained from small arrays. And in particular, an attempt to explain the formation of second knee as the birth and spread of cosmic rays in the galaxy and beyond. [3] E.G. Berezhko, S.P. Knurenko, L.T. Ksenofontov. Composition of cosmic rays at ultra high energies. // Astroparticle Physics 36 (2012) pp. 31-36. [4] Kumiko Kotera and Martin Lemoine. Inhomogeneous extragalactic magnetic fields and the second knee in the cosmic ray spectrum. // arXiv: 0706.1891v2 [astro-Ph] 4Jan 2008.

27. Conclusion Experimental data on CR spectrum obtained recently in compact arrays, indicate the existence of irregularities in the spectrum of the second CR (second knee) at energy  2  10 17 eV. Over the observation time (20 years of continuous observations at Yakutsk Small Cherenkov Detector) and event statistics of air showers with energy above 10 17 eV advance confirmation of existence of second knee most succeeded in Yakutsk, observing spectrum of Cherenkov "flashes" on the small Cherenkov array. There is reason to assume that the observed physics of the first and second breaks in the spectrum associated with astrophysical processes occurring both within our Galaxy, so beyond. Relatively smaller difference  between first and second break points can be explained by the transition boundary from galactic metagalactic to CR. Most possible sources of CR in the energy range 10 15 – 10 18 eV can be supernova remnants (SNR)

28. Contents Part 1: Cosmic Rays Spectrum in the Energy Range 1015 – 1018 eV and its Interpretation  History of Yakutsk array  Yakutsk array  Cosmic ray spectrum  Spectrum interpretation Part 2: Mass Composition of Cosmic Rays at Ultra High Energies  Methods for estimating mass composition  Mass composition of Cosmic rays data interpretation (recent results)

29. Methods for estimating mass composition  Method 1: A joint analysis of the average characteristics of the longitudinal development of the EAS and their fluctuations: X max, σ(X max ), dE/dX max Conclusion: Gradual increase of protons energy in the range: 3∙10 17 – 3∙10 18 eV. [1] Dyakonov, M. N., Egorova V. P., Knurenko S. P. et al. Proc. of science papers. Yakutsk, 1987, p. 29-56 (in Russian). [2] Dyakonov, M. N., Egorova V. P., Ivanov A. A., Knurenko S. P. et al. //Proc. of AS USSR, phys. ser., 50, 11, 1986, p.2168 – 2171. [3] Dyakonov M.N., Egorova V.P., Ivanov A.A., Knurenko S.P., et.al. //20th ICRC, Moscow, 1987, v.6, p. 147-150.  Method 2: Comparison of distribution asymmetry of Xmax at different energies by subtraction method. Conclusion: According to the Yakutsk array data in the energy ~10 17 – 10 19 eV observed systematic increase in the fraction of protons 60±10% M. N. Dyakonov, V. P. Egorova, A. A. Ivanov, S. P. Knurenko, V. A. Kolosov, S. I. Nikolsky, V. N. Pavlov, I. Ye. Sleptsov. // JETP letters (1989), 50, 10, p. 408- 410.

30. Method 3: Experimental data are compared with theoretical predictions of models QGSJET in the case of different primary nuclei with criterion χ 2. χ 2 determined by:  2 (Х m ) =  (N exp (Х max ) - N theory (Х max )) 2 / N theory (Х max ), (1)

31. Conclusion Thus, the model QGSJET01 can assume that the mass composition of of PCR changes from the energy (5  30)  10 17 to (3  )  10 18 eV. At E 0  3  10 18 eV PCR is ~70% proton and He, other nuclei in ankle region (~10 19 eV) less than ~30% (  10**19 эВ). 1. S.P. Knurenko, A.A. Ivanov, V.A. Kolosov et al. // Intern. Jour. of Modern Physics A, V.20, №29, (2005), p. 6894 – 6897. 2. S. P. Knurenko, A. A. Ivanov, I. Ye. Sleptsov. //Bull. RAS. Phys. ser. 2005, 69, № 3, p. 363 – 365. 3. S.P. Knurenko, V.P. Egorova, A.A. Ivanov et al. // Nucl. Phys. B (Proc. Suppl.). 151 (2006), р. 92-95.

32. Method 4: Multicomponent analysis using measurements of Cerenkov light and charged particles EAS Conclusion The analysis is performed for three energies 2,4∙10 17 eV, 9,8 ∙ 10 17 eV and 4,8 ∙ 10 17 eV. Cloud of points reflects standardized values. Lines – reflects borders of these zones. In this case, line m1 & line m2 divides nuclei to groups (p + He), C and (Si + Fe). [1]A. A. Ivanov, S. P. Knurenko, A. A. Lagutin, M. I. Pravdin, A. V. Sabourov, I. Ye. Sleptsov. Bul. RAS, Phys. ser., 2007, 71, 4, p. 467-469. [2]S.P. Knurenko, A.A. Ivanov, M.I. Pravdin et al. // Nucl. Phys. B (Pros. Suppl.), 175- 176, 2008, p. 201 – 206. [3]A.A. Ivanov, S.P. Knurenko, A.A. Lagutin, R.I. Raikin and I.E. Sleptsov. Proc. Altay Univ.,1, 2008, с. 63– 65.

33. Method 5 : Analysis of the proportion of muons, depending on the length of the particles track in the atmosphere Dependence of fraction of muons from track length in individual showers (θ = 0 – 50°, E≥10 18 eV) Distribution of ρ µ /ρ ch normalized to track length equal 500 g/cm 2 according to the model EPOS (UrQMD)

34. Conclusion Comparison of the distribution of muons share with the calculation results indicate a mixed composition at energies above 10 18 eV. Large fluctuations in the relationship does not allow a good accuracy to allocate separate groups of nuclei and make them their opinion on the percentage. But the use of "pure" response of the muon detectors (calculations Dedenko et al 2010) leads to the conclusion that a lightweight composition at energies 10 18 - 10 19 eV. 1. S. P. Knurenko, A. K. Makarov, M. I. Pravdin, A. V. Sabourov. Bul. RAS, phys. ser., 2011, 75, 3, p. 320 – 322 2. S.P. Knurenko, I.T. Makarov, M.I. Pravdin, A.V. Sabourov. Proc. XVI International Symposium. Chicago. 2010. arXiv: 1010.1185v1 [astro-ph. HE]

35. Method 6 : Evaluation of the mass composition by the maximum depth of air shower Xmax and model QGSJETII 03. Interpolation method.  Nature of the dependence of the changes with increasing energy, reaching a maximum at (5 - 30) ∙ 10 16 eV.  MC of CR is changing after the first break in the spectrum at ~3 ∙10 15 eV  Reaches more heavier nuclei in the energy range (3 - 30) ∙ 10 16 eV.  3 ∙ 10 17 eV MC become lighter. E. G. Berezhko, S. P. Knurenko, L. T. Ksenofontov. //Astroparticle Physics 36 (2012) pp. 31-36. [ S.P. Knurenko, A.V. Sabourov. //Astrophys. Space Sci. Trans., doi:10.5194/astra-7-251-2011

36. Contents Part 1: Cosmic Rays Spectrum in the Energy Range 1015 – 1018 eV and its Interpretation  History of Yakutsk array  Yakutsk array  Cosmic ray spectrum  Spectrum interpretation Part 2: Mass Composition of Cosmic Rays at Ultra High Energies  Methods for estimating mass composition  Mass composition of Cosmic rays data interpretation (recent results)

37. Comparison Yakutsk data with HiRes & Auger. Models QGSJETII-03 & SIBYLL 2.1. Calculation (Berezhko et al. 2012) for 2 scenarios  Scenario 1: Within the interval 3∙10 17 - 3 ∙10 18 eV the shape originates from unknown component (possibly – from the interaction of CR with galactic wind and shock re-acceleration) Above 3∙10 18 eV – from meta- galaxy  Scenario 2: Supernovae play a certain role in generation and subsequent acceleration of CR up to energy ~10 18 eV Thus, a combined analysis of the spectrum and CR MC with computational results doenst exclude the hypothesis of supernova – related origin of CR with energies up to 3 ∙ 10 18 eV.

38. QGSJETII-04. 2013 The curves correspond to the 3 versions of the calculation of Kotera work.

39. Conclusion Long-term observations at Yakutsk array shows: There are 2 features in the spectrum of CR in the energy range 10 15 – 10 19 eV. It can be assumed that these features are the result if: 1.Heavy component of CR for example iron escaping from our galaxy 2.Indicates an increasing role of metagalactic CR

40. Thank you for your attention!