The primary energy spectrum measured by using the time structure of extensive air showers with compact EAS arrays (ID441)

H. Matsumoto1, A. Iyono1, I. Yamamoto1,K. Okei2, S.Tsuji2, T. Nakatsuka3, N. Ochi4,S. Ohara5, T. Konishi5, N. Takahashi6 　

and the LAASLAAS experiments1 Okayama University of Science, Okayama 700-0005, Japan2 Kawasaki Medical School, Kurashiki 701-0192, Japan3 Okayama Shoka University, Okayama 700-8601, Japan4 Toyo University, Bunkyoku, Tokyo 112-8606, Japan5 Nara Sangyo University, Nara 636-8503, Japan6 Hirosaki University, Hirosaki 036-8560, Japan

ECRS2012, July 3-7, 2012, Moscow, Russia1

Linsley’s method

•Extensive air shower (EAS) thickness depends on the core distance r, the zenith angle θ and the primary energy E0.

•EAS thickness is defined by using the dispersion σt of arrival time distribution of EAS particles.

•The dispersion <σt> of arrival time distribution of EAS particles as a function of EAS core distance r.

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Aims of this applications

• The estimate of the primary energy spectrum by using Linsley’s EAS time structure method with compact EAS arrays.– Primary energy region : 10PeV – ~10EeV– Single observation (OUS1 array)– Coincidence observation (OUS1 array and OUS4

array)

• Observation for the primary cosmic ray by using multi EAS arrays spread over in Japan.

(LAAS experiment)– Investigation the Gerasimova-Zatsepin events– Anisotropy

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OUS1&4 array

•Detector•Plastic scintillation counter (50cm×50cm×5cm, PMT:HAMAMATSU H7195)

•Data acquisition system•ADC (Lecroy 2249W)•TDC (Kaizuworks 3781)•GPS module (Kaizuworks 3051A)•Shift register (MPK NIM-ANY)

•Trigger condition•More than 3 detectors (within 2.5μs time window)

•Array size•~200m2

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OUS4 detectors•Detector

•Plastic scintillation counter Top&bottom:(40cm×50cm×1cm, PMT:HAMAMATSU H7195)Side:(50cm×50cm×5cm, PMT:HAMAMATSU H7195)

•Data acquisition system•ADC (Lecroy 2249W)•TDC (Kaizuworks 3781)•GPS module•Shift register

•Trigger condition•Top&bottom (within 2.5μs time window)

•Restriction angle•25.6 [deg.]

Linsley’ method can be applied in case of rather large core distance, but arrival direction can not be determined

Zenith angle restriction are needed. This was pointed out at ISVHECR2006 in Weihai 5

Estimation procedure of primary energy

1. Arrival time distribution : ti (i=1,2,…,n)• Gamma distribution

2. Estimation of the dispersion σt • estimator : median

•

3. σt → r

•

4. Lateral distribution function• (r, n)→E0

data: (ti, n)

OUS1 array

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Detector simulation: energy distribution

@0.32EeV

@3.2EeVFWHM: 101.73 (1017.16-1018.89)

100.50 (1018.18-1018.68)•AIRES•QGSJETII-3 / Hillas Splitting Algorithm•Proton•Single power-law spectrum

Restriction of the zenith angle θ

•OUS1 θ<60 [deg.]

•OUS1+4 θ<25.6 [deg.]

FWHM: 101.58 (1016.39-1017.97)100.66 (1017.07-1017.73) 7

Systematics of energy resolutions

@0.32EeV OUS1: 195%OUS1+4: 69%

@3.2EeVOUS1: 145%OUS1+4: 51%

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Acceptance

~1/50

OUS1

OUS1+4

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Conversion of spectral indices

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Conversion table from observed spectral index |’| to primary spectral index |.

Data analysis

Observation time [day]

Total number of events

Number of selected events

OUS1 1741 1.1×107 2.0×105

OUS1+4 1150 1.5×105 1.4×103

Data period: 　 (one more year data period added after ICRC2011)

•OUS1 2006/4 – 2011/12

•OUS1+4 2008/8 – 2011/12

Restriction of the zenith angle θ

•OUS1 θ<60 [deg.]

•OUS1+4 θ<25.6 [deg.]

Noise rate ~0.4%

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Time difference between OUS1 and OUS4

GPS time accuracy : 1μsec.

Coincident events (within 10μsec. )

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Observed flux at OUS1 array

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Results: Primary energy spectrum (OUS1)

1016-1019.5eV 1016-1018.5eV 1018-1019.5eV

α’ -2.46±0.13 -2.74±0.19 -2.08±0.08

α -2.51(+0.23 -0.27) -3.20 (+0.53 -0.97) -2.09 (+0.10 -0.11)

Single power-law spectrum

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Observed flux at OUS1+4 array

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Results: Primary energy spectrum (OUS1+4)

Single power-law spectrum

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1016-1019.5eV 1016-1018.5eV 1018-1019.5eV

α’ -3.01±0.21 -3.27±0.22 -

α -3.13 (+0.33 -0.39) -3.60 (+0.42 -0.51) -

Derived by the application of Linsley’s EAS time structure method to LAAS-OUS1 single array observations.

Comparison of E ・ Flux [m-2s-1sr-1]

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Compiled by Tsunesada-san at UHCR2012

Conclusions

• The primary energy spectrum is determined by compact EAS arrays according to the Linsley’s EAS structure method.

• The primary energy resolution of OUS1 have been improved by restricting the EAS zenith angle by using OUS4. But its acceptance decreased ~1/50.

• The obtained spectral index values:– OUS1: -2.51 (+0.23 -0.27) (1016-1019.5eV)

-3.20 (+0.53 -0.97) (1016-1018.5eV) -2.09 (+0.10 -0.11)(1018-1019.5eV)

– OUS1+4: -3.13 (+0.33 -0.39)(1016-1019.5eV) -3.60 (+0.42 -0.51)(1016-1018.5eV)

• The primary energy spectrum of the OUS1+4 is steeper than that of the OUS1 due to minimizing systematic errors.

• Observed EJ spectra is consistent with other experiments. 19

EJ, E3J

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Observed flux at OUS1 and OUS1+4

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UHCR2012 compiled by Tsunesada-san

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1 2 3 Log10(EA) Log10(Es)AGASA 3.16(0.08) 2.78(0.3) - 19.01Yakutsk 3.29(0.17) 2.74(0.20) - 19.01(0.01) -HiRes 3.25(0.01) 2.81(0.03) 5.1(0.7) 18.65(0.05) 19.75(0.04)Auger 3.27(0.02) 2.68(0.01) 4.2(0.1) 18.61(0.01) 19.41(0.02)TA 3.33(0.04) 2.68(0.04) 4.2(0.7) 18.69(0.03) 19.68(0.09)