33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R IO DE JANEIRO 2013
T HE A STROPARTICLE P HYSICS C ONFERENCE

An anisotropy of galactic cosmic rays observed with GRAPES-3
A.O SHIMA1 , H.KOJIMA3 , S.S HIBATA1 , Y.H AYASHI2 , H.A NTIA4 , S.D UGAD4 , T.F UJII7 , S.K.G UPTA4 ,S.K AWAKAMI2 , M.M INAMINO2 , P.K.M OHANTY4 , I.M ORISHITA9 , T.NAKAMURA6 , T.N ONAKA7 , S.O GIO2 ,H.TAKAMARU1 , H.TANAKA2 , K.TANAKA5 , N.I TO2 , A.JAIN4 , T.M ATSUYAMA2 , B.R AO4 , K.YAMAZAKI2 ,N.YOSHIDA1
1 Chubu University
2 Osaka City University
3 Aichi Institute of Technology
4 Tata Institute of Fundamental Research
5 Hiroshima City University
6 Kochi University
7 Tokyo University
8 Institute for Cosmic Ray Research University of Tokyo
9 Asahi University

oshimaak@isc.chubu.ac.jp
Abstract: Anisotropy in arrival direction of galactic cosmic rays were reported by several group Nagashima etal. and Hall et al., or more recently by ground based experiments such as Milagro and Tibet AS-gamma in subTeV energy region, and also IceCube in a few hundreds TeV region. A large scale anisotropy could be caused
in several ways; the motion of the earth, large scale magnetic field structures, discrete distribution of cosmic
ray sources, and so on. We have also reported a sidereal anisotropy of low energy cosmic rays in GeV energy
observed with the large tracking muon detector of GRAPES-3 [1] [2]. Here we report a galactic cosmic ray
anisotropy observed with GRAPES-3 air shower array in high energy region, specially in combination with themuon detectors for suppression of the contamination of primary hadrons.
Keywords: galactic, cosmic ray, anisotropy

1Introduction

Observation of sidereal anisotropy of the high energy
galactic cosmic ray is an effective method for determining the structure of magnetic field in the interstellar spacenear boundary of the heliosphere which is not understood
yet very well. An anisotropy of galactic cosmic rays isthought to be reflect the general characteristic of propagation of galactic cosmic rays in the galactic magnetic fields.
Nagashima et al. have studied sidereal time variation ofcosmic rays for various energies. A number of experiment
have studied the anisotropy, such as air shower experiment
at Mt.Norikura (2750 m a.s.l), atmospheric muon measurements at Nagoya (sea level), at Sakashita (underground)and Hobart(underground, Australia). These measurements
have indicated the existence of two kinds of sidereal timeanisotropies, namely an intensity excess at sidereal timeof 6 hour RA and deficit at 12 hour RA. These are called
Tail-IN (TI) and Loss-cone (LC) anisotropies respectively.
In addition, Hall et al. (1999) have analysed the data frommany muon stations all over the world (sea level and underground) and have obtained results similar to that of Nagashima et al.
The observation of the sidereal variation of galactic cosmic rays at low rigidities is required for understand of thethree dimensional direction of galactic anisotropy, whichcannot be realised by observations only in the high rigidity region. From this point of view, it is worthwhile to investigate the existence of the sidereal variation of galactic
cosmic rays.

The anisotropy of galactic cosmic ray intensity in theenergy region of ≥ 10 TeV gives us an important information on the structure of galactic magnetic field of theheliosphere and the local interstellar space around the heliosphere, where cosmic rays propagate to the Earth. The
anisotropy of galactic cosmic rays were measured via thesidereal daily variation of cosmic ray intensity by several
ground based experiments. On the basis of the sidereal
variation observed in the TeV region, most of the previous
investigations reported that small amplitude and a phaseof maximum somewhere between 23-3hours in the local
sidereal time(LST). these observations are consistent withthe large scale diffusive propagation of cosmic rays.
Here we present the new results of an anisotropy of
the galactic cosmic gamma rays at very high energy region using the data of GRAPES-3 air shower array of 9years from 2000 to 2008. We’ve used the muon data of
GRAPES-3 large area tracking muon detector to discriminate the gamma induced air showers from nuclear induced
ones effectively. For this purpose we applied a hadron rejection criteria on all the recorded air shower events in order to suppress a contamination of nuclear primary particles.

2

GRAPES-3 experiment

The experimental system of the GRAPES-3 (Gamma RayAstronomy at PeV EnergyS Phase-3) experiment consistsof a densely packed array of scintillator detectors and a

Galactic cosmic ray
33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R IO DE JANEIRO 2013

arranged in the shape of an inverted pyramid to provide
adequate shielding up to a zenith angle of 45◦ .
At higher energies, where observations have to be necessarily made with ground based particle detector arrays
at high altitudes, the muon content of showers offers itself
as a possible parameter to discriminate against air showers initiated by nuclear cosmic rays. As mentioned above,muon station of GRAPES-3 can measure the direction ofmuon content of a shower.

Figure 1: The GRAPES-3 experimental system with 257
scintillator detectors and 16 muon detector modules
large area tracking muon detector. The EAS array consistsof 350 plastic scintillator detectors shown in Fig. 1, eachof 1 m2 in area. These detectors are deployed with an interdetector separation of only 8 m. The array is being operated at Ooty in south India (11.4◦ N, 76.7◦ E, 2200 m altitude).
In order to achieve the lowest possible energy threshold,a simple 3-line coincidence of detectors has been used togenerate the Level-0 trigger, which acts as the fast GATE
and START for the analog to digital and time to digitalconverters (ADCs and TDCs), respectively. As expected,
this trigger selects a large number of very small and local
showers and also larger showers whose cores land very
far from the physical area of the array. Therefore, it isalso required that at least 10 out of the inner 127 detectors
should have triggered their discriminators within 1 µ s of
the Level-0 trigger. This Level-1 trigger with an observed
EAS rate of 13 Hz is used to record the charge (ADC) andthe arrival time (TDC) of the pulses from each detector[4]. The pulse charge is later converted into the equivalent number of minimum-ionizing particles (MIPS) using
the most probable charge for a single MIP measured using the trigger from a small area (20×20 cm2 ) scintillation
counter telescope.

One of the most critical parameter in the study of direction of primary cosmic rays using a particle detector arrayis good angular resolution. This requires an accurate determination of the relative arrival time of the shower front
at various detectors. The high density of the detectors inGRAPES-3 enabled an angular resolution of 0.7◦ to be obtained at energies as low as 30 TeV. Angular resolution ofthe GRAPES-3 was estimated by 2-D Gaussian fit to theMoon shadow data.
During the period of this analysis from 2000 to 2008,
GRAPES-3 air shower array keeps stable good performance on the angular resolution. Fig. 3 shows the Moonshadow clearly seen in the all cosmic rays flux detectedby GRAPES-3 air shower array at energies above 100TeV.
The Moon shadow gives us a reasonable estimation of anangular resolution of GRAPES-3 air shower array of about∼ 0.7◦ above 100 TeV.

Figure 3: The deficit in cosmic ray intensity by an effect ofthe moon shadow in high energy region of above 100TeV.
The data used for this analysis is from 2000 to 2008.
Figure 2: A muon station has four muon detector modules
each consisting of 232 proportional counters. There arefour muon stations inside the air shower array (Fig. 1).

3The 560 m2 GRAPES-3 muon detector [9] consists of
4 super-modules in Fig.2 , each in turn having 4 modules.
Each module with a sensitive area of 35 m2 consists ofa total of 232 proportional counters (PRCs) arranged in
4 layers, with alternate layers placed in orthogonal directions. Two successive layers of PRCs are separated by 15
cm thick concrete. The energy threshold of 1 GeV for vertical muons, has been achieved by placing a total of 15layers of concrete blocks (total absorber thickness ∼550g.cm−2 ) above the Layer-1. The concrete blocks have been

The air showers that we observe on the earth are almost induced by charged cosmic rays and they have some amountof muon content with a certain lateral distribution depending on their energies. Therefore the air showers due tocharged cosmic rays could provide huge noise for thegamma ray astronomy by the ground based experiments.
However a charged cosmic rays could be identified precisely as a nuclear origin by measuring their muon content in the air shower using tracking muon detector. The
GRAPES-3 tracking muon detector can record the hit pattern of muons and reconstruct the direction of the muons

Hadron rejection and Analysis

Detected NµGalactic cosmic ray
33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R IO DE JANEIRO 2013

10310210

1

3.5

4

4.5

5

5.5

6Shower size (log Ne)
10Figure 4: Distribution of detected muons of proton induced air showers in the CORSIKA simulation. The figure
has been made with the following event selection; core location within the area of 20m from the center of muon station and zenith angle is witin 20 degrees.
with an angular resolution of about 6◦ . This implies thatthe tracking muon detector has a potential ability to distinguish primary gamma rays from large amount of charged
primary nuclei. Then we tried to develop a strong and aneffective hadron rejection criteria by taking advantage thetracking muon detector using CORSIKA monte carlo simulation.
This work was fully supported by COSIKA(COsmic
Ray Simulations for KAskade) simulation code[8], which
is widely used by high energy cosmic ray experiments
around the world. The CORSIKA was set up with the parameter of QGSJII for high energy hadronic interaction
and with GEISHA for low energy. The energy range ofprimary particles in the simulation was set from 5TeV to
1PeV with the spectral index of -2.7 for both proton andgamma primaries. All the simulated showers at the observation level of GRAPES-3 were reconstructed as real
shower data, and the number of muons were counted bysimulated GRAPES-3 tracking muon detectors. Since thehigh energy gamma rays can produce a certain amount ofmuons in air showers, just selecting muon zero showers
as a gamma candidate can cause an elimination of gamma
rays. Therefore we have to consider the distribution of thedetected number of muons for both gamma primary andhadron primary and utilize the difference of shape of the
distribution and optimize the detectable muon numbers inboth gamma and proton induced showers. Fig.4 shows thedetected muon distribution which were produced by thesimulated proton induced air showers for different energies corresponding shower size.
Because the possible detected number of muons could
be varying depending on their shower core location, primary energy and their zenith angles, we classified all therecorded air showers into bins with combinations of theircore location, their estimated primary energies and theirincident zenith angles as shown in the image of Fig.5, Inthe Fig.5, all the core location have been divided into 11
area bands of 10m width from the center of muon detector to the edge of the array. The zenith angle have also
been classified into 5 bins of 10 degrees width from thezenith to 50 degrees. For each classified data set of air

Figure 5: Classification of shower events for optimizing
the performance of hadron rejection in this analysis.

showers, we obtained the distribution of number of muonsrecorded by the muon detectors. We optimized the numberof muons by considering both the hadron rejections andthe gamma detections using CORSIKA simulation as mentioned above. Fig.6 shows a variation of both the detection
efficiency for gamma ray induced showers and rejection
efficiency for proton induced showers for different shower
size and different distance from the muon detector. Thiscondition will allow to detect above ∼90% gamma rays
for all energy region above 100 TeV for primary gamma
rays of a vertical incidence, while it can reject 99.999%
protons. This means that the showers which survived ourhadron rejection would be the candidates primary gamma
rays.

4Results

A total of 2.1×109 showers have been collected over a total live time of 27.8×107 s, spread over a 9-year period,
from 2000 to 2008. After the hadron rejection, only the
number of events of 1.8×105 was remained. For each EAS,
the core location, the shower age ‘s’ representing the steepness of the Nishimura-Kamata-Greisen (NKG) lateral distribution function and the shower size Ne have been determined using the observed particle densities, following theminimization procedure discussed in detail by Tanaka et al[6]. Also, for each shower, the zenith (θ ) and the azimuth(φ ) angles have been calculated using the time information from the TDCs, also following the minimization procedure described by Tanaka et al [6].
The direction of each air shower events have been transformed into the equatorial coordinates, and have beenbinned from 0◦ to 360◦ in right ascension and from -90◦
to 90◦ in declination into square cells with a bin size of3◦ . Each cell has been normalized by the averaged countsof its declination bands. The map data has been converted
to the 2-D visualized map with smoothing process. The
GRAPES-3 air shower array can observe the equatorial
sky from -40◦ to 60◦ in declination. Fig.7 and Fig.8 show
the sky map of significance values of the gamma ray candidates in the energy region of 100TeV and 290TeV.

Galactic cosmic ray33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R IO DE JANEIRO 2013

Figure 8: The sky map of gamma ray candidates of energy
290 TeV with an application of hadron rejection.

Figure 6: The conditions of hadron rejection have been
applied with 99.999% rejection for above 100 TeV, performed for various shower size, area and zenith angles. Triangle shows the efficiency of primary gamma ray detection
and diamond shows the proton rejection efficiency. Squareshows the number of muons detected by the muon detector.

timized hadron rejection criteria for each air showers andobtain a strong and effective hadron rejection efficiency
and effective detection efficiency for gamma ray induced
air showers. In this analysis using the data set of 9-years
from 2000 to 2008, we calculated an anisotropy sky mapof gamma ray candidates in the energy region of above
100 TeV and above 290 TeV with hadron rejection criteria. It has been founded that the uniform distribution of thearrival directions of the primary gamma ray candidates athigh energy. The performance of the hadron rejection efficiency of the GRAPES-3 muon detector in this analysis
can achieve 99.999% of hadronic primary cosmic rays.
Acknowledgment: We thank all the GRAPES-3 collaborator
in India for operation of the experiment. The air shower simulation and data analysis were carried out on the general-purpose
PC farm and analysis servers at Center for Computational Astrophysics, National Astronomical Observatory of Japan.

References

Figure 7: The sky map of gamma ray candidates of energy
100 TeV with an application of hadron rejection.

5Summary

We developed a strong and an effective hadron rejection
criteria by using CORSIKA monte carlo code. The hadron
rejection criteria have been applied to all the recorded airshower events to suppress a contamination of hadron induced air showers. Because air showers due to primary nuclei contain large amount of muons with a certain lateral
distribution, they play a large disturbance for the gamma
ray astronomy performed by a ground based experiments.
However very high energy gamma rays can produce air
showers with some amount of muon contents too. Therefore simply selecting muon-zero air showers as gamma ray
candidates which we reported previously [10] can possibly
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