32 ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , B EIJING 2011

Cosmic ray anisotropy observed by GRAPES-3 air shower array
A. O SHIMA1 , H. A NTIA2 , S. D UGAD2 , S. G UPTA2 , Y. H AYASHI3 , N. I TO3 , A. I YER2 , P. JAGADEESAN2 , A. JAIN2 ,S. K AWAKAMI3 , H. KOJIMA5 , K. K URAMOTO3 , I. M ORISHITA3 , D. M ATSUMIYA3 , M. M INAMINO3 , P. M OHANTY2 ,T. M ATSUYAMA3 , S. M ORRIS2 , P. NAYAK2 , T. N ONAKA4 , S. O GIO3 , T. O KUDA3 , B.S. R AO2 , K.C. R AVINDRAN2 ,S. S HIBATA6 , K. S IVAPRASAD2 , T. TAKAMARU6 , H. TANAKA4 , S. T ONWAR2 , F. T OSHIHIRO3 , Y. YAMASHITA3 , K.
YAMAZAKI31
National Astronomical Observatory Japan
2
Tata Institute of Fundamental Research
3Osaka City University
4Tokyo University
5Aichi Institute of technology
6Chubu University
7Asahi University
akitoshi.oshima@nao.ac.jp'2,,&5&9
Abstract: So far a genuine sidereal anisotropy of galactic cosmic rays was reported by Nagashima et al. and Hall etal., more recently by Milagro and Tibet groups in sub-TeV energy region. We have also reported a sidereal anisotropy oflow energy cosmic rays in GeV energy region observed by GRAPES-3 large tracking muon detector. GRAPES-3 muon
detector is also used to measure muon content of an air shower which is induced by high energy galactic cosmic rayof ≥10TeV. Here we report a sidereal anisotropy of galactic cosmic rays observed by GRAPES-3 air shower array incombination with muon detector using the data from 2000 to 2006.
Keywords: anisotropy, sidereal, cosmic ray

1

Introductionin the form of a contour map on the celestial sphere.

Observation of sidereal time anisotropy of the cosmic ray
is an effective method for determining the structure ofmagnetic ﬁeld in the interstellar space near boundary of
the heliosphere which is not understood yet very well. A
possible anisotropy would reﬂect the general charactaristicof propagation of cosmic rays in the galactic magnetic
ﬁelds. Nagashima et al. have studied sidereal time
variation for various energies of cosmic rays. A numberof experiment such as air shower measurements at Mt.
Norikura (2750 m a.s.l), muon intensity measurements
at Nagoya (sea level), muon measurements at Sakashita(underground) and Hobart(underground, Australia). Thesemeaurements have indicated the existence of two kinds ofsidereal time anisotropies, namely an intensity excess at
sidereal time of 6 hour RA and deﬁcit at 12 hour RA. These
are respectively called Tail-IN (TI) and Loss-cone (LC)anisotropies. In addition, Hall et al. (1999) have analyzed
the data from many muon stations all over the world (sealevel and underground) and have obtained results similar
to that of Nagashima et al. and have displayed their results

The observation of the sidereal variation at low rigidities
is indispensable for the determination of the three dimensional direction of galactic anisotropy, which cannot berealized by observations only in the high rigidity region.
From this point of view, it is worthshile to examine theexsistence of the sidereal daily variation of galactic cosmic
rays.
The anisotropy of galactic cosmic ray intensity in the energy region of ≥ 10 TeV gives us an important informationon the structure of magnetic ﬁeld of the heliosphere andthe local interstellar space around the heliosphere, wherecosmic rays propagate to the Earth. The anisotropy ofgalactic cosmic rays were measured via the sidereal daily
variation of cosmic ray intensity by several experiments.
On the basis of the sidereal variation observed in the TeV
region, most of the previous investigations reported that
small amplitude and a phase of maximum somewhere
between 23-3hours in the local sidereal time(LST). theseobservations are consistent with the large scale diffusive

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A. O SHIMA et al. C OSMIC RAY ANISOTROPY OBSERVED BY GRAPES-3 AIR SHOWER ARRAY

counter telescope.

Figure 2: A muon station has four muon detector modules
each consisting of 232 proportional counters. There are
four muon stations inside the air shower array (Fig. 1).
Figure 1: The GRAPES-3 experimental system with 257
scintillator detectors and 16 muon detector modules
propagation of cosmic rays.
Here we present the new results of a sidereal anisotropy of
the galactic cosmic rays using the data of GRAPES-3 airshower array of 7-years from 2000 to 2006. Furthermore
measuring muon content in an air shower by GRAPES-3
large area tracking muon detector, we tried to apply muon
cut condition on all air shower events to suppress hadronic
component to see an anisotropy of galactic cosmic rays
with low hadronic component.

2GRAPES-3 experiment

The experimental system of the GRAPES-3 (Gamma Ray
Astronomy at PeV EnergyS Phase-3) experiment consistsof a densely packed array of scintillator detectors and alarge area tracking muon detector. The EAS array consistsof 257 plastic scintillator detectors shown in Fig. 1, eachof 1 m2 in area. These detectors are deployed with aninter-detector 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 GATEand 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 μs 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[5]. The pulse charge is later converted into the equivalent
number of minimum-ionizing particles (MIPS) using themost probable charge for a single MIP measured using
the trigger from a small area (20×20 cm2 ) scintillation

The 560 m2 GRAPES-3 muon detector J[10](B 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 totalof 15 layers of concrete blocks (total absorber thickness∼550 g.cm−2 ) above the Layer-1. The concrete blocks
have been arranged in the shape of an inverted pyramid toprovide adequate shielding up to a zenith angle of 45◦ .
At higher energies, where observations have to be necessarily made with particle detector arrays at high altitudes,
the muon content of showers offers itself as a possible
parameter to discriminate against showers initiated by
nuclear cosmic rays. Fig.3 shows charactalistic showerparticle distribution pattern in shower plane for different
primaries. Each shower was produced by simulation using
CORSIKA code. As mentioned above, muon station of
GRAPES-3 can measure the direction of muon content ofa shower.

Figure 3: Images of particle distribution in different
shower. Data used for the shower image has been createdby CORSIKA. Left is gamma induced shower and right is
proton induced one.
Because the area spread of shower particle distribution, thenumber of muons detected by muon detector is dependent
on location of the shower core which lands inide the air
shower array. Therefore an efﬁciency of the detecor forboth gamma ray detection and hadron rejection will alsochange. Fig.4 shows the variation of both the detection
efﬁciency for gamma rays and rejection efﬁciency forhadrons for different shower size and different area which

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32 ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , B EIJING 2011

Figure 5: Moon shadow seen in various annular regions
centered on the direction of the Moon for four energy regions, with the data period from 2000 to 2006.

Figure 4: Muon cut conditions for several shower size andarea in the GRAPES-3 air shower array. Circle shows thegamma ray detection efﬁciency and triangle shows the proton rejection efﬁciency.
is distant from the muon detector.
One of the most critical parameter in the study of directionof primary cosmic rays using a particle detectorarrayis good angular resolution. This requires an accurate
determination of the relative arrival time of the shower
frontat various detectors. The high density of the detectors
in GRAPES-3 enabled an angular resolution of 0.7◦ to beobtained at energies as low as 30 TeV. Angular resolutionof the GRAPES-3 was estimated by 2-D Gaussian ﬁt to theMoon shadow data.
During the data period of this analysis from 2000 to2006, GRAPES-3 air shower array keeps stable good
performance mainly on the angular resolution. Fig. 5shows the Moon shadow clearly seen in the all cosmic rays
ﬂux detected by GRAPES-3 air shower array. The Moonshadow gives us a resonable estimation of an angularresolution of GRAPES-3 air shower array of about ∼ 1.1◦
above thereshold energy.

3using the observed particle densities, following the minimization procedure discussed in detail by Tanaka et al [7].
Also, for each shower, the zenith (θ) and the azimuth (φ)
angles have been calculated using the time informationfrom the TDCs, also following the minimization proceduredescribed by Tanaka et al [7].
The direction of each air shower events are transformed
into the equitorial coordinates, and binned from -0◦ to360◦ in right ascension and from -90◦ to 90◦ in declination
into square cells with a bin size of 3◦ . Each cell arenormalized by the average of its declination bands. Themap data has been converted to the 2-D vizualized map
using General Mapping Tool which is an open source
map creating tool(Fig.6). In the all sky map, near neighbor cells are smoothed by special algorithm of GMT.
Fig.7 shows the variation of signiﬁcance value in the rightascension which were calculated in each declination bands.
As mentioned in the Section 2, to pick up gamma raysfrom a large amount of cosmic rays, we have to measure
the muon content in an air shower and apply suitable
cut condition, the number of muons, on each cosmic rayinduced shower. In this analysis, we applied ”zero-muon”
criteria for the cut condition on each shower in orderto suppress hadronic component of cosmic rays. Thiscondition will be able to detect ∼95% gamma rays forall energy region above threshold energy, while it rejects
only about 90% hadrons, which means there are still large
amount of hadronic cosmic rays.

Data and Analysis

A total of 1.9×109 showers have been collected over a
total live time of 11.9×107 s, spread over a 7-year period,
from 2000 to 2006. For each EAS, the core location,the shower age ‘s’ representing the steepness of theNishimura-Kamata-Greisen (NKG) lateral distributionfunction and the shower size Ne have been determined

4 Conclusion
In this analysis using the data set of 7-years from 2000
to 2006, we obtained a large scale anisotropy of galactic
cosmic rays for all particles in the energy region of above
about 10 TeV. In the all sky map of all particle(Fig.6), twoVol. 1, 111

A. O SHIMA et al. C OSMIC RAY ANISOTROPY OBSERVED BY GRAPES-3 AIR SHOWER ARRAY

References

Figure 6: all sky map seen in all cosmic rays detected byGRAPES-3 air shower array in the energy of about 1013 eV[1] Kojima H.,et al., Proc 28th ICRC, Tsukuba,Japan, 7,3957 - 3960.,2003[2] Kojima H.,et al., Proc 29th ICRC, Tsukuba,Japan, 2,81 - 84.,2005[3] S.K. Gupta et al., Astrophys. Space Sci. 115 (1985)
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[5] S.K. Gupta et al., Nucl. Instr. Methods A 540 (2005)
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[6] K. Greisen, Ann. Rev. Nuc. Sci. 10 (1960) 63.
[7] H. Tanaka et al., GRAPES-3 Preprint (2006).
[8] S.C. Tonwar, Workshop on Techniques in UHE γray Astronomy, La Jolla, Eds: R.J. Protheroe & S.A.
Stephens, Univ. of Adelaide (1985) 40.
[9] http://www-ik.fzk.de/corsika[10] Y. Hayashi et al., Nucl. Instr. Methods A 545 (2005)
643.

Figure 7: Variation of signiﬁcance value in the right ascension for several declination bands.

Figure 8: all sky map of cosmic rays with the ”zero-muon”
criteria detected by GRAPES-3 air shower array in the energy of about 1013 eVkinds of anisotropy are visible, intensity excess at around
6 hour RA and deﬁcit at around 12 RA. Further more weapplied ”zero-muon” criteria on each air shower to suppress hadronic component of cosmic rays and also produced the all sky map in the same procedure as in the caseof all particle(Fig.4). The sky map of hadron suppressedcosmic rays also has an anisotropy with somewhat smooth
structure, not like all particle sky, with an excess of intensity at around 6 hour RA. Since we know that there is stilllarge amount of cosmic rays remained which survived the”muon-zero” criteria, we have to do still more clearance ofcosmic rays to approach the low hadronic cosmic ray sky
which means the sky of gamma ray candidates.
We reported a sidereal anisotropy of both all cosmic rays
and cosmic rays with hadron suppression ,which would include less hadronic cosmic rays, observed by GRAPES-3
air shower array in combination with muon detector using
the data from 2000 to 2006.

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