30 TH I NTERNATIONAL C OSMIC R AY C ONFERENCE

Angular resolution of GRAPES-3 array obtained from the shadow of Moon and Sun
in extensive air showers
A. O SHIMA2 , S.K. GUPTA1 , K. HAYASHI2 , Y. H AYASHI2 , N. I TO2 , A. I YER1 , P. JAGADEESAN1 ,A. JAIN1 , S. K ARTHIKEYAN1 , S. K AWAKAMI2 , H. KOJIMA3 , T. M ATSUYAMA2 , M. M INAMINO2 ,P.K. MOHANTY1 , S.D. M ORRIS1 , P.K. NAYAK1 , T. N ONAKA2 , S. O GIO2 , T. O KUDA2 , B.S. R AO1 ,K.C. RAVINDRAN1 , M. S ASANO2 , N. S HIMIZU1 , K. SIVAPRASAD1 , H. TANAKA1 , S.C. T ONWAR1 ,T. YOSHIKOSHI2
1Tata Institute of Fundamental Research
2Osaka City University
3Nagoya Women’s University
oshima@alpha.sci.osaka-cu.ac.jp
Abstract: The GRAPES-3 experiment observes extensive air showers using a high-density array ofscintillation detectors and a large area tracking muon detector. The array consists of 300 scintillation
detectors (each 1m2 in area) and 16 modules of muon detector having a total area of 560m2 . Good
angular resolution of the array is a key requirement for detection of point sources of gamma rays. Since
the angular size of Sun and Moon are small compared to the expected angular resolution of our array, we
have used the deficit in shower rate caused by the shadow of these two objects to determine the angularresolution as a function of shower size. Here we report the results of our analysis on observation of theshadow of Moon and Sun.

Introduction
GRAPES-3 experiment is a wide field of view
gamma ray observation operating at Ooty (E76.7 ◦ ,N11.4◦ , 2200m asl), India. Mean atmosphericdepth at Ooty is about 800g/cm2 . The array of
scintillation detectors observes the distribution ofsecondery particles in the extensive air showers toreconstruct the direction and energy of the primaryparticles and 16 large area muon detectors measure
the muon content accompanying the air showers.
The array of 256 scintillation detectors is operating with trigger rate of 13Hz in the period used inthis analysis (Figure1).
One of the most important goal of GRAPES-3
experiment is the search for gamma ray pointsources. We must know the angular resolution ofthe array, because if the array has the better angular resolution, the smaller region around sources
will be required to search thus the lower background. High energy primary cosmic rays produce
extensive air showers in the atmosphere and are de-Figure 1: GRAPES-3 air shower array in the periodof 2000 - 2003. 256 scintillation detectors and 16
large area muon detectors

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tected by the array of scintillation detectors whena suitable trigger condition is satisfied. Because
the threshold energy of primary cosmic ray particles detected by GRAPES-3 air shower array isa few TeV, all extensive air showers detected are
originated from extragalactic primary cosmic rays
passing through the solor system in the the almost
straight line. So we can expect to see the shadowof the Moon and Sun in the isotropic distributed
air showers induced by those primary cosmic rays
when we observe the area around the Moon andSun both spreading with 0.5◦ diameter on celestial
sphere. The observation of the shadow of Sun andMoon is usefull way to know the angular resolution directly of air shower array. Here we report on
the results of the analysis of the shadow of Moonand Sun.

MethodWe used the equal solid angle method to observe
the shadow of the Moon and Sun. The center ofthe circular search window with radius of 4 ◦ wasset on the Moon/Sun, and the search area was devided into several bins of annuli each having same
solid angle (Figure2). If the angular resolution ofarray is good, the shadow of the Moon and Sun appears as a clear deficit of shower rate at the centerof search area. But for the Sun shadow, it is difficult to see a clear deficit in this method becausethat the shape of the Sun shadow and its position isaffected by the magnetic field of the surface of theSun. This method can’t follow such an irregularshape of the Sun shadow.
We selected another region on the sky as a background area where there is no object preventing
the cosmic rays from reaching earth. The background region was placed by rotating the actual
Moon/Sun position by ±8◦ in the azmuth direction
with the same search window. Two backgrounds
located at symmetrical place with respect to the actual Moon/Sun position were added togather to bea background data.

Event selection
To select properly reconstructed showers we applied some quality cuts for both the Moon and Sun

Figure 2: Circular search window used in this analysis. Each annuli has same solid angle. Moon/Sun
is on the center of this area.
analysis. Ratio of number of particles detected in
the outer 2 rings of scintillation detecter array to a
total number of particles should be less than 0.35
for the purpose that the shower core was estimatedproperly. The estimated shower core must be inside the hexagonal area masked with red color in
the Figure1. The zenith angle of reconstructed air
shower direction is required to be less than 40 ◦ .
The data which seems to have noisy counters wasremoved from analysis. And the data set with anamplitude of azimuthal angle distribution of morethan 1.5% was not used in this analysis, because
the angular resolution is not good in that period.
The data set in this analysis was recorded between
2000 March and 2003 May. After applying all cuts,
the total number of events for the Moon becomes
5.7×105 and for the Sun becomes 7.3×105 . Furthermore we removed the data in the period 2000
- 2001 from the Sun shadow analysis, because inthat period the solor magnetic field is so strongthat we couldn’t see the shadow of the Sun in theequal solid angle method. Finally 3.8×105 events
remained for the Sun.

Shadow of the Moon
In the Figure3 the shadow of the Moon appears as
a deficit of number of events at the center of searcharea. The flat back ground in the bottom figure
in which the no deficit can be seen indicates thedeficit of the shadow of the Moon is not artifact.
Figure4 shows the dependence of the angular resolution on the shower size. The angular resolution

30 TH I NTERNATIONAL C OSMIC R AY C ONFERENCEof thr air shower array improves as the shower size
increase.

Figure 3: Top: Shadow of the Moon. Bottom:
Background. Each bin in the figure has same solid
angle. Angular resolution was calculated to be
1.13◦ for the shower size of log10(Size) ≥ 3.2. The
two back ground regions were added togather.

Shadow of the Sun
As mentioned above, we removed the data set inthe period of 2000 - 2001, because the solor magnetic field in this period was strong that affects
the shadow of the Sun so that we couldn’t see theshadow of the Sun and measure the angular resolution properly. The shadow of the Sun is shownin the Figure5 with the back ground area. The calculated angular resolution is 0.75◦ for the shower
size of log10(Size) ≥ 4.25. The angular resolution
derived from the Sun shadow is large compared tothe Moon whose resolution is 0.52◦ for the same
shower size condition, this may be due to the effect of the solor magnetic field.

Figure 4: Shadow of the Moon. From top theshower size of log10(Size) ≥ 3.5, ≥ 3.75, ≥ 4.0, ≥4.25. The angular resolution is 1.05◦ , 0.89◦ , 0.70◦ ,0.52◦ respectively.

A NGULAR RESOLUTION

Acknowledgements
We thank all members in OOTY for help in construction and operation of GRAPES-3 experiment.

References[1] A. Oshima et al. 1054-search for point sourcesof gamma rays using GRAPES-3 experiment.
In 30th International Cosmic Ray Conference,Merida,2007.
[2] A.Borione et al. Obsevation of the shadows ofthe Moon and Sun usinf 100 TeV cosmic rays.
Physical Review D, 49:1171–1177, 1994.

Figure 5: Shadow of the Sun with the shower sizeof log10(Size) ≥ 4.25. The angular resolution wascalculated to be 0.75◦ .

Summery
We have observed the shadow of the Moon and Sun
using extensive air showers. The angular resolution of the array for the shower size of ≥ 3.2 isabout 1.13◦ which will be used for the point source
analysis[1]. We adopted the angular resolution derived from the Moon shadow analysis as the angular resolution of the air shower array, because in theSun shadow analysis the data in the period of highsolor activity was not used and it seems that the influence of the solor magnetic field on the shadow of
the Sun is still remained even after removing suchthe data set. So we concerned the Sun shadow tobe just an indicator for the validity of the Moonshadow analysis.