Observation of Cosmic Ray Anisotropy with GRAPES-3 Experiment · Observation of Cosmic Ray...
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Observation of Cosmic Ray Anisotropywith GRAPES-3 Experiment
Meeran Zuberi
(on behalf of P.K.Mohanty)
GRAPES-3 Collaboration
PoS(ICRC2019)354
July 31, 2019
Meeran Zuberi (TIFR) Cosmic ray anisotropy with GRAPES-3 July 31, 2019 1 / 18
Outline
1 GRAPES-3 Experiment
2 Cosmic Ray Anisotropy
3 Analysis Method
4 Summary
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GRAPES-3 Experiment
Located at Ooty, India (11.4◦ N, 76.7◦ E and 2.2 km msl)
EAS array: 400 plastic scintillation detectors (1 m2 each)total area of 25,000 m2
Muon telescope: 3712 PRCs arranged in 16 modulestotal area of 560 m2
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Air Shower Trigger
Two level trigger system :
Level 0 :-> OR of detectors in each line-> Coincidence of three
consecutive lines
Level 1 : Minimum 10 detectorsout of central core region
Trigger Rate : ∼42 Hz(3.6 million EAS/day)
Energy Range :few TeV to ∼10 PeV
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Cosmic Ray Anisotropy
* Prog. Part. Nucl. Phys. 94 (2017) 184
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Sky coverage of GRAPES–3
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Data Selection
Data used: 2000–2006
Selection cut: Only when full day data is available
Live Time: ∼1377 days (∼54%)
Total Showers: ∼2 billions
Median Energy: 15 TeV
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Analysis Method
For each shower, zenith (θ) and azimuthal (φ) angles are determinedby using a conical fit to the relative arrival times of shower particles.
Some of the shower parameters namely core location, age and size areobtained by fitting NKG function.
For each shower, θ and φ are converted into right ascension (α) anddeclination (δ).
α and δ are binned into 3o × 3o cells.
Each cell is normalized with the average by its respective declinationband.
A dummy cosmic ray distribution is calculated by shuffling the realdata event times.
Anisotropy is calculated by subtracting from real cosmic raydistribution with the dummy one and then the significance iscalculated by using Li–Ma formula.
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Anisotropy Significance Map
Right Ascension (degree)
De
clin
atio
n (
de
gre
e)
90−
60−
30−
0
30
60
90
4−
2−
0
2
4
090180270360
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Anisotropy in different Declination Bands
Right Ascension0 50 100 150 200 250 300 350
Sig
nific
ance
6−
4−
2−
0
2
4
6o
< Dec < 30o
15
o < Dec < 15
o0
o < Dec < 0
o15
o < Dec < 15
o30
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Recent Improvements in Air ShowerParameters
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Atmospheric Effect Corrections
Particle densities observed by plastic scintillation detectors showperiodic variations.
These variations are found to have correlation with temperature andatmospheric pressure.
Time (hours)0 50 100 150
(%
)U
C I/
I)∆(
20−
10−
0
10
Det. 286
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Atmospheric Effect Corrections
Temperature Coefficient Distribution Pressure Coefficient Distribution
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Atmospheric Effect Corrections
0 50 100 150
(%
)U
C I/
I)∆(
20−
10−
0
10D# 286 (a)
Time (Hours)0 50 100 150
(%
)P
TC
I/I)
∆(
20−
10−
0
10 (b)
Atmospheric effects corrected particle density for Det. 286.
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Timing Measurements
Since 2013, better TDCs (32–channel HPTDC) are used to record therelative arrival time of showers.
Hourly time offsets are measured by using Random Walk Method.
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Angular Resolution Improvement
Shower front curvature showsthe dependence over shower sizeand age.
After applying curvaturecorrections, angular resolution(AR) improved by ∼50 % forlog10(Ne)> 4.0.
At present, AR is 0.8o at∼ 4 TeV and 0.1o at ∼ 500 TeV.
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Summary
Anisotropy measurements with GRAPES–3 data are consistent withresults reported by Tibet and IceCube.
A better understanding of detectors performance has improved theangular resolution by > factor of 2.
With these improvements we hope to obtained precise measurementsof anisotropy overlapping both northern and southern hemispheres.
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Angular Resolution
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