Multi-TeV -ray Astronomy with GRAPES-3

Pravata K Mohanty

On behalf of the GRAPE-3 collaborationTata Institute of Fundamental Research, Mumbai

Workshop on Astroparticle Physics , Bose Institute, Darjeeling, 10 - 12 December 2009

High energy -ray astronomy

Cosmic Ray origin: a long standing problem

Conventional method: Energy spectrum and composition

More direct method: Detection of high energy -rays

High energy -ray astronomy is emerging as a very exciting field of

astronomy.

The detection of a large numbers of galactic and extra-galactic sources in

GeV - TeV energy range by the current generation of IACT experiments

such as HESS, VERITAS, MAGIC and very recent results of Fermi-LAT

space telescope completely changed the scenario and our perception of -

ray universe.

The region > 10 TeV is still unexplored.

High energy -ray astronomy with EAS arrays

EAS experiments ideal > 10 TeV

Large effective area

Large FOV

- ~ 100% duty cycle

Drawbacks – Poor angular resolution

Ideal for extended sources, flaring sources and sky survey

Present EAS experiments: GRAPES-3, ARGO-YBJ, Tibet AS-gamma, MILAGRO

Future experiments: HAWC, Tibet AS +MD, GRAPES-3 + Expanded MD …..

The GRAPES-3 Experiment

• Scintillation detectors - 400, 1m2 each - Inter-spacing 8 m - Particle density (ADC) - Timing (TDC)

• Muon detector - 35 m2 x 16 modules - 4 orthogonal layers of proportional counters to track muons - 1 GeV threshold

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Front view of two muon modules in a station

Trigger - by scintillation detectors - Rate ~ 30 Hz - Efficiency (90%): ~ 30 TeV for ~ 50 TeV for P

-ray Astronomy with GRAPES-3

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GRAPES-3 Location:76.7E, 11.4N, 2200m a.s.l

Advantage of location: Can view both northern and southern skies

Target:Observation of sources detected by

HESS and MILAGRO like HESS J1908+063 MGROJ1908+06

Search for extended sources

Search for diffuse -ray flux

GRAPES-3 Field of View

Many TeV sources in GRAPES-3 Field of View

The unique advantage of GRAPES-3 for - ray astronomy is its large area compact tracking muon detector for CR background rejection

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Duty Cycle of GRAPES-3 D

uty

cycl

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)

GRAPES-3 Angular Resolution

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Even – Odd Method

Space angle ->

Division of the array to two overlapping sub arrays with odd and even numbered detectors and determine the angle by each sub array

The systematic errors may be common to both

and will cancel out by the difference

GRAPES-3 Angular Resolution

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Left – Right Method

Space angle ->

Division of the array to left and right half through the line joining the core and the center of the shower.

Moon Shadow Method

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angle from moon center ---->

(a) Ne > 103.2 , (b) Ne > 103.5 , (c) Ne > 103.75 , and (d) Ne > 104.0

GRAPES-3 Angular Resolution

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Comparison of the 3 methods

Paper submitted to

Astroparticle Physics,

A Oshima et al.

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Muons in EAS Data

20m

40m

60m

80m

Detected Muons

CR Rejection Efficiency

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DataMC

Observation of CRAB Nebula

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On source region:

~2.7σ, after back ground

rejection

Off source region ±8° in the direction

of right ascension

Observation period:

Mar 2000 –Sep 2004

Diffuse -ray flux Upper Limit

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GRAPES-3

Enhancing the GRAPES-3 sensitivity

sAim: To increase the background rejection by doubling the muon detector area i.e. 560 m2 -> 1120m2

Already planned for this

Expanding of Muon Detector area

Increasing the density of scintillation detectors

Aim: To reduce the triggering threshold energy

(Simulation shows 8m to 4m detector separation reduces threshold from 30TeV to 15 TeV at 90% trigger efficiency. More simulation required to conclude)

Construction Plan for New Muon Detector

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Civil construction

Layout of the modules Side view of one module

2.5 m height of soil

Difference from existing muon detector:

(1) Single hall for ease of working (2) soil as absorber to save cost and time

courtesy: Mr. B.S.Rao

Construction Plan for New Muon Detector

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Detector ~ 4000 proportional counters exists from the KGF experiment

will be used to make 16 modules

But all of them to be remade. The major operation required are cleaning, evacuation, filling gas and testing.

Design and procurement of necessary equipments for this work already began.

Electronics DAQ logic would be same.

More compact design using latest electronics like FPGA

Optimistic time frame for completion ~ 2 years

Simulation for Expanded Muon Detector

CORSIKA QGSJET1 (Version 6.72)

-ray showers: 30-1000TeV

proton showers: 50-1000TeV

CR rejection efficiency:

CR = Showers with N> 0

Total number of showers

-ray retaining efficiency:

= Showers with N = 0

Total number of showers

Cosmic ray Rejection Efficiency

Present

Expanded

-ray retention efficiency

Expanded

Present

GRAPES-3 sensitivity to CRAB

Statistical significance

A T F

= A T FCR (1-CR)

A -> core selection area

T->Observation time of Crab (4 hour/day)

F -> Integral -ray flux (30-1000TeV)

FCR -> Integral CR ray flux (50-1000TeV)

-> Solid angle of view (=2)

one year observation

present

Expanded

Summary

More efficient background rejection and higher sensitivity with expanded muon detector

Enhanced potential for detection of new sources in the multi-TeV region with expanded muon detector

THANKS

Sensitivity

Sensitivity depends on gamma ray flux from source, effective collection

area and efficiency of charged cosmic ray background rejection

Gamma ray flux extremely low > 10 TeV. Not in our control

Sensitivity can be increased by

Increasing collection area

Rejecting large fraction of cosmic ray background

High background rejection

High angular resolution, not much can be done in EAS experiments as the limit comes

due to shower fluctuation

Gamma – hadron discrimination through muon content

The GRAPES-3 Experiment• Scintillator detectors ~ 400, 1m2 each with

8m inter-detector separation Measures particle densities and relative arrival times

to estimate primary energy and direction• Muon detector 16 modules, 560 m2 area

consists of 3712 proportional counters

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Triggering Threshold < 10 TeV Duty Cycle: March 2000 - Sep 2004