Multi-TeV  -ray Astronomy with GRAPES-3 Pravata K Mohanty On behalf of the GRAPE-3 collaboration Tata Institute of Fundamental Research, Mumbai Workshop on Astroparticle Physics, Bose Institute, Darjeeling, 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, 1m 2 each - Inter-spacing 8 m - Particle density (ADC) - Timing (TDC) Muon detector - 35 m 2 x 16 modules - 4 orthogonal layers of proportional counters to track muons - 1 GeV threshold s 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 s 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 J MGROJ 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

s Duty Cycle of GRAPES-3 Duty cycle (%)

GRAPES-3 Angular Resolution s 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 s 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 s angle from moon center ----> (a) N e > , (b) N e > , (c) N e > , and (d) N e >

GRAPES-3 Angular Resolution s Comparison of the 3 methods Paper submitted to Astroparticle Physics, A Oshima et al.

s Muons in EAS Data 20m 40m 60m 80m Detected Muons 

CR Rejection Efficiency s Data MC

Observation of CRAB Nebula s 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 s GRAPES-3

Enhancing the GRAPES-3 sensitivity s Aim: To increase the background rejection by doubling the muon detector area i.e. 560 m 2 -> 1120m 2 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 s 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 s 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: TeV proton showers: TeV 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 F CR  (1-  CR ) A -> core selection area T->Observation time of Crab (4 hour/day) F  -> Integral  -ray flux ( TeV) F CR -> Integral CR ray flux ( TeV)  -> 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, 1m 2 each with 8m inter-detector separation Measures particle densities and relative arrival times to estimate primary energy and direction Muon detector 16 modules, 560 m 2 area consists of 3712 proportional counters s

s Triggering Threshold < 10 TeV Duty Cycle: March Sep 2004