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GAW Gamma Air Watch A Large Field of View
Imaging Atmospheric Cerenkov Telescope
Giancarlo Cusumano Ist. Astrofisica Spaziale
e Fisica Cosmica, Palermo, Italy on behalf
of the GAW Collaboration
20thEuropean Cosmic Ray Symposium in
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The promise of exciting discoveries of yet
unknown VHE transient phenomena in the Universe
fully justifies the efforts towards the
construction of large field-of-view ground-based
gamma-ray detectors. (F. Aharonian, ph/0511139
4 Nov 2005)
GAW is Path finder experiment to test the
feasibility of a new generation of Imaging
Atmospheric Cerenkov Telescopes characterized by
large field of view and adequate sensitivity.
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The GAW Collaboration
Istituto di Fisica Spaziale e Fisica Cosmica di
Palermo, IASF-INAF, Palermo
Italy
Spain
Portugal
Instituto de Astrofisica de Andalucia,
Granada and Universidad de Sevilla, DFAMN-US,
Sevilla
Laboratório de Instrumentação e Fisica
Experimental de Partículas, LIP, Lisbon
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Why Large field of view in VHE Astronomy ?
Presently, the existing and planned ground-based
Imaging Atmospheric Cerenkov Technique IACT
observatories aim to fulfil two main objectives

• Lower Energy Threshold (few tenths of GeV),
• Higher Flux Sensitivity (in the entire VHE
  region).

Excellent Background Rejection High Spatial
Resolution Studies of known sources Survey of
limited sky region Discover serendipitous sources
IACT Telescopes as CANGAROO III, HESS, MAGIC,
VERITAS,
allow
but
small Field of View (3-5)
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Mirrors and Large Field of View
To collect light, the present IACTs use large
reflector mirrors (up to 17 m Ø) characterized by
FoV of the order of few degrees (no more than 5
full angle). They cannot reach larger FoV due
to
How to enlarge the FoV ?
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Lenses and Large FoV
To enlarge the FoV, a solution can come from the
use of refractive optics,
as a Fresnel lens is.
reflective light collector
refractive light collector
Incident radiation
Detector at the focal surface
Mirror (reflective)
Detector at the focal surface
Lens (refractive)
Detector at the focal surface
Incident radiation
A refractive Fresnel lens can work as an
efficient light collector for IACT Cerenkov
telescopes where no strong optical constraints
are required.
Advantages
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GAW and its Optics System
GAW uses a non-commercial Fresnel lens as light
collector.
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The GAW Telescope Unit
GAW is conceived as an array of three identical
telescopes disposed at the vertexes of an
equilateral triangle, 80 m side.
80 m
Each telescope is equipped with a refractive
optics, the Fresnel lens, and with a focal
surface detector formed by a grid of MultiAnode
PhotoMultiplier Tubes, MAPMT. Each telescope
will be equipped with an alt-az mounting.
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Focal Surface Detector
The focal detector is formed by a grid of
MultiAnodePhotoMultiplierTubes, MAPMT Hamamatsu
R7600-03-M64, 88 pixels each (baseline). The
number of active channels (order of 104)
constituting the detector at the focal surface
makes it basically a large UV sensitive digital
camera with high resolution imaging capability.
Hamamatsu R7600-03-M64
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Focal Detector and Light Guides
The MAPMT large dead area induces a low
geometrical efficiency factor of 50 on photon
detection.
In order to correct this loss, each MAPMT pixel
is coupled to an UV transparent pyramidal Light
Guide which allows to cover the FoV with an
uniform transmission efficiency (80 for photons
incoming within 25 from the vertical). At the
same time, the pixel size becomes equal to 4.4
arcmin 4.4 arcmin.
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GAW Detector Working Mode
Instead of the usual charge integration
method, GAW front-end electronics design is based
on single photoelectron counting mode
Trigger Threshold Trigger Threshold gt 20 pixels
on / GTU / Area Where GTU
(Gate Time Unit) 10 ns
Area 4 MAPMT
ltNSBgt 2200 photons m-2 ns-1 sr-1
? 2-3 pe / GTU / Area
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GAW the project
GAW array of three identical telescopes
disposed at the vertexes of an equilateral
triangle, 80 m side.
Site Calar Alto Observatory, Spain, 2150 m
a.s.l.
Phase 1 (2007-2008) testing and calibration
phase GAW in testing configuration 66 FoV
Moving the detector along the FoV, the
sensitivity of GAW will be tested observing the
Crab Nebula with on-axis and off-axis pointing up
to 12 with energy threshold of 700 GeV.
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Expected Performance Collecting Area
GAW collecting area vs energy. Each point is
computed multiplying the fiducial area (15201520
m2) by the ratio between triggered and generated
events (mono-energetic gamma-ray events, source
on-axis with respect to the telescope). Error
bars are only due to the statistics of the sample.
GAW and the Crab Nebula. GAW collecting area has
been convolved with a Crab-like spectrum. The
figure shows the differential detection rate of
the Crab Nebula vs energy, which peaks at 0.7
Tev. Error bars are only due to the statistics of
the sample.
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Expected Performance Sensitivity
66 FoV
GAW sensitivity with 66 FoV. Sensitivity
reachable under the configuration of GAW phase
1. The sensitivity limit is evaluated in the
case of source with a Crab-like spectrum.
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in conclusion
Technological aspects
GAW is an RD path-finder experiment for g-ray
astronomy (TeV), to test the feasibility of a
new generation of IACT Cerenkov telescopes, which
joins good sensitivity with large Field of View,
using a Fresnel lens as refractive light
collector, and the single photoelectron
counting mode as detection working method,
under 3-stereoscopic observational approach.
Scientific objectives

• Search for counterparts of unidentified g-ray
  sources (transient or stable)
• VHE activity from Flaring Blazars
• Search for serendipity sources
• Deep sky survey 36060 sky region

Planned time schedule
2007 phase 1 installation 2008 test and
calibration 2009 starting phase 2
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GAW Gamma Air Watch
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GAW Baseline (prototype and Large FOV)
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Arrival Direction Reconstruction
as in the standard stereoscopic analysis, it is
obtained superimposing the Cerenkov images of the
telescopes in a common plane and determining the
point of minimal distance among the major axes of
the image ellipses.
In GAW the direction of each major axis is
determined by minimizing its distance with
respect to the photoelectrons position after
suitable suppression of the more external points,
mainly due to the diffuse light
background. Exploratory data analysis
techniques are used to enhance the useful good
candidate points.
20thEuropean Cosmic Ray Symposium in
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