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Extending the
Sensitivity Of
Air-Cerenkov
Telescopes
Steve Biller, Oxford University
(de la Calle & Biller – astro-ph/0602284)
Most effort in ACTs today is generally directed at
lower energies (~200 GeV) using small pixels and,
typically, relatively narrow fields of view
What about higher energies and wider fields of view ??
IR Background  ~5-20 TeV gammas probe IR regime of interest
Tests of Lorentz Invariance  want high energies & moderate redshift
AGN Jet Dynamics  spectral shape & variability at high energies
TeV Sky Survey  wide field of view perhaps as important as threshold
Gamma-ray Induced Shower
First Interaction
Cerenkov Light Pool
Shower Maximum
~ 10km a.s.l.
Observation Level
~120m
10TeV, 15Tev, 20TeV
500GeV, 1Tev, 2TeV
10 Km
SHOWER AXIS
5 Km
100
(°) : 0.57-1.14
200
300
1.14-2.29 1.72-3.43
400
2.29-4.57
500
2.86-5.71
X (m)
• Optical Reflector:
Mount: Davis-Cotton design
10 m diameter
Mirror Area: ~87 m2
271 hexagonal facets
Focal length: 10 m (f1 optics)
Mirror Facet
0.61 m
Simulation of Single Facets
Ray Tracing
Facet Shape
: Hexagonal
Facet Area
: ~0.32 m2
Facet Separation: 0.61 m
Facet Diameter : 0.61 m
Facet Reflectivity
Misalignment
: YES
• Camera:
935 pixels (PMT)
0.30 /PMT
10 FOV
QE according to wavelength
Light Cones
QADCs and TDCs included in the simulations
0.0012 m
Cone
17.3°
0.052 m
0.0125 m
Cone
0.046 m
PMT
PMT
Light Cones
Ray Tracing
Hexagonal + Straight
PMT Separation : 0.052 m
PMT Radius
: 0.026 m (0.15 °)
Cone Angle
: ~17.3°
Cone Height
: 0.0125 m
Cone Reflectivity : 80%
Alpha
Length
Width
TSlope
Arrival time of first photon (ns)
2 TeV Gamma-ray
shower
(0° zenith angle,
125m core distance, using
only pixels with > 7 phe)
Radial distance on camera (m)
Fit for E and R as a function of
Displacement, Summed light and TSlope/D
2
c 
S
3
i=1
[mi – mio(E,R)]2
[sio(E,R)]2
2
cj 
S
4
[mi – mio(E,X,Y)]2
[sio(E,X,Y)]2
i=1
2
c
S
n
array

j=1
2
cj
t = (1/c)(hmax2 + r2)
Dt 
10 Km
(r22 – r12)
2 c hmax
5 Km
hmax
SHOWER AXIS
d
r
100
(°) : 0.57-1.14
200
300
1.14-2.29 1.72-3.43
400
2.29-4.57
500
2.86-5.71
X (m)
shape
orientation
time-based “depth”
(independent by construction)
Integral Sensitivity
50 hours
Integral Sensitivity
1 hour
Assumptions:
(the small print)
1) That the simulations are accurate and that the analysis techniques applied
are valid. In this regard we have verified the simulation by comparing
with analytical calculations, alternative simulations and actual experimental
data. We have been able to reproduce trends seen in other, independent
studies and replicate parameter distributions and published sensitivity
curves from existing experiments.
2) That the repeated sampling of showers employed does not lead to significant
biases. To this end, we have explicitly tested this with regard to image
selection and have found no evidence of any bias to within the limits our
statistical uncertainties. Furthermore, throughout this analysis, we have
specifically checked to insure that resulting distributions were not unduly
influenced by a handful of independent showers with unusually high ``weights.'‘
3) That the background rejection factor for the tandem WFOV design
can be factorised into groups of largely independent contributions
which can be separately assessed and combined as a product. However,
in addition for there being logical arguments for why this ought to be
the case for the parameters used, we have also explicitly verified the
independence of these parameters to within the limits of our statistical
uncertainties.
4) That the predicted rejection factors due to newly introduced timing parameters
are accurate, even though this is yet to be experimentally tested. We find no
reason to doubt these factors given that the photon timing is largely governed by
basic air-shower development and geometry, though we certainly encourage
future experimental efforts to explicitly explore this.
Conclusions:
(the big print!)
These results predict the 2-telescope
design considered here to be more than
3 times more sensitive than existing/planned
arrays in the regime above 300 GeV for
continuously emitting sources; up to
10 times more sensitive for hour-scale
emission; significantly more sensitive in the
regime above 10 TeV; and possessing a sky
coverage which is roughly an order of
magnitude larger than existing instruments.
Next Step: Bigger mirrors, even wider field-of-view,
investigate simple array configurations.
Controversial Proposals:
1) Telescope performance has NOT been optimised
in current instruments. This should be done before
designing large arrays.
2) Work towards any “all-sky” instruments should
start by actually building at least one wide-angle
instrument with conventional methods before
looking at radical solutions.
3) Should work towards a prototype via modification
of one of the HESS telescopes.
Note: Lower intensity gamma images means we’re
competing with lower energy background showers
 Must improve rejection by at least an order
of magnitude to be in the ball park!
Also note: Most of the information we’ve obtained
from AGN in the TeV regime has come
from data taken during strong flares
Not so much background limited!
(effective area could buy more than rejection)
Gammas:
~ 5x106 effective Showers
300GeV - 20TeV
Crab-like spectrum  = 1.5 (Integral)
0 zenith Angle
Core Distance < 800m
Protons:
106 – 107 effective Showers
200GeV - 20TeV
 = 1.7 (Integral)
0- 15° zenith Angle
Core Distance < 800m
YCORE (m)
EAS Simulation Code*
Observation Level: 2300 m a.s.l. (763 g/cm2, same as Whipple Observatory)
TELESCOPE
800m
XCORE (m)
Distribution of Core Locations
*by S Biller, based on EGS4 & SHOWERSIM
Simulations of the Whipple 10m gamma-ray telescope
Length
Distance
Whipple 10m Crab Data
EAS Simulations
SC2000
Width
Alpha
Cherenkov Signal : ADC i (d.c.) ≡ # detected photoelectrons (pe)
Telescope Trigger Condition : at least 2 pixels  20 pe
Image Cleaning : RMSNoise = 3 pe
Picture Pixel : Signal is 4.25 s above RMSNoise (from night sky background)
Boundary Pixel: Signal is 2.25 s above RMSNoise and borders a picture pixel
Image Parameterization : Number of picture pixels in the image  5
Image Moment parameters (Hillas 1985)
Background Rejection : Size and distance dependent cuts on:
Length(s,d), Width(s,d), Alpha(s,d), TSlope(s,d)
so as to arbitrarily keep 95% after each individual cut