Concept(s) for very low energy observations (=<10 GeV)

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Concept(s) for very low energy observations(lt10
GeV)

• John Finley, Alexander Konopelko
• Department of Physics, Purdue University,
  525 Northwestern Avenue, West Lafayette, IN
  47907

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Rationale

• A first 100 GeV stereoscopic array - H.E.S.S. -
  has been taking scientific data since Dec03.
  H.E.S.S. delivers exciting physics results!
• CANGAROO, MAGIC, VERITAS are close to complete
  construction and/or performance tests.
• H.E.S.S. collaboration has started thorough
  developments for the 2nd phase.
• Discussion on the next-generation instrumentation
  is ongoing!

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Major Physics Goals

• Further observation of SNR Origin of Cosmic Rays
• Detailed studies of physics of AGN jets
• Cosmology link EBL gamma-ray absorption
• Resolving morphology and spectra of gamma-rays
  from PWN
• Detection of pulsed gamma-ray emission
• Search for Dark Matter
• Observation of Gamma-ray Bursts
• etc

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General Physics Requirements

• Achieve energy threshold of 10 GeV
• Reasonable angular (lt0.5 degree) and energy
  resolution (lt50)
• Sufficiently large collection area, providing
  high gamma-ray rate
• Upgrade sensitivity above 100 GeV
• Improve quality of stereo analysis (large image
  size ph.e.)
• Drastically increased collection area
• Widen dynamic energy range, up to 10 TeV
• Keep relatively large sensitive scan window
• Shorten a response time for transients
• Simultaneous observation of a few objects

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Alternatives

• Extended version of H.E.S.S./CANGAROO/VERITAS
  arrays a farm of up to 200 tel.-s of the
  same art OR MAGIC ARRAY 20 tel.-s
  of 17 m each
• Single stand-alone very large telescope
  reflector area of about 1000 m2 ECO-1000
• 5_at_5
  five of 20 m tel.-s at 5 km
  a.s.l.
• Stereo Array
  a few 30 m tel.-s at 2-3 km a.s.l.
• etc

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Constrained Choice
Single or Stereo?
Which Stereo?
()
Single Stand-Alone Telescope
Farm of 12-17 m Telescopes

• Large collection area gt 50-100 GeV
• Low energy threshold needs to be proven!
• Conventional angular energy resolution

• High muon rate timing needs to be proven
• Modest angular energy resolution
• Large collection area at low energies

Stereo Array
Stereoscopic System

• Low energy threshold 10 GeV!
• Improved CR rejection, angular energy
• resolution gt 100 GeV

• Suppressed muon rate
• Advanced shower reconstruction
• Improved sensitivity at low energies!
• Detailed systematics
• Proven by HEGRA and H.E.S.S. at
• higher energies

5_at_5

• Very low energy threshold 5 GeV
• Reduced sensitivity at higher energies
• Technically difficult and very expensive!

() Kruger Park Workshop (1997)
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High Altitude Site

• Photon density is higher at Rlt100 m!
  unfavorable region for imaging
• Images/Time pulses are broader reduced
  signal/n.s.b.l. ratio per pixel
• Centroid further displaced from the center of
  FoV requires larger camera
• Possibly, enhanced n.s.b.l. flux requires a
  higher threshold
• Higher flux of secondary charged particles
  muons, electrons etc
• Perhaps, all that needs some test measurements!

5 km
2.2 km
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Energy Threshold

• Minimum image size 40 ph.-e.
• Basic telescope parameters
• Reflector area, Ao
• Efficiency of photon-to-ph.-e.
• conversion, ltegt()
• Altitude of observational site ()
• Effective area of a reflector
• ltAgtltegtAo

() in recent years extremely slow progress in
development of advanced photodetectors. ()
robotic telescopes for high altitude sites need
further inverstigations, but they are apparently
very expensive!
Lateral distribution of mean image size in 10,
102, 103 GeV gamma-ray showers simulated for a 30
m telescope.
One needs a 30 m telescope to detect gamma-ray
showers of 10 GeV!
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We need something large to collect and focus
radiation!
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Telescope Design
Energy threshold
Experiment Reflector size QE ltAgt Altitude
Whipple 10 m 0.25 8 m2 2.3 km
HEGRA 5x3.5 m - 1 m2 2.2 km
H.E.S.S. I 4x12 m - 11 m2 1.8 km
VERITAS 4x12 m - 11 m2 2.3 km
CANGAROO III 4x10 m - 8 m2 160 m
H.E.S.S. II 28 m - 61 m2 1.8 km
MAGIC I 17 m - 23 m2 2.2 km
MAGIC II 17 m 0.8 73 m2 2.2 km
ECO 1000 36 m - 325 m2 2.2 km
5_at_5 5x20 m 0.25 31 m2 5 km
STEREO ARRAY 5x30 m - 70 m2 1.8 km
0.5-1 TeV
100 GeV
sub 100 GeV
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Reflector

• A 30 m dish-mount is technically feasible! 600
  tonne
• Focal length of 36 m
• Parabolic dish is preferable
• Small time spread of reflected light
• Good PSF for off-axis light (lt1.5o)
• Glass mirrors are ok
• Automatic mirror adjustment
• Camera auto focus dislocation by 20 cm
• High slewing speed 200 deg/min
• Approximate cost 5 MUS

Prototype H.E.S.S. II telescope parabolic dish,
diameter of 28 m, focal length of 36 m, 850
mirror facets of 90 cm each Courtesy of W.
Hofmann
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What about optical astronomy?
VLT Very Large Telescope 48 m (16 m
equiv.) ELT Extremely Large Telescope 25 m CELT
California Extremely Large Telescope 30 m GSMT
Giant Segmented-Mirror Telescope 30m TMT
Thirty-metre Telescope (US Canada ?) Euro50
Finland, Ireland, Spain, Sweden UK OWL A 100 m
optical near-infrared telescope
Future plans for large telescopes...
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Camera

• FoV of 3.0o diameter
• Limited by broad PSF at the large off-sets
• Low energy events are close to the camera center
• Scan window of about 2o diameter
• Small pixels of 0.07o
• Reduce n.s.b. contamination
• Better imaging of low energy events
• Limited by PSF for a 30 m parabolic dish
• Homogeneous design
• Custom PMs
• Fast electronics e.g. SAM (Swift
  Analog Memory) readout of lt10 ms, made in
  France
• Approximate cost 5 MUS

PMs pattern in a 1951 pixel camera. Superimposed
is the image of a 30 GeV g-ray shower.
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Contemporary Array Layout

• Constrained by the size of C-light pool 100
  m
• Similar to HEGRA H.E.S.S.
• No optimization done so far!

HESSII
100 m
Total costs 10
MUS x Number of Telescopes
100 m
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Simulations Stereo Array
Altitude 1.8 km a.s.l.
Atmosphere Tropical
Reflector size 30 m
Reflector design Parabolic (F/1.25)
Focal length 37.5 m
Number of telescopes 5
Distance between telescopes 100 m
Conversion efficiency 0.1 ph.-e./photons
Trigger Signal of any 3 PMgt6 ph.-e.
Tail cut 3/5 ph.-e.
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Input Energy Spectra

• Gamma-rays HEGRA collaboration, ApJ, 539 317
  (2000)
• Electrons Du Vernois et al. ApJ, 559 296 (2001)
• Cosmic-Ray Protons Nuclei Sanuki et al. ApJ,
  5451135 (2000)
• lt 30 GeV
• gt 30 GeV

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Gamma-Ray Detection Area

• Energy threshold is about 8-10 GeV
• Effective radius at 10 GeV is 200 m
• 2-fold coincidences dominate at low energies
• Coll. area for 5 tel.-s is by a factor of 2-3
  larger than for 2 tel.-s

System of 2 (curve 1) 5 (curve 2) 30 m
telescopes. A 30 m single stand-alone telescope
(dashed curve).
System of 5 30 m telescopes for a trigger
multiplicity of 2, 3, 4, 5 telescopes (curves 1,
2, 3, 4).
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Detection Rates
Raw background rate Single stand-alone tel. 1.7
kHz System of 2 tel.-s 1.0 kHz Array of 5
tel.-s 3.2 kHz
Integral rates after cuts R(gtEth)
Eth, GeV Rg, Hz Re, Hz RCR, Hz
5 5.5 2.5 1.0
10 4.7 1.5 1.0
30 2.5 0.18 0.9
50 1.7 0.06 0.68
100 1.0 0.01 0.34
Detection rates of g-ray showers (1), electrons
(2), and cosmic rays (3).
Event trigger rate of 3.2 KHz can be easily
maintained by advanced readout system!
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Low Energy Events
Longitudinal development, C-light emission of a
10 GeV g-ray shower.
Average time pulses of the C-light emission from
a 10 GeV g-ray shower.
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Time-Dependent Imaging
R 150 m
Qx, deg

• Centroid is close to the center of FoV
• Small angular size
• Very high fluctuations in image shape

Qy, deg
C-light image of a 10 GeV g-ray shower averaged
over a sample of events.
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Single Telescope Analysis
Straightforward approach

• Standard image parameters
• Simultaneously orientation shape
• Non-parametric estimation of multi-variate
  probability density
• Bayesian decision rules
• Test on MC simulated events

In the energy range of 10-30 GeV the maximum
achieved Q-factor is 2.7 for the g-ray acceptance
of 50 which is not very different from
supercut
3D visualization of the signal background
samples.
Courtesy of Chilingarian, A., Reimers, A.
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Angular Resolution in Stereo

• 63 radius at 10 GeV is 0.3o
• Q-factor is about 3.1
• 3-fold resolution is better by 30

Angular resolution of g-ray showers with two (2)
three (1) telescopes.
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Analysis by Mean Scaled Width

• Cut 0.91
• Background rejection 12.5
• Q-factor 1.2

Joint Q-factor 3.8 (2 tel.-s) 5.0 (3 tel.-s)
Distributions of simulated signal background
events weighted according to the spectra.
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Sensitivity Estimates
Conditions exposure of 50 hrs, confidence level
of 5s, number of g-rays gt10.
Setup Rg, Hz Rraw, kHz Fmin(gt5 GeV), cm-2s-1
Single stand-alone tel. 16.8 1.7 1.46x10-10
Stereo system of 2 tel.-s 11.2 1.0 6.45x10-11
Stereo array of 5 tel.-s 20.1 3.2 2.94x10-11
Summary

• Single stand-alone telescope yields high g-ray
  rate
• Stereo system of two tel.-s provides sensitivity
  higher by a factor 2.2 than single tel.
• Stereo array gives further improvement by a
  factor of 2.2
• Sensitivity of stereo array is by 5 times better
  than single tel.

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Sensitivity of Stereo Array
For observations at zenith.

• Energy threshold 10 GeV
• Raw trigger rate 3.2 kHz
• Crab g-ray rate after cuts 4 Hz
• Background rate after cuts 8 Hz
• S/N per hour 85 s
• Crab can be seen in 12 sec
• Corresponding number of g-rays 50

Summary

• Improved sensitivity in 10-100 GeV region
• Better than GLAST above few GeV
• Unique for short time phenomena

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Conclusions

• The move to lower energy threshold is likely to
  remain a significant drive for the VHE gamma-ray
  astronomy
• The next generation of ground-based imaging
  atmospheric Cherenkov detectors is widely belied
  to be a system of 30 m class telescopes
• Such a detector meets most of the physics
  requirements to achieve the scientific goals as
  currently perceived by gamma-ray astrophysics
  community!