Low Energy Array of A Major CT Facility

1
Low Energy Array of A Major CT Facility

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

2
Physics Results from HESS

• First astronomical image of SNR RXJ1713.7-3946
  Nature 432 (2004)
• A new population of VHE g-ray sources in the
  Milky Way Science 307 (2005)
• Discovery of VHE g-rays from an X-ray binary
  Science 309 (2005)
• Discovery of VHE g-rays from the Galactic Center
  ridge Nature 439 (2006)
• A low level of EBL as revealed by g-rays from
  blazars Nature 440 (2006)

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Image of RXJ 1713.7-3946
Nature 432 (2004)
4
Galactic Plane Survey
8 new sources Science, March05 6 new sources
ApJ, Jan06
RXJ1713.7-3946 Nature, Nov04 Galactic
Center AA, Fall05 LS5039 Science, July05
Just two telescopes were operational!
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Upper Limits on SED of EBL
Measurements!
lower limits from galaxy counts (HST)
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After HESS Summary

• g-ray sky is bright in 100 GeV30 TeV energy
  range
• VHE g-ray sources touch upon a diverse set of
  physics topics
• Firm predictions of sub-100 GeV g-ray emission
  based on observations
• Multi-wavelength studies are indeed important
• Theory is often behind the observations

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Beyond HESS
Horan Weeks 2003
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Major Physics Goals

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

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g-Rays on the Ground
10 - 100 GeV 100 GeV 10
TeV 10 100 TeV
HESS, VERITAS CANGAROO III MAGIC
Better sensitivity!
There is a need for a combination of various
specialized techniques!
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Sub-100 GeV g-Ray Sources

• Mainly extragalactic point-like sources
• Northern hemisphere is preferable
• No strong need for surveys, i.e. pointing
  observations (narrow fov!!! except GRB)
• Strongly variable as well as pulsed emission
• Short time, high-flux emission (transients)

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CTA Major European Project
Eth 1-2 TeV
Eth 50-100 GeV
Eth 10-20 GeV
Proposed by W. Hofmann Berlin (2006)
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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 0.1()
• 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 g-ray showers simulated for a 30 m
telescope
Go for a 30 m telescope to detect g-ray showers
of 10 GeV!
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Telescope Design
Energy threshold
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 7 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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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 Low Array Layouts

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

Total costs 12
MUS x Num. of Telescopes
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Northern Observatory
Apache Point, NM
Courtesy of J.P. Finley
Three 30 m IACTs
50-70 m
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Simulations Stereo Array
Konopelko, Astroparticle Phys. (2005)
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g-Ray Collection 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 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)
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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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 3.2 for the g-ray acceptance
of 50 which is not very different from
supercut
3D visualization of the signal background
samples
Konopelko, A., Chilingarian, A., Reimers, A.
J.Phys. G, in press (2006)
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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.
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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Cross-calibration of Arrays
Images recorded by a 30 m telescope help to
improve signal/background rejection!
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Sensitivity of CTA
GLAST
Crab
W. Hofmann CTA Talk (2006)
10 Crab
MAGIC
20 wide-angle 10 m telescopes de la Calle
Perez, Biller, astro-ph 0602284
30 m stereo telescopes Konopelko Astropart.Phys.
24 (2005) 191
H.E.S.S.
Current Simulations
1 Crab
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Conclusions

• Extension of g-ray observations towards lower
  energies remains to be a driving force!
• CTA is a possible prototype of a future major
  facility for ground-based g-ray astronomy
  primarily a mix of the arrays of 30 m 12 m
  IACTs!
• Such a detector meets most of the physics
  requirements to achieve the scientific goals as
  currently perceived by g-ray astrophysics
  community!
• Detailed design studies on Stereo Array of 30 m
  telescopes (possibly as a component of CTA-type
  detector) are needed!

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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.