Concept(s) for very low energy observations (=PowerPoint PPT Presentation

UCLA, October 21, 2005 Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

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Concept(s) for very low energy observations

(=<10 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 Dec’03. 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 (<0.5 degree) and energy resolution (<50%) 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@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 Stand-Alone Telescope

Stereoscopic System

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

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

Single or Stereo?

Farm of 12-17 m Telescopes

5@5

Stereo Array

Which Stereo?

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

• Low energy threshold: 10 GeV!• Improved CR rejection, angular & energy resolution > 100 GeV

• 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

2.2 km

5 km

Photon density is higher at R<100 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!


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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,

Altitude of observational site

Effective area of a reflector:

<A>=<>Ao

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!

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!


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We need something large to collect and focus radiation!


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Experiment: Reflector size: QE: <A> 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@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

Energy threshold:

Telescope Design

Under construction!

MAGIC votes for Stereo!


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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 (<1.5o)

Glass mirrors are ok Automatic mirror adjustment Camera auto focus [dislocation by ~20 cm] High slewing speed: 200 deg/min Approximate cost: ~5 M$US

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 4×8 m (16 m equiv.)ELT: Extremely Large Telescope 25 mCELT: California Extremely Large Telescope 30 mGSMT: Giant Segmented-Mirror Telescope 30mTMT: Thirty-metre Telescope (US + Canada + ?)Euro50: Finland, Ireland, Spain, Sweden & UKOWL: 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 <10 s, made in France]

Approximate cost: ~5 M$US

PMs pattern in a 1951 pixel camera. Superimposed is the image of a 30 GeV -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!

100 m

100 m

HESSII+

80 m

80 m

VERITAS+

Total costs: 10 M$US x Number of Telescopes


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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 PM>6 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, 545:1135 (2000)

< 30 GeV

> 30 GeV

14 3 2.47 0.1 0.05 (0.11 0.10)log( )( ) / (2.67 0.01 0.5) 10 ( /10 ) EdJ E dE E GeV 2 1 1[ ]cm s GeV

3 1 2.2 1 1 1 1 1( ) / 1.36 10 (1 ( / 5 ) ) [ ]edJ E dE E E GeV cm s GeV sr

][))775.1/(1(3907.0/)( 1112141.33016.0 srGeVscmGeVEEdEEdJCR

][)10/(106.9/)( 11127.239 srGeVscmGeVEdEEdJCR


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Gamma-Ray Detection Area

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).

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


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Detection Rates

Detection rates of -ray showers (1), electrons (2), and cosmic rays (3).

Raw background rateSingle stand-alone tel.: 1.7 kHzSystem of 2 tel.-s: 1.0 kHzArray of 5 tel.-s: 3.2 kHz

Eth, GeV R, 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

Integral rates [after cuts] R(>Eth)

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 -ray shower.

Average time pulses of the C-light emission from a 10 GeV -ray shower.


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Time-Dependent Imaging

x, d

eg

y, deg

C-light image of a 10 GeV -ray shower averaged over a sample of events.

R = 150 m

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

x, d

eg

y, deg

7.25-7.5 ns 8.25-8.5 ns 9-9.5 ns 10-12 ns


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Single Telescope Analysis

Standard image parameters

Simultaneously ‘orientation’ & ‘shape’

Non-parametric estimation of multi-variate probability density

Bayesian decision rules

Test on MC simulated events

‘Straightforward’ approach:

3D visualization of the signal & background samples.

Courtesy of Chilingarian, A., Reimers, A.

In the energy range of 10-30 GeV the maximum achieved Q-factor is 2.7 for the -ray acceptance of 50% [which is not very different from supercut]


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Angular Resolution in Stereo

Angular resolution of -ray showers with two (2) & three (1) telescopes.

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


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Analysis by Mean Scaled Width

Distributions of simulated signal & background events weighted according to the spectra.

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

Joint Q-factor: 3.8 (2 tel.-s) 5.0 (3 tel.-s)


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Sensitivity EstimatesConditions: exposure of 50 hrs, confidence level of 5, number of -rays >10.

Setup: R, Hz Rraw, kHz Fmin(>5 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

Single stand-alone telescope yields high -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.

Summary:


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Sensitivity of Stereo Array

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

For observations at zenith.

Stereo Array

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!

Documents

Concept(s) for very low energy observations (=PowerPoint PPT Presentation