1. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 1 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

2. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 2 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!

3. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 3 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

4. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 4

5. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 5 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

6. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 6 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 m 2 ; 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

7. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 7 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)

8. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 8 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!

9. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 9 Energy Threshold Minimum image size: ~40 ph.-e. Basic telescope parameters:  Reflector area, A o  Efficiency of photon-to-ph.-e. conversion,    Altitude of observational site  Effective area of a reflector: = A o Lateral distribution of mean image size in 10, 10 2, 10 3 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!

10. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 10 We need something large to collect and focus radiation!

11. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 11 Experiment: Reflector size:QE: Altitude: Whipple10 m~0.258 m 2 2.3 km HEGRA5x3.5 m -1 m 2 2.2 km H.E.S.S. I4x12 m -11 m 2 1.8 km VERITAS4x12 m -11 m 2 2.3 km CANGAROO III4x10 m -8 m 2 160 m H.E.S.S. II28 m -61 m 2 1.8 km MAGIC I17 m -23 m 2 2.2 km MAGIC II17 m ~0.873 m 2 2.2 km ECO 100036 m -325 m 2 2.2 km 5@55x20 m~0.2531 m 2 5 km STEREO ARRAY5x30 m -70 m 2 1.8 km 0.5-1 TeV 100 GeV sub 100 GeV Energy threshold: Telescope Design Under construction! MAGIC votes for Stereo!

12. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 12 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 (<1.5 o ) 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

13. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 13 What about optical astronomy? VLT: Very Large Telescope 4×8 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...

14. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 14 Camera FoV of 3.0 o diameter  Limited by broad PSF at the large off-sets  Low energy events are close to the camera center  Scan window of about 2 o diameter Small pixels of 0.07 o  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.

15. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 15 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 HESSII+ 80 m VERITAS+ Total costs: 10 M$US x Number of Telescopes

16. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 16 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.

17. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 17 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

18. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 18 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

19. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 19 Detection Rates Detection rates of  -ray showers (1), electrons (2), and cosmic rays (3). 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 E th, GeV R , Hz R e, HzR CR, Hz 55.52.51.0 104.71.51.0 302.50.180.9 501.70.060.68 1001.00.010.34 Integral rates [after cuts] R(>E th ) Event trigger rate of ~3.2 KHz can be easily maintained by advanced readout system!

20. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 20 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.

21. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 21 Time-Dependent Imaging  x, deg  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, deg  y, deg 7.25-7.5 ns 8.25-8.5 ns 9-9.5 ns 10-12 ns

22. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 22 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]

23. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 23 Angular Resolution in Stereo Angular resolution of  -ray showers with two (2) & three (1) telescopes. 63% radius at 10 GeV is 0.3 o Q-factor is about 3.1 3-fold resolution is better by 30%

24. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 24 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)

25. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 25 Sensitivity Estimates Conditions: exposure of 50 hrs, confidence level of 5 , number of  -rays >10. Setup:R , HzR raw, kHzF min (>5 GeV), cm -2 s -1 Single stand-alone tel.16.81.71.46x10 -10 Stereo system of 2 tel.-s11.21.06.45x10 -11 Stereo array of 5 tel.-s20.13.22.94x10 -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:

26. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 26 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 in12 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

27. UCLA, October 21, 2005Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future" 27 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!