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Future Prospects: Ground-based Gamma-ray Astronomy Workshop

Explore concepts for very low-energy observations (<10 GeV) in gamma-ray astronomy. Delve into the physics goals, requirements, alternatives, and constraints to drive future advancements in this exciting field.

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Future Prospects: Ground-based Gamma-ray Astronomy Workshop

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  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 Workshop: "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! Workshop: "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 Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  4. Workshop: "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 Workshop: "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 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 Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  7. Constrained Choice Single or Stereo? Which Stereo? (*) Single Stand-Alone Telescope Farm of 12-17 m Telescopes • Large collection area > 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 > 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@5 • Very low energy threshold: 5 GeV • Reduced sensitivity at higher energies • Technically difficult and very expensive! (*) Kruger Park Workshop (1997) Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  8. High Altitude Site • 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! 5 km 2.2 km Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  9. Energy Threshold • Minimum image size: ~40 ph.-e. • Basic telescope parameters: • Reflector area, Ao • Efficiency of photon-to-ph.-e. conversion, <e>(*) • Altitude of observational site (**) • Effective area of a reflector: <A>=<e>Ao (*) 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! Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

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

  11. Under construction! MAGIC votes for Stereo! Telescope Design Energy threshold: 0.5-1 TeV 100 GeV sub 100 GeV Workshop: "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 (<1.5o) • 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 Workshop: "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... Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  14. 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 ms, made in France] • Approximate cost: ~5 M$US PMs pattern in a 1951 pixel camera. Superimposed is the image of a 30 GeV g-ray shower. Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  15. VERITAS+ 80 m 80 m 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 M$US x Number of Telescopes 100 m Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  16. Simulations: Stereo Array Workshop: "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 Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  18. 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). Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  19. 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(>Eth) 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! Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  20. 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. Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  21. Qx, deg Qy, deg 10-12 ns 8.25-8.5 ns 9-9.5 ns 7.25-7.5 ns 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. Workshop: "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: 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. Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  23. 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. Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  24. 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. Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  25. Sensitivity Estimates Conditions: exposure of 50 hrs, confidence level of 5s, number of g-rays >10. Summary: • Single stand-alone telescope yields high g-ray rate • Stereo system of two tel.-s provides sensitivity higher by a factor 2.2than single tel. • Stereo array gives further improvement by a factor of 2.2 • Sensitivity of stereo array is by 5times better than single tel. Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

  26. Stereo Array 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 Workshop: "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! Workshop: "Ground-based Gamma-ray Astronomy: Towards the Future"

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