1. 1 Observation of GRBs at tens of GeV with a full-coverage air shower array at 5000-6000 m elevation Zhaoyang Feng (Speaker), Yiqing Guo, Yi Zhang, Hong Lu, Hongbo Hu Institute of High Energy Physics, CAS, Beijing, China Tianlu Chen Tibet University, Lhasa, China ICRC2011, Beijing, China

2. 2 Gamma-ray bursts: the most violent explosions in the universe Compact Star Merger Massive Star Core Collapse

3. 3 Still a relatively young field, many open questions: -- Classification(how many physically distinct types?) -- Progenitors(massive stars vs. compact stars; others?) -- Central engine( black hole, magnetar, quark star?) -- Ejecta composition (baryonic vs.magnetic?) -- Energy dissipation mechanism (shock vs.magnetic reconnection) -- Particle acceleraton & radiaton mechanisms (synchrotron, inverse Compton,quasi-thermal) -- Afterglow physics (medium interaction vs. long-term engine activity) Due to their elusive feature, GRBs are still not fully observationally uncovered in all the temporal and spectral regimes Major advance is made whenever a new temporal or spectral window is unveiled ---Bing Zhang The GRB field

4. 4 Why Study GRBs at Very High Energy (GeV-TeV)?  A new temporal window  Need to understand acceleration mechanisms, energetics, and therefore constrain the progenitors and jet feeding mechanism  Constrain local environment characteristics: Doppler factor, seed populations, photon density, B field, acceleration and cooling timescales, …  Understanding progenitor then leads to an understanding of cosmology & stellar evolution required to support progenitor population  Extragalactic background light induced absorption (EBL absorption) of high energy photons  Potential ultra high energy cosmic ray sources  Limits on Lorentz Invariance Violation

5. 5 Gamma-Ray Telescopes Pair Production Telescopes EGRET/Fermi Atmospheric Cherenkov Telescopes HESS/VERITAS/MAGIC Particle Detection Arrays Milagro/HAWC 0.1 - 100 GeV Space-Based (small area) Background free Large Aperture/High Duty Cycle Small acceptance: difficult to extend to high energies 50 GeV - 100 TeV Large Area Excellent background rejection Small FOV, small duty circle, not fast slew speed Low altitude, High threshold 100 GeV - 100 TeV Large Area Good background rejection Large Aperture & Duty Cycle

6. 6 Let’s go to 5000-6000 m altitude, with  Lower energy threshold  Better energy resolution  Better angular resolution Cosmological GAmma rays Observatory (CGAO) Observation of GRBs at tens of GeV

7. 7 Candidate sites ( please refer to next talk #1348 ） A site survey team (7 physicists) investigating 7 candidate sites at June 2011. Two excellent sites are found  Pumajiangtang Township (~5100m) : suitable for HAWC-like detector  Sheka(4300-5400m): Weather is good for IACTs Sheka Pumajiangta ng Township YangBaJing

8. 8 HAWC-like CGAO @5100m ( Pumajiangtang Township) Preliminary configuration in MC study: Instrumented Area: 150m*150m = 22,500m 2 841 PMTs (29x29) Single layer with 4m depth 5.0m spacing Trigger Condition: Nhit > 10 Trigger ~42 kHz 4m 5 m Sincerely thank HAWC collaboration for allowing us to use HAWC simulation and reconstruction code

9. 9 Effective Area 5100m CGAO: ~ 5-6x effective area VS 4100m ~ 100x effective area VS Fermi-LAT @40GeV ~ 1000x effective area VS Fermi-LAT @80GeV Very preliminary!

10. 10 Sensitivity as function of spectral index and cutoff Simulated GRB: T = 1 s Zenith = 20 deg Power law spectrum with energy cutoff Very preliminary!

11. 11 Sensitivity with different Z Very preliminary!

12. 12 Another possibility: CGAO (IACTs) at Sheka （ 4300m-5400m), A new 5@5 project ? Traditional IACTs, Magic, HESS,…, but with very fast slew speed: 5-10 o /s? or GAW, Gamma Air Watch – a new generation of IACTs with large field of view?

13. 13 In summary, we proposed a “Cosmological GAmma rays Observatory ” Observation of GRBs at tens of GeV at 5000- 6000 m elevation in Tibet * Nature of GRBs (central engine, radiation mechanisms, et al.) * Ultra high energy cosmic ray sources * Cosmology, EBL absorption * Lorentz Invariance Violation... Also an excellent telescope for detection and study of gamma ray sources

14. 14 Thank you!

15. 15 ~30 GRB have been seen by LAT above 100 MeV; Both long (>2 sec) and short (<2 sec) bursts have been seen; Some bursts are only visible in LAT Low Energy events; Most of the bursts show high- energy emission afterglow and delayed high-energy onset; Constraint: lower limit of bulk Lorentz factor of the colliding shells: ~1000; Some bursts have an extra spectral component (a different mechanism at high energy?); These short, distant and bright flashes can be used as tools to probe basic physics… PRELIMINARY 15 Fermi-LAT GRB catalog

16. 16 GeV GRBs observation EGRET: detected 4 GeV photos. For GRB940217, 2 photons with energies 3 GeV, and one photon with energy 18 GeV 90 minutes later. Fermi-LAT: detected 32 GRBs in three years’ run, six of which have gamma-rays with energies up to tens of GeV. Ground-based experiments: Tibet ASgamma experiment, ARGO-YBJ,Milagro,Pierre Auger Observatory, Wipple, MAGIC, HESS, VERITAS without positive result. only the prototype of Milagro (Milagrito) reported a possible detection of signals associated with GRB970417 with 3 confidence level.

17. 17 Limits on Lorentz Invariance Violation Some QG models violate Lorentz invariance: v ph (E ph ) ≠ c Some QG models violate Lorentz invariance: v ph (E ph ) ≠ c A high-energy photon E h would arrive after (or possibly before in some models) a low-energy photon E l emitted together A high-energy photon E h would arrive after (or possibly before in some models) a low-energy photon E l emitted together GRB080916C: highest energy photon (13 GeV) arrived 16.5 s after low-energy photons started arriving (= the GRB trigger)  a conservative lower limit: M QG,1 > (1.50 ± 0.20)×10 18 GeV/c 2 GRB080916C: highest energy photon (13 GeV) arrived 16.5 s after low-energy photons started arriving (= the GRB trigger)  a conservative lower limit: M QG,1 > (1.50 ± 0.20)×10 18 GeV/c 2 min M QG (GeV/c 2 ) 10 16 10 17 10 18 10 15 1.8x10 15 Pulsar (Kaaret 99) 0.9x10 16 1.8x10 17 0.2x10 18 4x10 16 GRB (Ellis 06) GRB (Boggs 04) AGN (Biller 98) AGN (Albert 08) GRB080916C Planck mass 10 19 1.5x10 18 1.2x10 19 n = 1,2 for linear and quadratic Lorentz invariance violation, respectively (Jacob & Piran 2008)

18. 18 Moderate Angular and energy Resolution Very preliminary! Energy resolution is under studied