1. Water Cherenkov Technology in Gamma-Ray Astronomy Gus Sinnis Los Alamos National Laboratory

2. Complementarity of Gamma-Ray Detectors Large Aperture/High Duty Cycle Milagro, Tibet, ARGO, HAWC Large Area Good Background Rejection Large Duty Cycle/Aperture Sky Survey Extended Sources Transients (AGN, GRB) Highest Energies Galactic Diffuse Emission Low Energy Threshold EGRET/GLAST Space-based (Small Area) “Background Free” Large Duty Cycle/Aperture Sky Survey AGN Physics Transients (GRBs) High Sensitivity HESS, MAGIC, VERITAS Large Area Excellent Background Rejection Low Duty Cycle/Small Aperture High Resolution Spectra Study of known sources Limited Surveys Fast Flaring Distant AGN

3. Goals of a TeV Gamma-Ray Survey Instrument Galactic cosmic-ray origins –Galactic diffuse emission –Highest energies (>10 - 100 TeV) Particle acceleration in astrophysical jets –Gamma-ray bursts –Active galaxy transients –Multi-wavelength/messenger campaigns All-sky survey –Discovery potential –IACT alert system

4. Galactic Cosmic Rays Measure Galactic accelerators to >100 TeV Measure diffuse emission spatial and spectrally resolved –Large area (100,000 m 2 ) –High duty factor (~100%) –Large field-of-view (~2 sr) EGRET Milagro pion channel Inverse Compton channel EGRET all sky (100 MeV) Strong & Moskalenko Cygnus Region

5. Absorption by EBL requires –Low energy threshold –200 GeV for z = 0.5 horizon –1-2 TeV for z = 0.1 horizon Extragalactic transients Fermi/LAT ScienceExpress 2/19/2009 GBM LAT Gamma-Ray Bursts –GeV ≥ 0.1 x MeV fluence –10 -7 ergs/cm 2 @ 10 sec –4000 m 2 @ 200 GeV GeV = 0.1 100keV GeV = 100keV MAGIC collab.

6. Extensive Air Shower Arrays http://www.ast.leeds.ac.uk/~fs/photon-showers.html 4 km 1 TeV gamma-ray shower Longitudinal Profile 7.7 km 30 km meters gammas electrons gamma:electron ratio ~6:1 em particles sparse at low energies need enclosed area ~ active detector area 200 m Tibet AS  Active detectors Milagro

7. Effect of Altitude on Response 30% eff 7% eff

8. E-M Energy on Ground 5200 m Observatory ~10% of energy reaches the ground Error on Mean

9. Background rejection in EAS arrays  ’s within a 10 5 m 2 area of core Large fluctuations of shower size manifest as fluctuations in muon content

10. Milagro – 1 st Generation 8 meters e  80 meters 50 meters 2600m asl 898 detectors – 450(t)/273(b) in pond – 175 water tanks 4000 m 2 (pond) / 4.0x10 4 m 2 (phys. area) 5-40 TeV median energy (analysis dependent) 1700 Hz trigger rate 400 Gbyte/day 0.3 o -1.2 o resolution (0.75 o average) 95% background rejection (at 50% gamma eff.)

11. Background Rejection in Milagro Proton MC Data  MC Hadronic showers contain penetrating component:  ’s & hadrons – Cosmic-ray showers lead to clumpier bottom layer hit distributions – Gamma-ray showers give smooth hit distributions

12. Background Rejection (Cont’d) mxPE:maximum # PEs in bottom layer PMT fTop:fraction of hit PMTs in Top layer fOut:fraction of hit PMTs in Outriggers nFit:# PMTs used in the angle reconstruction Apply a cut on A 4 to reject hadrons: A 4 > 3 rejects 99% of Hadrons retains 18% of Gammas S/B increases with increasing A 4 Background Rejection Parameter

13. TeV Observations of Fermi Sources 34 Fermi BSL Galactic sources above declination of -5 o 14 detected by Milagro above 3  –FDR Miller 2001 estimates 1% false positive rate 5 new TeV sources Geminga 6.3  as extended source (2.6 o fwhm) BoomerangCygnus Region MGRO 1908+06 HESS 1908+063 Geminga Crab Nebula Fermi Sources

14. Geminga Pulsar Milagro C3 Pulsar (AGILE/Fermi) MGRO 2019+37 Fermi Pulsar  Cygni SNR Fermi Pulsar HESS 2032+41 MGRO 2031+41 MAGIC 2032+4130 Fermi Pulsar Milagro (C4) 3EG 2227+6122 Boomerang PWN IC433 SNR MAGIC VERITAS Radio pulsar J0631+10 (new TeV source) unID (new TeV source) unID (new TeV source) Fermi Pulsar MGRO 1908+06 HESS 1908+063 W51 HESS J1923+141 SNR G65.1+0.6 (SNR) Fermi Pulsar (J1958) New TeV sources

15. HAWC: The Next Generation The base of volcán Sierra Negra latitude : 18º 59’ longitude: 97º 18’ altitude : 4100m Inside Parque Nacional Pico de Orizaba 2 hours from Puebla (INAOE) 15x Milagro sensitivity 5x larger active detector area optical isolation of detector elements 10x larger muon detector improved angular resolution improved energy resolution higher altitude (4100 m) 1/3 median energy of Milagro

16. The HAWC Collaboration Los Alamos National Laboratory B. Dingus, J. Pretz, G. Sinnis University of Maryland D. Berley, R. Ellsworth, J. Goodman, A. Smith, G. Sullivan, V.Vasileiou University of New Mexico J. Matthews University of Utah D. Kieda Michigan State University J. Linnemann Pennsylvania State University Ty DeYoung NASA/Goddard J. McEnery Naval Research Lab A. Abdo UC Santa Cruz M. Schneider Instituto Nacional de Astrofísica Óptica y Electrónica Alberto Carramiñana, L. Carasco, E. Mendoza, S. Silich, G. T. Tagle, Universidad Nacional Autónoma de México R. Alfaro, E. Belmont, M. Carrillo, M. González, A. Lara, Lukas Nellin, D. Page, V. A. Reese, A. Sandoval, G. Medina Tanco,O. Valenzuela, W. Lee Benemérita Universidad Autónoma de Puebla C. Alvarez, A. Fernandez, O. Martinez, H. Salazar Universidad Michoacana de San Nicolás de Hidalgo L. Villasenor Universidad de Guanajuato David Delepine, Victor Migenes, Gerardo Moreno, Marco Reyes, Luis Ureña UC Irvine G. Yodh University of New Hampshire J. Ryan

17. HAWC Design 900 tank array 4.3m high x 5m diameter tanks 100 MeV  photons shown Through-going Muon 150 m

18. HAWC: Background Rejection Gammas Protons Size of Milagro deep layer Energy Distribution at ground level Size of HAWC Rejection Parameter: nPMT/cxPE nPMT = # PMTs in event cxPE = Maximum # Pes in PMT > 30 m from fit core location

19. Background rejection Background rejection improves improves with increasing energy S/B 5x at E> 5 TeV (with rejection vs. no rejection) Essentially background free near 100 TeV hadrons gammas Milagro HAWC Fraction bkgd remaining

20. HAWC: Effective Area

21. HAWC DC Sensitivity: 5-Year Survey IACTs 50 hrs (~0.06 sr/yr) 1 yr EAS 5 yrs (~2  sr) 2000 km 2 sr hr

22. Survey Sensitivity 4 min/fov 7 min/fov 1500 hrs/fov

23. Sensitivity vs. Source Size Large, low surface brightness sources require large fov and large observation time to detect. EAS arrays obtain ~1500 hrs/yr observation for every source. Large fov (2 sr): Entire source & background simultaneously observable Background well characterized

24. Brenda Dingus HAWC Review - December 2007 AGN Monitoring Measure TeV duty factors and notify other observers of flares in real time. Unbiased survey for TeV “orphan” flares All sources within ~2  sr will be observed every day for ~ 5 hrs. Continuous observations – no gaps due to weather, moon, or solar constraints. HAWC’s 5  sensitivity is (10,1,0.1) Crab in (3 min, 5 hrs, 1/3 yr) Worldwide Dataset of TeV Observations by IACTs of Mrk421 1 month

25. Tank Details PMT at bottom of tank Non reflective interior surfaces Roto-molded tank issues –Largest tanks available not deep enough –Too large for road transport (build on site) Steel pipe with bladder No size limitations, easy transportation (in pieces) @ Sierra Negra In CA

26. Conclusions Water Cherenkov Technology enables a “low”-threshold all-sky gamma-ray capability (sub-TeV) First generation instrument built at moderate altitude demonstrated the capability of the technique –Discovery of Galactic diffuse emission at 10 TeV (large excess observed) –Discovery of extended sources of TeV emission –Discovery of an anomalous component to the local cosmic rays –TeV counterparts to Fermi GeV sources (5 new TeV sources) The next generation instrument will have ~15x greater sensitivity –Build at high altitude (4100m) Scientific Goals –Origin of Galactic Cosmic Rays –Understanding Galactic accelerators (Pevatrons) –Extragalactic accelerators via multi-wavlength/messenger study of transients Active Galaxies (10x Crab in 3 minutes) Gamma-ray bursts Funding received for R&D and site development ($1M) –3 tanks operating on site –All permits for full array in place Proposal at NSF and DoE awaiting PASAG (Summer 2009)