The future of ground-based gamma ray astronomy Where do we go?

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Panic !

Where do we stand ? And how did we get here?

Whipple 1968

H.E.S.S. 2003

Horan & Weeks 2003

Experimental sensitivity in Crab Units late 1980’s late 1990’s mid-2000’s Theory prediction (SN 1006) “Heinz Völk Units” Inefficient injection High B, low density

Radio Optical UV X-Ray Gamma IR Energy [eV] Peak detected flux / Detection threshold Neutrinos (?) Flux sensitivity

Is there a future of ground-based gamma ray astronomy ?  Physics issues  Instruments to address these issues

Wish list Sensitivity ~ E -0.8 A eff 1/2  bg -1/2  -1

Horan & Weeks 2003 Sensitivity: a no-brainer

Angular resolution: Crab viewed with EGRET Crab viewed with HEGRA-CT

Angular resolution: Source structure Chandra SN 1006 Typical TeV beam size J. Hiraga ASCA/Chandra

Wide energy coverage: Acceleration mechanism

Interaction with extragalactic background light (EBL) Cosmology and structure of space-time AGNs Wide energy coverage: Gamma ray horizon, IR & cosmology Blanch & Martinez 2004 Simulated measurements Different EBL models

Large solid angle coverage  Surveys: New sources not visible in other wavelengths  Monitoring of bursts and transients (AGN, GRB, Quasars,…) HEGRA unidentified Cygnus source

Large effective area / rate: MWL correlation of flux and index in AGNs Whipple Mkn 421

Need a smart new idea! If I had one, I wouldn’t tell you … from now on: brute force approach …

will concentrate on future beyond current generation will concentrate on future beyond current generation  A biased view! 

Sensitivity, angular resolution: seems hard to beat Cherenkov telescopes  I believe that Cherenkov telescopes are good for at another Generation beyond CANGAROO / H.E.S.S. / MAGIC / VERITAS  Should know: where are the fundamental limits of the technique?

Ultimate limit: use all photons Fit to distribution (x,y, x, y,t) here: using crude representation of distr. function, can probably do better

Shower fluctuations: angular resolution: fit to all photons O /√E TeV with geomag. field in bending plane

How many photons are needed? relative to PMT quantum efficiency 1 TeV 100 GeV 10 GeV 1 GeV shower fluctuations photon statistics

Actual shower images with NSB Hillas parameters

Shower fluctuations: background rejection 1 TeV  2 TeV p  x [m]  y [m] Rejection: few  y [m]  x [m]  y [m] 100 GeV  200 GeV p Rejection: ~ 10 -2

Conclude:  With enough light (few 10 p.e./GeV), should be able to gain factor ~3 from angular resolution  Similar factor from background rejection (p)  Larger telescopes  (Dense) telescope arrays for low energies  Small pixels  advanced photon detectors  High altitude  Bonus at low energy: geomagnetic cutoff

Optimum telescope size 10 m20 m30 m Cost per area Fixed costs dominate (Control, camera) Dish cost dominates Dish size > shower size, depth of field problem ? Triggering on low-energy showers becomes very complex

Focus & depth of field Example: Cherenkov images in a 20 m telescope Focus at infinity Focus on shower head Optimum focus Focus on shower tail Practical limit around 30 m diameter ? Telescope size = shower width

Practical thresholds A. Plyasheshnikov 30 m km Effect of geomagnetic cutoff A. Plyasheshnikov

Large Telescopes Technical design complete Super CANGAROO (M. Mori) see also: ECO 1000

Improved photon detectors PMT GaAs NIM A518, 615 Thinned CCD PMTGaAsCCD Signal: 1 :2.2 : 4.5 S/√B: 1 : 1.1 : 1.4 Self-quenching Geiger-mode avalanche cells Russian groups, MPI Munich/Semicond. Lab

ALMA site ? 10 GeV Gamma 5 km 2 km A. Konopelko

Courtesy NRAO/AUI and ESO VHE physicists dream ?  High-resolution mode  Survey / monitoring mode  Large-area mode  Halo of nano-telescopes for 10+ TeV

Survey instruments  Cherenkov telescopes with large cameras and Gascoigne aspheric corrector plate … could imagine 10 o to 15 o diameter  Fresnel lens wide-angle instruments nontrivial Fresnel lensnontrivial Fresnel lens huge focal plane ( channels)huge focal plane ( channels) would probably want several (stereo) instruments of 10 m classwould probably want several (stereo) instruments of 10 m class

HAWC: A Next Generation All-Sky VHE Gamma-Ray Telescope from G. Sinnis

Median Energy 180 GeV (Milagro ~3 TeV)  Angular resolution ~1 o  Sensitivity 50 mCrab / y for steady sources, ~ 10 h for 1-Crab flare (H.E.S.S.: 30 sec) ~ 10 h for 1-Crab flare (H.E.S.S.: 30 sec)

Conclusions  Try to get (at least) one Cherenkov telescope system with sub-mCrab 100 GeV to TeV energies, O(10 GeV) threshold and one wide-angle 100 GeV survey instrument  Unite community  Develop low-cost, no-frills production techniques Honeycomb foil mirrors a la Durham ?Honeycomb foil mirrors a la Durham ? ASIC for signal storage, trigger, digitisationASIC for signal storage, trigger, digitisation …

no magnetic field (< few 10 nT) atmospheric depth adjustable from 2 r.l. up combine perfect angular resolution (no low- energy stragglers) with large detection area

Gamma- ray ~ 10 km Particle shower ~ 1 o Cherenkov light ~ 120 m Veto (Drift chamber)

replaces geostationary communication satellites

Summary I’m afraid I have nothing really substantial to say … but nevertheless it is hard to fit within less than ½ hour !