1. On A Large Array Of Midsized Telescopes Stephen Fegan Vladimir Vassiliev UCLA

2. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Primary Science Goals Detect and measure VHE transients at cosmological distances  Self triggering  Measure lightcurve from Mrk-421 at z=1 with few minute resolution Survey of VHE sky to level of 1-2 mCrab Detailed observations of Galactic sources in the energy range 20GeV to >50 TeV

3. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Requirements From Science Goals To resolve a few min variability time scale in the emission of a Mrk 421-like AGN at z=1 the collecting area must be ~1km 2. To survey the sky to 1-2 mCrab over a few years of operation, must have VERITAS sensitivity over full sky 1km 2 collecting area → Crab Nebula rate of ○ 1  /min >10 TeV○ 2  /hr >100 TeV E Interval Rate [GeV] [min `1 ] 25-501.3 50-1000.7 100-2000.3

4. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Baseline Design Distance From Center Of Array [m] Array 1.217 telescopes 2.8 hexagonal rings + 1 3.80m separation Telescope and Detector 1.ø10m equivalent 2.QE = 0.25 (Bialkali) 3.15º field of view Facts and Figures 1.Outer radius: 640m 2.Single cell area: 5543m 2 3.Total area: 1.06km 2

5. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Collecting Area vs. Field Of View Field of view [π sr] Field of view [deg] Collecting Area [km 2 ] Current IACTAs Narrow field of view <0.01 km 2 @ 40 GeV 0.05-0.1 km 2 @ 100 GeV 0.2-0.3 km 2 @ 10 TeV Square KM Array Continuum of modes Trade area for solid angle Parallel mode Narrow field of view 1 km 2 @ 40 GeV 2 km 2 @ 100 GeV 4-5 km 2 @ 10 TeV “Fly’s Eye” mode Wide field of view 0.02-0.03 km 2 @ 40 GeV 0.1-0.2 km 2 @ 100 GeV 3-4 km 2 @ 10 TeV

6. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA All Sky Coverage: “Fly’s Eye” Mode Each telescope points in different direction. If position of telescope n on ground is (x n,y n ) Zenith = ξ × ( x n 2 + y n 2 ) 1/2 Azimuth = tan -1 ( x n / y n )

7. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Question(s) How can an array of mid-sized telescopes operate in the E~30 GeV range? Why is the collecting area of instruments like VERITAS so large at E>100GeV? OR

8. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Gamma-rays INSIDE detector Instrument has efficiency ε(E) such that effective area is: A I =ε(E)πR I 2 Gamma-rays OUTSIDE detector Instrument detects γ’s to radius R O (E) such that effective area is: A O =ε(E)πR O (E)(2R I +R O (E)) Energy Dependence ε(E):0.4 @ 20 GeV 0.8 @ 40 GeV R O (E):<80m @ 20 GeV 600m @ 10 TeV Cell Effect – Collecting Area INSIDE DETECTOR OUTSIDE DETECTOR RIRI RORO

9. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Cell Effect – Small Impact Parameter Infinite Array Of Telescopes 1.3500m ASL →R Cherenk = 85m 2.D Scopes = 80m Distance [m] Geometry Dictates That 1.Impact point of every shower is in some cell 2.B Max = 47m 3.At least 3 telescopes contained in Cherenkov light pool

10. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Cell Effect – Cherenkov PE Density PE density after: 1.Atmosphere 2.Mirror reflection 3.Photocathode Cell Geometry Consider only the density within 80m of core Midsized telescopes ø10m, A=78m 2 E=32 GeV, b=80m →n PE =78 Distance from shower core [m] Photoelectron density [PE/m 2 ]

11. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Trigger – Threshold vs. Pixel Size Trigger Pixel Size [degree] (n th -N nsb ) / Q QE=0.25, FoV=15 o, R nsb =0.1 kHz QE=0.25, FoV=10 o, R nsb =0.1 kHz QE=0.25, FoV=15 o, R nsb =1.0 kHz QE=0.5, FoV=15 o, R nsb =0.1 kHz QE=0.5, FoV=10 o, R nsb =0.1 kHz QE=0.5, FoV=15 o, R nsb =1.0 kHz QE=0.25, FoV=10 o, R nsb =1.0 kHz QE=0.5, FoV=10 o, R nsb =1.0 kHz Importance of effect: 1) QE 2) Rate 3) FoV Normalized trigger threshold for given QE, NSB rate and FOV

12. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Trigger – Efficiency vs Pixel Size – I Central Telescope Trigger Efficiency Trigger Pixel Size [degree] Parameters: E  =42 GeV FoV=15 o Rnsb=1kHz Optimum trigger sensor pixel size is 0.07 o -0.3 o Weak dependence on QE, D, El (Central Telescope) QE: 1.0, D=7m QE: 0.5, D=10m QE: 0.5, D=7m QE: 0.25, D=10m El: 3.5 km

13. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Trigger – Efficiency vs Pixel Size – II Array Trigger Efficiency Photon Energy [GeV] Array Trigger: Three telescopes above operational threshold Array Parameters: Elevation: 3.5 km QE: 0.25 Reflector: 10 m FoV: 15 o p=0.05 o p=0.08 o p=0.10 o p=0.13 o p=0.16 o p=0.20 o (Full Array)

14. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Trigger – Peak Detection Energy Trigger Efficiency Photon Energy [GeV] Diff. Rate El=4.5km, QE: 1.0, D=7m El=4.5km, QE: 0.5, D=10m El=3.5km, QE: 1.0, D=7m El=3.5km, QE: 0.5, D=10m El=4.5km, QE: 0.5, D=7m El=4.5km, QE: 0.25, D=10m El=3.5km, QE: 0.5, D=7m El=3.5km, QE: 0.25, D=10m Parameters: Trigger pixel size: 0.146 o Un-localized source (FoV=15 o ) R nsb : 1kHz Diff. spectral index: 2.5 12 GeV 15 GeV 20 GeV 27 GeV Effects: 1) “Cell operation” mode 2) Optimum trigger pixel size 3) QE, Reflector Size 4) Elevation 5) R nsb

15. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Trigger – Proton Rate Proton Energy [GeV] Differential CR rates Integrated Rate: 32 kHz FoV: 15 o

16. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Trigger – Telescope Multiplicity Average Number of Telescopes in Trigger Photon Energy [GeV] El=4.5km, QE: 1.0, D=7m El=4.5km, QE: 0.5, D=10m El=3.5km, QE: 1.0, D=7m El=3.5km, QE: 0.5, D=10m QE: 0.5 El: 4.5 km, D=7m QE: 0.25 El: 4.5 km, D=10m QE: 0.5 El: 3.5 km, D=7m QE: 0.25 El: 3.5 km, D=10m 30 GeV  triggers 5 telescopes

17. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Cleaning – Sample Event Photon direction [deg] Event 1 (42 GeV)Event 2 (42 GeV)

18. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Cleaning – Voronoi Diagram Event 1 (42 GeV)Event 2 (42 GeV) 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -0.4-0.3-0.2-0.10.00.10.20.30.4-0.10.00.10.20.30.5 0.60.70.8 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 Photon direction [deg]

19. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Cleaning – P.E. Separation Scales  : 21 GeV NSB: 150  /deg 2  : 42 GeV NSB: 150  /deg 2  : 100 GeV NSB: 150  /deg 2 QE: 0.25 Reflector Diameter: 10m Elevation: 3.5 km Trigger pixel size: 0.146 o Voronoi Diagram P.E.-P.E. separation scales in Image: 0.015 o -0.045 o Diff. density [Arbitrary] P.E. separation [deg]

20. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Reconstruction – Angular Acceptance  radius [deg] Event containment fraction [1] 21 GeV 42 GeV 100 GeV CR Optimum cut: 4 photons within circle of 0.02 o radius

21. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Cleaning – Sample Event Single 42 GeV event View from 4 telescopes Optimal cleaning (from consideration of angular reconstruction) keeps only photons near core Multiple cleaning schemes may be appropriate. ●Shower axis ●Shape cut ●Energy estimate

23. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Background Rejection – Shape Cut 21 GeV 42 GeV 100 GeV CR N PE [1] Mean cascade radius [m] Parameter gives the width of the emission region in space Electromagnetic showers: tightly confined along shower axis, have small Hadronic events: reconstructed cascade radius is larger than for gamma-rays

24. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA  < 0.2 o Background Rejection – Shower Max Mean emission height above array [m] Ln(N  ) [1]  cut Parameter gives the mean emission height of the Cherenkov photons. 21 GeV 42 GeV 100 GeV CR 1 st interaction Proton: 70g/cm 2 Gamma: 37g/cm 2 Distribution of reconstructed different for each species.

25. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Background Rejection – Shower Max Integrated flux [p/deg 2 /min] Mean emission height above array [m] S/N=S/SRT(B) [arbitrary]  containment [fraction] Integrated flux [p/deg 2 /min]  containment [fraction] S/N=S/SRT(B) [arbitrary] 21 GeV 42 GeV 100 GeV CR 21 GeV 42 GeV 100 GeV 21 GeV 42 GeV 100 GeV 21 GeV 42 GeV 100 GeV

26. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Lessons “Cell Effect”: operation in 20-30 GeV range possible with midsized telescopes Optimum trigger pixel size is ~0.1º Optimum image pixel size is ~0.01º Very “hard” cleaning required to optimize reconstruction of shower axis Multiple cleaning regimes is suggested Reconstructed emission height can be used to reject protons

27. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Comparison – Collecting Area Energy [GeV] Collecting Area [m 2 ]

28. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Comparison – Differential Rate Energy [GeV] Differential Rate [arbitrary]

29. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA All Sky Survey – One Year Sensitivity Sensitivity [mCrab]

30. Towards A Future Ground Based Gamma Ray Observatory October 20, 2005, Malibu, California On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA Summary Of Array Parameters Array of 217 telescopes Elevation 3.5km Telescopes’ coupling distance 80m Area ~1.0km2 (~1.6km 2 ) Single Telescope Field of View ~15º FoV area ~177deg 2 Reflector Diameter ~7m Reflector Area ~40m 2 QE 50% (200-400nm) Trigger sensor pixel size 0.146º Trigger Sensor Size ~31.2cm NSB rate per Trigger pixel ~3.2pe / 20ns Single Telescope NSB Trigger Rate 1kHz Energy Range 20–200GeV Differential Detection Rate Peak ~30GeV Single Telescope CR trigger rate ~30kHz Image pixel size – 0.0146º Readout image – 128 x 128 pixels Readout Image size – 1.875º x 1.875º NSB per pixel – 0.032 (20 nsec gate) ADC – 8 bit (S/N improved, 10– >8) Pixel dimension 12mm x 12mm Sensor area – 12.3mm x 12.3mm Shutter exposure – a few msec Image integration time - 20ns Optical system TBD Array trigger protocol TBD Data Rates ~80 Mb/secper node Online data processing TBD