VERITAS Design. Vladimir Vassiliev Whipple Observatory Harvard-Smithsonian CfA

VERITAS Design Vladimir Vassiliev Whipple Observatory Harvard-Smithsonian CfA

VERITAS design goals VERITAS is a ground-based observatory for gamma-ray astronomy VERITAS design is derived from scientific goals of the project to perform Spectroscopic measurements in the photon energy range 100 GeV 10 TeV Detection of photons with energies from 50 GeV to 50 TeV VERITAS design is based on the properties of the Cherenkov radiation generated by atmospheric cascades VERITAS design utilizes already proven photon detection technologies: imaging and stereoscopic observation techniques VERITAS design is a compromise dictated by different scientific objectives

VERITAS Scientific Objectives Active Galactic Nuclei continuous monitoring, collecting area, energy threshold, angular resolution Extragalactic Background Light collecting area, energy threshold Gamma Ray Bursts prompt response, collecting area, energy threshold, angular resolution Shell-type Supernova Remnants exposure, field of view, angular resolution Galactic Diffuse Emission exposure, field of view Gamma-ray Pulsars energy threshold, collecting area, angular resolution Plerions exposure, angular resolution Unidentified Galactic EGRET Sources collecting area, field of view, angular resolution Dark Matter (Neutralino) energy resolution, exposure, angular resolution Lorentz symmetry violation (Quantum Gravity) energy resolution, collecting area

Design Requirements Collecting Area Exposure Angular Resolution Energy Threshold Field of View Energy Resolution Prompt Response Design constraints derived from the scientific goals of VERITAS are fuzzy. The best design should allow tuning of the instrument to a particular scientific objective. Due to the project cost limitation the compromises in the design of VERITAS are necessary to satisfy different observational tasks. Majority of the design parameters are driven by the properties of the atmospheric Cherenkov radiation.

Atmospheric Cherenkov Technique advantages and limitations γ A=0.8 m 2 Ω=2.6 sr p 8-10 km A=50,000 m 2 Ω=0.003 sr (Galbraith & Jelley, 1953, Nature, 171, 349) No anticoincidence shield!

Cherenkov radiation (characteristic scales) Atmospheric height µ 9 18 km θ θ θ= 0-1.27 o E c =+ -23 MeV R = 130 m τ = 6-8 nsec θ 1000 Photon density 100 10 1 0.1 0.01 1 21 41 61 81 101 121 Radius (m)

Detection of photons Shower size 100000 10000 1000 100 10 1 0.1 100 TeV 10 TeV 1 TeV 100 GeV 0.01 10 GeV 0.5 5.5 10.5 15.5 20.5 Atmospheric depth (r.l.) PEs per 10m reflector 1000 100 10 1 0 100 150 250 350 200 GeV 100 GeV 50 GeV Radius (m)

Telescope Aperture 15m 10m 8m Low energy threshold to accomplish VERITAS scientific goals Reliable spectroscopy below 100 GeV Efficient calibration with GLAST in the energy window 50 GeV 100 GeV VERITAS Telescope Aperture Possibility to operate several smaller aperture telescopes as one large aperture instrument Potential photo-detector upgrade to higher QE Cost limitation

Background 10 TeV 1 TeV 100 GeV 10 GeV 0.1% 1.0% 10.0% 100.0% Fluxes: F γ =2.1x10-7 (E/1 TeV) 1.50 m -2 sec -1 (Crab) F e =4.2x10-5 (E/1 TeV) 2.26 m -2 sr -1 sec -1 (CR e) F p =6.3x10-2 (E/1 TeV) 1.75 m -2 sr -1 sec -1 (CR) Area: A=50,000 m 2 Solid angle: Ω=0.003 sr (FoV=3.5 o )

Structure of the Cherenkov radiation C ~2 m ~60 m γ θ γ γ γ γ

Cherenkov radiation imaging concept p γ p Atmospheric height 20 km 1.4 km 5o e _ e + µ _ µ+ (Kertzman & Sembroski, 1994, NIM A, 343, 629) http://maat.physics.depauw.edu/gamma/kascade.html

Imaging 10m Whipple telescope legacy Crab Nebula Weekes et al., 1989, ApJ, 342, 379 Markarian 421 Punch et al., 1992, Nature, 358, 477 Markarian 501 Quinn et al. 1996, ApJ, 456, L83 VERITAS Telescope: Aperture- 10m Focal length- 12m Mirror- Davies-Cotton Camera: Field of view- 3.5 o Number of pixels- 499 Pixel spacing- 0.148 o

Telescope Camera Field of View 5.0 o 3.5 o 2.5 o Avoid severe truncation of images for efficient event reconstruction and background rejection Effective operation of a single telescope Sky surveys Mapping of extended sources Increasing of collecting area at high energies VERITAS Telescope Camera FoV Cost limitation : high angular resolution and low energy threshold require smaller camera pixel size and consequently smaller FoV, if the number of channels in the camera remains constant

0.10 o Pixel Size Number of Pixels Matching intrinsic characteristic scale of fluctuations in the shower images for efficient background rejection Low energy threshold High angular resolution 0.15 o VERITAS Camera Pixel Size Number of Pixels is 499 Cost limitation : decreasing FoV and collecting area at high energies, if the number of channels is fixed 0.25 o

Telescope Optical System f/1.5 f/1.2 Bearable aberrations of the telescope optical system (f=focal length / telescope aperture) Off-axis: global aberration at the edge of the FoV limits f-number On-axis: astigmatism restricts the size of the mirror facet VERITAS Telescope Optical System Rapidly growing cost as a function of f-number f/0.7

Stereoscopic observations γ Aharonian et al. 1993, TeV Workshop, 81

VERITAS layout Low energy threshold Low energy threshold and large collecting area

150m Spacing between Telescopes Signal to noise ratio Collecting area at high energies 80m VERITAS Spacing between Telescopes Array energy threshold Sub-array collecting area 50m

VERITAS integral sensitivity Weekes et al., 1999, VERITAS proposal

VERITAS summary of basic characteristics 100 1 % 10 1 degree 0.1 0.1 Non-rejected background 0.01 Angular resolution 1000 100 GeV 100 % 10 10 Energy threshold 1 Energy resolution hour 1000 100 10 1 0.1 0.01 5σ Crab exposure 10 1 0.1 0.01 0.001 5σ 50 hours minimal flux Whipple(single pmt) Whipple VERITAS Crab

VERITAS differential flux sensitivity Flux error: 20% Exposure: 5 hours d ln(eγ)=0.57

VERITAS differential energy flux sensitivity Flux error: 20% Exposure: 50 hours d ln(e γ )=0.57

VERITAS differential flux sensitivity Flux error: 20% Exposure: 1 minute d ln(e γ )=0.29

Crab spectrum by GLAST 1 year exposure 1000 100 N VERITAS operation region Trigger affected region Peak of the differential rate 10 1 0.010 0.013 0.018 0.024 0.032 0.042 dn/n < 0.2 0.056 0.075 0.100 0.133 0.178 0.237 0.316 0.422 0.562 0.750 1.000 E γ (TeV) An array of seven 10m telescopes provides optimal configuration for VERITAS calibration using GLAST spectroscopic measurements

Conclusions VERITAS is a giant leap in the development of ground-based observatories for the highest energy gamma-ray astronomy. VERITAS design has no risk involved because it is based on two already proven technologies: imaging and stereoscopy. VERITAS is a 7-telescope array designed to complement GLAST mission at high energies and maximize scientific return.