A Future All-Sky High Duty Cycle VHE Gamma Ray Detector

Gus Sinnis/Los Alamoswith

A. Smith/UMdJ. McEnery/GSFC

Scientific Motivation• High-Energy (>25 GeV) GRB Spectra

– Understanding of acceleration in GRBs– Tests of Lorentz invariance at Planck mass

• Monitor AGN– Long-term continuous studies– Long-term multi-wavelength correlations– Observe short (<10 minute) flares from many AGN

• Discover new sources– Source statistics needed to understand AGN– New objects unseen at other wavelengths?

Milagro: Current State of Art• Moderate altitude: 2200 m asl• Moderate area: 4000 m2

• Sensitivity: Crab ~4-5/sqrt(year)• Threshold for GRBs: ~300 GeV• Median energy: ~2 TeV

ARGO: Next Generation

• High altitude: 4300 m asl• Moderate area: 6500 m2

• Sensitivity: Crab 10/sqrt(year) [before background rejection]

Design Goals• GRBs

– Complement GLAST– Energy threshold near ~20 GeV

• Near known energy• Able to see large distances (z~1)

– Lorentz invariance studies require ability to see prompt emission• AGN

– Ability to detect/study short (~10-20 minute) flares– Ability to detect distant AGN (z~0.3)– Ability to continuously monitor all AGN

• Large field of view (~2 sr)• ~100% duty cycle• Crab sensitivity ~7/sqrt(day) - 140/sqrt(year)

Is Such an Instrument Possible?Approximation B: Effect of altitude

Strawman Design• 40,000 m2 water Cherenkov Detector• High altitude: 4500 m2

• Two layers of photodetectors– Top layer for direction and triggering– Bottom layer (calorimeter) for background rejection & energy

determination• 8” PMTs (same as Milagro)• 3 m detector spacing (~10,000 PMTs)• 50 PMT trigger (in top layer)• Corsika to Generate air shower• GEANT in water• Use standard Milagro reconstruction for events

200 m

200

m

Event Timing

Angular Resolution

0.75o resolution

Preliminary resolution function

Milagro

StrawmanTriggered

Fit in Bin

Effective Area vs. Energy: Gamma Rays

Background Rejection

• Estimate total rate (due to background) by scaling proton efficiency from Milagro– 120 kHz trigger rate expected

• As in Milagro use bottom layer information to detect penetrating component of hadronic showers.

• Small clumps of intense light indicate presence of penetrating component

Gam

mas

Prot

ons

Pulse Heights in Bottom Layer260 GeV

660 GeV 350 GeV

170 GeV 2.5 TeV 2.2 TeV

Background Rejection

Use 4 parameters: nTop, nBot2, nBot8, sumPEBot

MARS: J. Friedman

Reject 95% of protons

Accept 55% of gammas

2.5x improvement in sensitivity

D.C. Sensitivity: Sky Survey• Crab Spectrum: dN/dE = 3.2x10-7 E-2.49

– 0.22 (0.12) Hz of gammas from Crab raw (cut)– Whipple 0.025 Hz– Veritas 0.8 Hz

• Background rate 80 (4) Hz raw (cut)• 3 /sqrt(day) raw data• 7.5 /sqrt(day) cut data

– 140 /sqrt(year)• 35 mCrab sensitivity (all sky) in one year

– Whipple: 140 mCrab per source– VERITAS: 15 mCrab per source

Transient Sensitivity

Absorption of TeV Photons

e+

e-

~eV

~TeV

E (TeV)

z (

= 1

)

Effect of IR Absorption on Distant Sources

z = 0.03z = 0.1z = 0.2

z = 0.3

z = 0.0

Energy Distribution of Fit Events

AGN Sensitivity

Gamma Ray Burst Sensitivity

Conclusions• A large area, high altitude all sky VHE detector can:

– Instantaneous sensitivity comparable to Whipple– D.C. sensitivity approaching VERITAS– AGN sensitivity to z = 0.3– GRB sensitivity to <50 GeV– GRB sensitivity to z~1

• Continuing work– Background rejection (low energy)– Improved event reconstruction– Detailed detector design (electronics, DAQ, infrastructure)– Reliable cost estimate needed (~$30M???)– Site survey