Whats new on the ground Running Cherenkov gamma ray telescopes

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Whats new on the ground?Running Cherenkov gamma
ray telescopes
Thémis
Les Arcs 23 January 2001
CELESTE
David A. Smith Centre dÉtudes Nucléaires de
Bordeaux-Gradignan, In2p3/CNRS
CAT
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Very motivating Cosmic accelerators, probing
the extragalactic medium, and all that c.f.
Sundays talks. A big problem 300 gamma
sources at 1 GeV. Few well-studied sources at 200
GeV. Cherenkov imager sensitivity is good BUT the
accelerators run out of gas, and absorption kicks
in.
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Example 1 Crab nebula and pulsar.

• Cornerstone of the SynchrotronInverse Compton
  paradigm, key to AGNs.
• Supernova remnants held to be source of high
  energy cosmic rays.
• Acceleration site in pulsars could be deduced
  from the energy of the spectral cutoff.
• But no other known steady source is as intense.

Convenient number to remember 1 Crab 10 -10
erg/cm2/s, around 100 GeV. Cas A, IC443, g
Cygni, CTB80 Pulsars very promising, see E.
Durand talk.
(M. de Naurois thesis, astro-ph/0010264, 265, ApJ
in preparation)
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Example 2 The blazar Mrk 501.

• Seen by Whipple before Egret Cherenkov
  sensitivity is good.
• Blazars intrinsically very variable.
• Egret in the hole. Imagers on the IC bump, or
  beyond.
• Prediction from keV and radio results is that
  nearly no other blazars are so bright at 200 GeV.
• For 421 and 501, redshift z0.03.
• Bigger z gt X-galactic infrared absorption.

Looking for things like 1ES 1101-232, 1ES195965,
1ES 1426528, 2EG J02224253,...
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Egret is out of the hole, and the sharp cut-off
is beyond the imager range.
J. Buckley, Astropart. Ph. 11 p.119 (1999)
Markarian 421
CAT, Whipple
Egret
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• The crying need in the field is to improve
  sensitivity and/or to lower the minimum energy
  threshold.
• This talk
• Imager designs converging to an optimum
• Big mirrors, fine cameras, and stereo.
• Solar farms are growing up.
• Milagro a wide field-of-view without getting
  lost in space.
• Review of sources see individual talks...

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• Glossary
• Imagers Whipple (Arizona), CAT (Pyrenees), Hegra
  (Canary Islands),
• Cangaroo (Australia), Durham (Australia), Grace
  (India),
• Telescope array (Utah).
• Solar farms Celeste (Pyrenees), Stacee (New
  Mexico), Solar-II (California), Graal (Spain).
• Milagro Water Cherenkov, instead of atmospheric
  (New Mexico).
• Future imagers HESS array (Namibia), Veritas
  array (Arizona),
• Magic (Canary Islands), Cangaroo III (Australia)
• Satellites Egret on the Compton GRO, AGILE,
  gamma AMS, GLAST.

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• How to improve an imager
• Thresholds presently around 250 GeV, determined
  by Cherenkov signal to night sky noise ratio.
• Example 1 Whipple had biggest mirror (10 meters)
  and oldest camera.
• CATs mirror 5x smaller, but smallest pixel size
  (2 mr) and fastest electronics (few ns
  coincidence), for same threshold.
• Example 2 Cangaroo extended their
• 7 m mirror to 10 meter diameter.
• Energy threshold in 200 GeV range.
• Sensitivity increases
• finer cameras improve
• gamma/proton image separation
• stereo,
• for better alpha resolution
• for muon rejection

CAT Mrk 501 flare A. Djannati et al, Astron.
Astrophys. 350 (1999) 17-24.
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Alpha perspective angle of parallel lines viewed
from an offset position. Like, looking up at tall
trees. Or looking at meteor paths in the sky.
Digitally combined composite of nine 8-minute
exposures, November 18th 1999, 1h29-2h46 TU,
Sharm El Sheihk, Egypt, by Nigel Evans, courtesy
of Sky Telescope, June 2000. All Leonid
meteors radiate from a point just inside the
sickle of Leo, whose bottom star, Regulus, is the
brightest star at lower left
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Alpha Angle between ellipse major axis and line
from image center to camera center.
Protons fat, irregular images. Gammas
uniform, narrow.
A. Konopelko,
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A. Konopelko,
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Remaining background in alpha plot is mainly
muons.
A muon passing through the imager mirror appears
as a full circle, radius Cherenkov
angle. Circle center position gives muon
direction relative to imager pointing
direction. Muon at edge of mirror
half-circle. Muon distance D from mirror, arc
length 1/D. For CAT few pixels at 8
meters. Can look just like low energy g. For
CAT 20 Hz trigger rate, of which 12 Hz is
muons. Invisible beyond 12 meters. Future big
mirrors with low thresholds gt 1 kHz
muons/mirror. Two mirror coincidence rejects
muons.
Rare event muon arc together with hadron image.
G. Vacanti et al, Astropart. Phys. 2 (1994) 1-11.
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• SUMMARY of developments from the principal
  imagers
• Whipple now has a fine camera like CAT
• CANGAROO now has a 10 meter mirror like Whipple
• CAT is studying poor mans stereo using
  CELESTE,
• to be a little bit like HEGRA (more on this
  later).

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There is more to life than just imagers Or the
choice of lower energy instead of higher
sensitivity.
Sandia Laboratory, New Mexico site of the STACEE
experiment
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Wavefront sampling, revisited
An imager measures the angular distribution of
Cherenkov light at one place in the light pool
(or multiple points, for stereo). A different
approach, wavefront sampling, was validated by
ASGAT and Themistocle. The spatial and temporal
light distributions are measured by mirrors at
many points in the light pool. In the TeV range,
imagers work best. BUT! Below 100 GeV, hadron
showers produce little Cherenkov light. At g
shower maximum, only Eg/2 electrons (15 electrons
at 30 GeV), so statistical fluctuations and
geomagnetic scattering dominate hadron/gamma
differences. Wavefront samplers are blind to
muons. Below 100 GeV the advantages of imagers
are diminished. The geometry of a solar plant is
a technical compromise (example aberrations
change as source is tracked) but allows fast,
cheap access to 30 GeV.
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40 heliostats since 1999. Trigger threshold 30
GeV Analysis threshold 50 GeV (at transit) 13
heliostats being added.
Thémis (Pyrénées)
CELESTE CAT imager ASGAT Themistocle
5 trigger groups
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• 30 ton boiler removed from tower, replaced by
  spherical secondary mirrors.
• Winston cones for sharp field-of-view (10 mr).
• Nanosecond phototubes electronics optimize the
  cherenkov signal vs. night sky light noise.
• Programmable delays track celestial rotation.

One of six cameras
Data acquisition based on 1 GHz Flash ADCs.
( See talk by E. Durand )
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Trigger rate versus threshold
Trigger 5 analog sums, 8 heliostats each. 3 of 5
logic coincidence.
Accidental coincidences from night sky light
Simulation for 4 p.e. per heliostat threshold,
assuming Crab spectrum, at transit.
data
Cherenkov
Monte Carlo
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( See Heidelberg proceedings, astro-ph/0010264,
265 )
Padding adapted to Flash ADC data, to decrease
sensitivity to background light.
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( Whipples sensitivity in 1991 )
( A hadron veto scheme being tested further
increase? )
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Poor mans stereo CAT and Celeste record same
showers, provides muon rejection for CAT.Improve
performance of both telescopes, study spectra
from 30 GeV to many TeV.
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Mrk421 by Celeste. (No detection of
501, 1ES0219428, 1ES2344514. IC443 most
promising supernova remnant. Pulsed study of Crab
and PSR195132, see E. Durand talk).
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• Four solar farms are on track. Besides Celeste,
• STACEE (Sandia, New Mexico)
• Crab detection at 190 GeV with preliminary
  detector (S. Oser et al, Ap. J. in press).
• Currently, shakedown of upgraded instrument,
  expect 50 GeV threshold.
• Solar-II (Barstow, California)
• First light have tracked the Crab while
  recording air showers.
• See talk by G. Mohanty.
• GRAAL (Almeira, Spain)
• No secondary optics. Instead, big phototubes mix
  light from several heliostats, for high energy
  threshold.
• See talk by M. Diaz Trigo.

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• Solar-II
• 2000 heliostats available (10x Celeste or Stacee)

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Milagro can provide sorely needed alerts for
Cherenkov telescopes. Pointing instruments (e.g.
air Cherenkov) generally have better sensitivity
than survey instruments. Counter example
Glast. Narrow field-of-view problems i) you
necessarily have an a priori discovery goal -
serendipity almost ruled out. ii) While looking
at one sleeping blazar, another can flare behind
your back. Cherenkov telescopes watch X-ray
satellites, optical monitors, and each other for
alerts. Help from Milagro ?
( See talk by Jordan Goodman )
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• CONCLUSIONS.
• After a great start (Crab, 421, 501),
  ground-based gamma-ray astronomy turns out to be
  a tougher business than some of us thought.
• We are getting more clever about which blazars
  SNRs might be the best bets (dont miss
  Thursdays talks!)
• While waiting for the next generation of
  instruments,
• the imagers are converging towards similar
  fundamental design choices.
• (improved sensitivity through hadron image
  rejection, muon rejection, and alpha resolution)
• solar farms have gotten their first results,
  with improvements in progress.
• (More sources available at lower energy, even if
  sensitivity is modest at first)
• Southern sky coverage constantly improving
• Milagro brings the advantages of air shower
  arrays (24 hour northern sky coverage) down to
  the atmospheric Cherenkov energy range (gt200
  GeV).
• The future imagers will go under 100 GeV with
  high sensitivity, from 2003.