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What’s new on the ground?
Running Cherenkov gamma ray telescopes
Thémis
CELESTE
Les Arcs
23 January 2001
David A. Smith
Centre d’Études Nucléaires de
Bordeaux-Gradignan,
In2p3/CNRS
CAT
Very motivating
Cosmic accelerators, probing the
extragalactic medium, and all that:
c.f. Sunday’s 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.
1ES 2344+514
Example 1: Crab nebula and pulsar.
•Cornerstone of the Synchrotron+Inverse Compton paradigm, key to AGN’s.
•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)
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
E. Pian, Ap J Lett 492 p.17 (1998)
bright at 200 GeV.
•For ‘421 and ‘501, redshift z=0.03.
Bigger z => X-galactic infrared absorption.
1025 Hz =40 GeV
Looking for things like 1ES 1101-232,
1ES1959+65, 1ES 1426+528, 2EG J0222+4253,...
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
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...
Babar’s wife
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.
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.
CAT’s 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.
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
Protons:
fat, irregular images.
Gammas:
uniform, narrow.
Alpha:
Angle between
ellipse major
axis and line from
image center to
camera center.
A. Konopelko,
A. Konopelko,
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 => ~1 kHz muons/mirror.
Two mirror coincidence rejects muons.
G. Vacanti et al, Astropart. Phys. 2 (1994) 1-11.
Rare event: muon arc together with hadron image.
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 man’s stereo” using CELESTE,
to be a little bit like HEGRA (more on this later).
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
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.
BUT!
In the TeV range, imagers work best.
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.
40 heliostats since 1999.
Trigger threshold: 30 GeV
Analysis threshold: 50 GeV
Thémis
(Pyrénées)
(at transit)
13 heliostats being added.
CELESTE
CAT imager
ASGAT
Themistocle
5 trigger groups
•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 )
Trigger: 5 analog sums, 8 heliostats each.
3 of 5 logic coincidence.
Trigger rate versus threshold
Accidental coincidences
from night sky light
Simulation for 4 p.e. per heliostat threshold,
assuming Crab spectrum, at transit.
Monte Carlo
data
Cherenkov
( See Heidelberg proceedings,
astro-ph/0010264, 265 )
“Padding” adapted to Flash ADC data, to decrease sensitivity to background light.
( Whipple’s sensitivity
in 1991 )
( A “hadron veto” scheme being tested: further increase? )
Poor man’s 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.
Mrk421 by Celeste.
2000
(No detection of ‘501, 1ES0219+428, 1ES2344+514.
IC443 most promising supernova remnant.
Pulsed study of Crab and PSR1951+32, see E. Durand talk).
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.
Solar-II
2000 heliostats available (10x Celeste or Stacee)
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 )
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 & SNR’s might be the best bets
(don’t miss Thursday’s 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 (>200 GeV).
 The future imagers will go under 100 GeV with high sensitivity, from 2003.