Data analysis techniquesData analysis techniques
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CherenkovCherenkov TelescopesTelescopesfor GammaGamma--Ray AstrophysicsRay Astrophysics
Part II: Astrophysics with Part II: Astrophysics with Cherenkov Cherenkov TelescopesTelescopes
W. W. HofmannHofmannMPI fMPI für ür Kernphysik, HeidelbergKernphysik, Heidelberg
H.E.S.S. Telescope System (Photomontage)
Imaging atmospheric Cherenkov telescopes
Pioneered by theWHIPPLE group Perfectioned in
CAT telescope
WHIPPLE490 PMTcamera
Stereoscopywith HEGRA
Whipple, ...B~ 0.1 2001z=0.129XBLH 1426+428
HEGRAC~ 0.031999~ 3.5 kpcShell SNRCassiopeia A
CrimeanC~ 1.51998z=0.44RBL3C66Aup to ~ 2
up to ~ 2up to ~ 2
up to ~ 0.6up to ~ 10up to ~ 10
~ 0.4-
~ 0.7~ 0.5~ 0.7~ 0.5
1
Flux (CU)
MilagritoC1999GRBGRB 970417a
B
BBCAA
CC
BBBAA
Grade
Crimean2001z= 0.069XBLBL Lac
Tel. Array1999z=0.048XBL1ES 1959+650Durham1999z=0.116XBLPKS 2155-304Whipple1997z=0.044XBL1ES 2344+514Whipple, ...1995z=0.034XBLMarkarian 501Whipple, ...1992z=0.031XBLMarkarian 421
ExtragalacticDurham1999~ 8 kpcBinaryCentaurus X-3HEGRA2001~ 1.6 kpcShell SNRMonoceros
CANGAROO19991 – 6 kpcShell SNRRXJ 1713.7-3946CANGAROO1997~ 2 kpcShell SNRSN 1006CANGAROO19970.2 – 0.5 kpcPlerion ?VelaCANGAROO, ...1995~ 2 kpcPlerion ?PSR 1706-44Whipple, ...1989~ 2 kpcPlerionCrab Nebula
GalacticGroupDiscoveryDistanceTypeSource
Adapted from Weekes, astro-ph/0010431
A: > 5 σ, confirmed B: > 5 σ, unconfirmed C ≤ 5 σ
Catalog of TeV sources
The Crab Nebula with EGRET and HEGRA
4 degr. 4 degr.
EGRET on CGROat ~ 100 MeV
HEGRA CT Systemat ~ 1 TeV
Gamma rays are secondary productstheir emission traces primary particle populations• Hadrons
• π0 decay• Electrons
• Synchroton radiation• Inverse Compton scattering• Bremsstrahlung
• Heavy instable particles (strings, monopoles, ...)
Problem:to relate the gamma ray flux and the primary spectra,one needs to• Know the properties of the production “target”• Deconvolve energy spectra
Galactic Sourcesand the Origin of Cosmic RaysGalactic Sourcesand the Origin of Cosmic Rays
The quest for the sources of cosmic raysBest (only?) candidate:Supernova explosions
Argument 1:Energy balance
ECR ~ ρE V / τesc ~ 1041 erg/s
ESN ~ 1051 erg/30 y ~ 1042 erg/s
... need O(10%) efficiency
Argument 2:Shock acceleration as mechanism
dN/dEShock acc ~ E-2.1
dN/dEObs ~ dN/dEAcc τesc(E)
where τesc (E) ~ E-0.6
dN/dE ~ E-2.7
ρE ~ 1 eV/cm3
mostly nuclei;< 1% electrons(above 1 TeV)
Particle acceleration in Supernova remnants
Example:Cassiopeia A• about 300 y old• size about 5 pc• speed of ejecta ~5000 km/s
Gamma-ray flux from SNRDrury, Aharonian, Völk 1994 (DAV)
Fγ(> 1 TeV) ~ 5 θ E51 n1 dkpc
-2 [Crab]
θ EfficiencyE51 Energy [1051 erg]n1 Density [1/cm3]dkpc Distance [kpc]
SNR acceleration models give• Peak energies O(PeV)• Efficiencies θ up to 0.5
HEGRA Galactic Plane Survey
G. Pühlhofer,H. Lampeitl
First survey at TeV energies
Limits on TeV flux for over 60 SNR
Individual limits O(0.1 – 1) Crab
Combined limit 0.04 Crab, for an average distance of 5.5 kpc
DAV: 0.1 Crab for 50% eff.
HEGRA
HEGRA
CANGAROO
CANGAROO
Exp.
0.03
< 0.03
0.7
0.5
Flux [Crab Units]
1.4 – 2.1SN 1006
3.3 – 3.7Cassiopeia A
2.2 – 4.5Tycho
1 – 6RX J1713.7...
d [kpc]Supernova
All positive observations remain to be confirmed ...
SN 1006ASCA
Cassiopeia AChandra
TychoRosat
0.1~ 0.6
0.04~ 0.2
0.06~ 0.3
……
10%50%
SNR 1006 (CANGAROO)
CANGAROOTanimori et al. 1998Tanimori et al. 2001
• Signal observed with different generations of (single) telescopes3.8 m 7.7 σ10 m 6.5 σ
• Signal centered on NE rim of SNR
• Cannot really map source distribution, but signal appears wider than experimental resolution
Significance map
InterpretationCANGAROOTanimori et al. 1998Tanimori et al. 2001
Mastichiadis, Stecker 1996Aharonian, Atoyan 1999...
• SN 1006 accelerates electrons up to energies of ~100 TeV
• No evidence for (or against) proton acceleration
RX J1713.7-3964 (CANGAROO)
CANGAROOMuraishi et al. 2000Enemoto et al. 2001
αsource
Rumors about paper subm. to Nature,stating that X-rays and TeV spectra areNOT consistent for electron models …
Tycho SNR (HEGRA)HEGRAAharonian et al. 2001G. Rowell
Angular distribution ofphoton candidatesrelative to source
Very good upper limitof 0.03 Crab Units
Cuts into model parameter spaceof hadronic models
Cassiopeia A (HEGRA) HEGRAAharonian et al. 2001Ph.D. G. Pühlhofer
Very deep observation – 232 h5 σ signal at 0.03 Crab level
C.o.g. of gamma emssion(diameter of SNR only3-4 arc-min; cannotresolve details)
Cassiopeia A spectraInverse Compton radiation:model synchrotron spectrain a multi-zone model, then predict IC
Atoyan et al. 2000
IC, ...
π0 decay
Fit of spectral shape:probability forhadronic origin 98%electron origin 15%
Problems with injection ?Völk 2001
Inefficient injection in thewhen B fields parallel toshock front?Acceleration only near “poles”?
Monoceros: SNR Interacting with Cloudan EGRETsource
HEGRATeV gamma rays~ 5 σ for total excess
F. Lucarelli et al.,ICRC 2001
Conclusions
• Cherenkov telescopes have opened up TeV gamma ray astronomy
• A number of astrophysical objects produce gamma rays with energies well beyond TeV scales (Crab: > 50 TeV, SNRs > 10 TeV, AGNs > 20 TeV)
• Many of these objects emit as much or more energy at TeVenergies than in other wavelength ranges
• In current objects, data are consistent with a primary electron population (with energies up to and beyond 100 TeV)
• Shock wave acceleration is strongly supported, but...
• High energy electrons are much more efficient in producing gammas – O(10...100) at TeV energies – hence not obvious if electron sources will also accelerate sufficient cosmic rays
• No proof for nucleon acceleration... just around the corner?Need
Verifying nucleon acceleration
Need more sensitive instruments, which allow a precise spectral and spatial mapping of sources H.E.S.S., ...
... and neutrino observatories ...
Spectral andspatial distribution
of synchrotron radiation
Spectral andspatial distributionof VHE gamma
radiationInconsistency ?in particular at highenergy ... electrons
are hard to accelerate,see LHC vs LEP
Extragalactic Sources,and the Infrared BackgroundExtragalactic Sources,and the Infrared Background
Gamma-Rays from the AGNsMarkarian 421 and 501
Mkn 501Faint galaxy, about500 Mio light yearsfrom Earth
In 1997 the strongestsource of highest-energygamma-rays in the sky !
a strongly variable source
Whipple dataM. Catanese,T. Weekes,astro-ph/9906501
Mkn 421Whipple
Energy spectrum of gamma rays
Shape (almost) independent ofintensity
Interpretation of Blazar sources
Black hole of 108-109
solar masses
Relativistic jet
Small “Blob”accelerating particlesSize (c∆T) δ
Acceleration ofcharged particles
in the blob
Acceleration ofcharged particles
in the blob
Targets for photon production
within the blob
Targets for photon production
within the blob
Absorption within the blob
Absorption within the blob
Absorption in intergalactic
space
Absorption in intergalactic
space
Measurement of flux and spectrumMeasurement of
flux and spectrum
Electrons ? Protons ?Spectrum & cutoff ? Size of blob ?Variability of injection rate ? of spectrum ? ...
IC: on synchrotron photons ? Disk photons ?Background photons ?
Proton interactions with moving clouds? ...
Optically thick source ?If not, need to limit energy
density in source volume !Could generate cutoff ...
Pair creation on backgroundphotons limits path; could
generate cutoff ...Photon density not well known !
Energy resolution of 10-30% maysmear cutoff
Syst. error in scale may shift cutoff
Usetime dependence of
flux and spectral shapeand correlation with
other wavelengths (X-rays)
Acceleration ofcharged particles
in the source
Acceleration ofcharged particles
in the source
Spectrum ?Electrons,
protons, ... ?
CAT dataPiron et al. 2001
Looks like electrons!
TeVkeV
Origin of variabilityin synchrotron self-Compton(SSC) models
A. Mastichiadis, J. Kirkastro-ph/9610058
Electron injectionincreased by x 3;at 1, 2, ∞ τ
Maximum energy of electrons increasedby x 5
Magnetic fieldincreased by x 3
e
X-ray
γ-ray
Correlation between TeV gammas and X-rays
H. Krawczynski et al.astro-ph/9911224
HEGRA vs. RXTE
Expect TeV ~ (X-ray)2 for SSC• if electron spectrum does not change• in the static limit
Mkn 501
Correlation between X-ray flux and spectral shape
Harder X-ray spectrum for higher flux:either electron spectrum or B field or source boost δchange in addition to electron flux;Data constrain mainly δ/B
L. Maraschi et al.,astro-ph/9909059,astro-ph/0102295,
Correlation between VHE flux and spectral shape:Mkn 421
Long-termF. Piron, CATastro-ph/0109286
syst.error
Short-termD. Horns, HEGRA
Flux
Hardness ratio
F. Tavecchio et al.ApJ 554 (2001) 725
Fits using SSC models
F. Tavecchio,L. Maraschi,astro-ph/0002431
1010δ
22Index
0.010.02B (G)
6 x 106107γbreak
0.21Flux
2.51.8R (1016 cm)
LowHigh
Values for R, B, γbreakdiffer by more thanone order of magnitudebetween the two fits!
Targets for photon production within the source
Targets for photon production within the source
There is more than SSC:SSC +ECD (Direct disk radiation) +ECC (Reprocessed disk rad.)
3C 279
Hartmann et al.,2001
Absorption within the source
Absorption within the source
Source size ~ δ ∆tPair creation optical depth
~ Fopt ∆t –1 δ –6 Eγ
minimum doppler factor ~8.5 for Mkn 501
Absorption in intergalactic
space
Absorption in intergalactic
spaceVHE gammaBackgroundphoton
e+
e-
Largest cross section near threshold,4 EVHEEPhoton ~ (2 mec
2) 2
(Funk et al, 98)
Energy of γ-ray
Energy ε of background photonnear peak of X-section (eV)
Density of target photonsx ε2
Optical depth(Redshift z)
RedshiftMkn 421, 501
G. Hauser, E. Dwekastro-ph/0105539
Density of target photons x ε2
Recent Whipple and HEGRA results on Mkn 421
Mkn 501Mkn 421Energy cutoff
6.2 ± 0.4 TeV4.0 ± 0.4 TeVHEGRA
4.6 ± 0.8 TeV4.3 ± 0.3 TeVWhipple
Syst. errors ~ 30%
F. Krennrich et al.ApJ 560 (2001) L45
WhippleMkn 421
Mkn 421
HEGRAWhippleMkn 501 Mkn 501
HEGRA
Spectral cutoff and the IR/O background
Origin of the cut-off could be• Interaction with IR/O background depends only on zsource• Cutoff in electron spectra in source• KN regime of IC process different for each source• Optically thick source
Approaches in literature1. Assume power law source spectrum; neglect other effects
determine IR/O density2. Fit electron spectrum, B, δ from X-rays; predict source γ spectrum;
assume that source is transparent determine IR/O density3. Assume IR/O density, determine source spectrum from observed
spectrum consistency check: reasonable source spectrum ? (constant spectral index or steepening with energy)
ok ok not ok
The TeV-gamma-ray crisis (?)
R. Protheroe, H. Meyer,astro-ph/0005349
Solutions
Photon condensatesfake high-energy photons(Harwit et al. 1999)
Violation of Lorenz invariancemodifies photon propagation(Coleman & Glashow 1999, ...)
Measure height ofshower maximum, ~ log(Eγ)
Measurement of flux and spectrumMeasurement of
flux and spectrum
Reanalysis of HEGRA datawith improved energy resolution:consistent, but last point lower
IR crisis goes away, if systematic errors on VHE data and errors on IR data are taken into accountAharonian et al., A&A 366 (2001) 62
O. de Jager,F. Stecker,astro-ph/107103
H 1426+428XBL at z=0.129Synchrotron extends to > 100 keV
D. Horan et alICRC 2001
Detected byVERITAS
Also seenby HEGRAF. Aharonianet al., ICRC 2001see also CAT
Source clearlyidentified
HEGRAsourcelocation
Spectrum & Infrared Background
HEGRA spectrumconsistent with E-2.6
also consistent with stronglyabsorbed spectrum ...
AGNs
• Time variability in VHE data requires compact source and significant boost (~10)
• SSC models describe data reasonable well, but other (e.g.hadronic) models are not completely excluded
• AGN spectra deviate from power laws, with cutoffs around a few TeV for Mkn 421 and 501 (with z ~ 0.03)
• Data are (within statistical and systematic errors) consistent with IR absorption using typical IR spectra; no obvious crisis or need to invoke exotic phenomena
• More precise data (multiwavelength spectra, time variability) are needed to fix model details
• More sources are needed, at different z, luminosity, etc
Many new results expected for the next years ...with O(100) sources
Physics with non-imaging Cherenkov instruments
STACEE
CELESTE secondaryreflector and PMTs
Sample light distribution on theground, rather than angular distribution
32 - 64
4
32
40
No. of PMTs
?~ 101300 - 260032 - 64Barstow, USSOLAR 2
2507 - 12250063Almeria, SpainGRAAL
190~121200(→ 2400)
32 (→ 64)
Albuquerque, USSTACEE
60~102000(→ 2700)
40(→ 53)
Pyrenees, FranceCELESTE
Threshold(GeV )
F.o.V.mrad
Mirror area (sqm)
No. of heliostats
LocationInstrument
“CELESTE was designed to reach a very low energy threshold without a large expenditure in time and resources by exploitingthe mirrors of an existing structure...”(M. De Naurois et al., astro-ph/0107301; see also CELESTE Proposal, 1996)
CELESTESTACEE GRAAL
Problem of current experiments: very limited field of view
pγ
CELESTE detection rates
Crab:2.1 σ / √h3.4 σ / √h with “double pointing”
Rate ~ 4 /minQuality fact. ~ 1.5 (homogeneity)
Detection of Crab Nebula
CELESTEM. De Naurois et al.,astro-ph/0107301
GRAALF. Arqueros et al.,astro-ph/0108270
STACEES. Oser et al.,astro-ph/0006304
CELESTE
GRAAL
hopefully soon … detailed AGN data
The Near FutureThe Near Future
H.E.S.S. in Namibia 1st Tel. ready in Spring 20022nd late in 20023rd and 4th in 2003
MAGIC at La Palma 1st light in 2002
CANGAROO III in Australia
1st Tel. operational,2nd in Spring 20023rd in 20034th in 2004
Ideas for the Far FutureIdeas for the Far Future
Skip topic
Higher sites
F. Aharonian et al.,Astropart. Phys. 15 (2001) 335
20 m telescopes at 5 km provide sub 10 GeV thresholdand very high rate
Possible location:
Images still ok at 10 GeV
Better vision:novel photon detectors
ε ~ 15%
ε ~ 22%
ε ~ 62%
Wide-angle systems with Fresnel lenses
Potential problems• Size – lens diameter is about 2 x
effective aperture for most designs• Technical issues concerning lens
construction (stability, glare, stray light, ...)
• Number of pixels – O(105) for 60 degr. fov
D.J. Lamb(OWL)
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Uhuru
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GLAST
Cherenkovtelescopes
X-Ray (keV)