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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