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Tuning in to Nature’s Tevatrons

Stella Bradbury, University of Leeds

• TeV-ray Astronomy

• the atmospheric Cherenkov technique

• the Whipple 10m telescope

• ACTIVE galactic Nuclei

• the site of TeV -ray emission?

• multiwavelength clues to the emission mechanism

• the next 5 years

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• < 50 GeV e-e+ pairs produced in satellite volume and trapped

• > 250 GeV sample the Cherenkov light pool at ground calorimetric measurement

TECHNIQUE

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Background Rejection

• -ray generates “airshower” through e+e- pair production & bremsstrahlung

Simulated Cherenkov photon distribution at ground:

-ray proton

• cosmic ray and air nuclei collide 0 +

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• a single 12.5 mm Ø photomultiplier pixel subtends 0.12º

• width of a typical -ray Cherenkov image is 0.3º

• use a cluster trigger

-ray ? nucleon? local muon ?

The Whipple 10m reflector

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• humidity• unexpected loads!• temperature cycle• lightning

Nature’s Challenges• field stars, night sky light• moving targets!

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HST image of M87

Those detected at TeV energies are BL Lac Objects: • rapid optical variability + flat spectrum radio emission

• virtually featureless optical continuum - emission lines swamped by relativistically beamed radiation from jet?

Active Galactic Nuclei

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• Doppler beaming enhances luminosity Lobserved = p Lintrinsic where = [(1 - cos )]-1

• optical depth for TeV + UV/optical e± must be less than 1 limits ratio of rest frame luminosity to size of emission region

• 9 was derived from flare on right (Gaidos et al. Nature 383, 319)

-ray Emission Site?

Sub-hour TeV-ray flares - count rate more than doubled

causality requires time for disturbance to propagate

emission region only ~ size of solar system

plasma “blob” in jet? Whipple Telescope - Mkn 421

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-ray Production Mechanism?

• Synchrotron Self-Compton e- + synch e- + -ray

• External Inverse Compton e- + external e- + -ray

• photo-meson production p+ + 0, ± -rays,

e ± , n,

Assume emission region is associated with shock accelerated particles, then pick any combination of :

9Markarian 501 April ‘97Multiwavelength Observations

• might expect simultaneous TeV -ray and X-ray flares if due to the same e- population (Self-Compton)

• increase in e- density increase in ratio of Self-Compton to synchrotron emission?

• in External IC model -ray & optical flares could come from different sites time lag?

• proton induced cascade outbursts?

4.2

2.6

1.7

1.1

10Markarian 501 Spectral Energy Distribution

• Power in X-rays & -rays very similar - both much greater in 1997 • Synchrotron peak shifted from 1 keV to 100 keV during outburst

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TeV -ray detection of Active Galactic Nuclei 600 million light years away limits on IR background density 10 more restrictive than direct satellite measurement in 4 - 50m range

Possible IR contributors:

• early star formation

• Very Massive Objects (dark matter candidates)

• heavy light + IR

for 0.05 eV < m< 1 eV

-ray Horizon

(Biller et al. Phys. Rev. Lett. 80, 2992)

Extragalactic Infrared Background : may cut-off -ray signal from

distant sources as -ray + target e- + e+

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1ES1959+65 flared on 17/05/02

It was predicted to emit TeV -rays as it is bright in X-ray and radio

The Next 5 Years~ 70 Active Galactic Nuclei are known to emit -rays above 100 MeV

6 have been detected at ~ 1 TeV

We now have a basis for targeting particular objects

We need more sensitive instruments to expand the TeV catalogue

13The MAGIC Telescope on La Palma

Imaging telescope with a single 17m diameter dish.

Energy threshold < 20 GeV with future hybrid photodetectors

Operational early 2003?

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The VERITAS array of 12m telescopes in Arizona:

• 1st telescope on-line 2003

• 7 by end of 2006

• uses stereoscopic technique - viewing Cherenkov flash from different angles to improve background rejection

• energy resolution E/E ~ 15%

Same philosophy as H.E.S.S. and CANGAROO III - under construction

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Flux Sensitivity: bridging the gap between Cherenkov telescopes and satellites will allow cross-calibration and full coverage of spectrum

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• full coverage of the -ray sky from 100 MeV to > 10 TeV will be achieved in the next 5 years

• Cherenkov telescopes will exploit new technology common to particle and astroparticle physics e.g. hybrid photo-detectors, analogue optical fibre signal transmission

• based on known source spectra at longer wavelengths expect VERITAS to detect 30 BL Lac objects

• better source statistics determine emission mechanism and hence contribution to the flux of charged cosmic rays

• as distant-ray sources, Active Galactic Nuclei can be useful probes of the infrared background from the early universe

In Conclusion...