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1
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
2
• < 50 GeV e-e+ pairs produced in satellite volume and trapped
• > 250 GeV sample the Cherenkov light pool at ground calorimetric measurement
TECHNIQUE
3
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 +
4
• 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
5
• 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
7
• 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
8
-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+
12
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?
14
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...