1. 1 Tuning in to Nature’s Tevatrons Stella Bradbury, University of Leeds T e V  -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. 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. 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. 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. 5 humidity unexpected loads! temperature cycle lightning Nature’s Challenges field stars, night sky light moving targets!

6. 6 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. 7 Doppler beaming enhances luminosity L observed =  p L intrinsic 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. 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 :

9. 9 Markarian 501 April ‘97 Multiwavelength 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

10. 10 Markarian 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

11. 11 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 - 50  m 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. 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

13. 13 The 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. 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

15. 15 Flux Sensitivity: bridging the gap between Cherenkov telescopes and satellites will allow cross-calibration and full coverage of spectrum

16. 16 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...