1. H.E.S.S. observations of AGN “probing extreme environments with extreme radiation” Anthony Brown National Astronomy Meeting, 2006 H.E.S.S. = High Energy Stereoscopic System

3. Stereoscopic Method ~ 10 km Air shower ~ 1 o Cherenkov light ~ 120 m → Angular res better than 0.1 o event-by-event + Field of View → source location better than 20”

4. Active Galactic Nuclei

5. Table of known AGN at VHE SourceRedshiftType M870.004FR I Mkn 4210.031BL Lac Mkn 5010.034BL Lac 1ES 2344+5140.044BL Lac 1ES 1959+6500.047BL Lac PKS 2005-4890.071BL Lac PKS 2155-3040.117BL Lac H1426+4280.129BL Lac H2356-3090.165BL Lac 1ES 1218+3040.182BL Lac 1ES 1101-2320.186BL Lac PG 1553+1130.25 < z < 0.74BL Lac

6. Instrinsic properties X-ray emission in low state: source in quiescent state

7. Some interesting constraints Assuming single zone model (both VHE & X-ray emission originate from same ‘blob’ of relativistic material) Flux Variability (light cross time) – R ≤ cΔtδ / (1+z) Taking Δt ≈ 2000 s → Rδ -1 ≤ 5.4 10 13 cm Pair Production Opacity (Optical depth for VHE photons ≤ 1) –Equation 3.7 from Dondi & Ghisellini 1995 R -1 δ -6.4 ≤ 5.6 10 -24 cm -1 → δ ≥ 19.3

8. More constraints Consider the syn. radiation –Mean observed energy of an electron, γ e Take = 10keV → Bδ γ e 2 = 1.1x10 12 G SSC scenario, the same electrons Comptonize photons to VHE regime –δ γ e m e c 2 / (1+z) ≥ 3 TeV → Bδ -1 ≤ 0.03 G → B ≤ 0.58 G → γ e ≥ 1.3 x 10 6

9. VHE spectra & the EBL VHE photons are absorbed through pair production  VHE  EBL → e ± Attentuates the high energy tail of the VHE spectra until its cut off Assume a slope, compare to observed slope and deduce the amount of absorption Extra-galactic background (EBL), carries unique cosmological information about evolution and formation epochs of galaxies Poorly understood and difficult to measure directly

10. Spectra & E xtragalactic B ackground L ight 1 ES 1101  = 2.9±0.2 H 2356 (x 0.1)  = 3.1±0.2 EBL Source spectrum  = 1.5 Preliminary

11. Spectra & E xtragalactic B ackground L ight 1 ES 1101  = 2.9±0.2 H 2356 (x 0.1)  = 3.1±0.2 too much EBL Source spectrum  Upper limit on EBL Preliminary

12. Spectra & E xtragalactic B ackground L ight 1 ES 1101  = 2.9±0.2 H 2356 (x 0.1)  = 3.1±0.2 too much EBL Not really a solution: add huge amount of UV photons to EBL  problems with source energetics, X-ray/gamma- ray SED ratio UV EBL Preliminary

13. Spectra & E xtragalactic B ackground L ight lower limits from galaxy counts measure- ments upper limits Reference shape HESS limits X X EBL resolved Universe more transparent

14. Summary Catalogue of known extra-galactic VHE sources is growing at an unprecedented rate Able to use VHE spectra to probe the EBL → the universe is more transparent to VHE photons H.E.S.S. II will lower the energy threshold allowing us to increase the catalogue further and probe the EBL better

15. Poster Advert Optical spectroscopy of PKS 2155-304 Looking for emission lines → found evidence for H alpha Found evidence of unusual variability