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H.E.S.S. observations of AGN “probing extreme environments with extreme radiation”

H.E.S.S. observations of AGN “probing extreme environments with extreme radiation”. H.E.S.S. = High Energy Stereoscopic System. Anthony Brown National Astronomy Meeting, 2006. Air shower. ~ 10 km. ~ 1 o. Cherenkov light. +. ~ 120 m. Field of View. Stereoscopic Method.

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H.E.S.S. observations of AGN “probing extreme environments with extreme radiation”

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

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

  3. Active Galactic Nuclei

  4. Table of known AGN at VHE SourceRedshiftType M87 0.004 FR I Mkn 421 0.031 BL Lac Mkn 501 0.034 BL Lac 1ES 2344+514 0.044 BL Lac 1ES 1959+650 0.047 BL Lac PKS 2005-489 0.071 BL Lac PKS 2155-304 0.117 BL Lac H1426+428 0.129 BL Lac H2356-309 0.165 BL Lac 1ES 1218+304 0.182 BL Lac 1ES 1101-232 0.186 BL Lac PG 1553+113 0.25 < z < 0.74 BL Lac

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

  6. 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 1013 cm • Pair Production Opacity (Optical depth for VHE photons ≤ 1) • Equation 3.7 from Dondi & Ghisellini 1995 • R-1δ-6.4 ≤ 5.6 10-24 cm-1 →δ ≥ 19.3

  7. More constraints • Consider the syn. radiation • Mean observed energy <E> of an electron, γe Take <E> = 10keV → Bδ γe2 = 1.1x1012 G • SSC scenario, the same electrons Comptonize photons to VHE regime • δ γemec2 / (1+z) ≥ 3 TeV → Bδ-1 ≤ 0.03 G → B ≤ 0.58 G → γe ≥ 1.3 x 106

  8. VHE spectra & the EBL • VHE photons are absorbed through pair production gVHEgEBL → 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

  9. Source spectrum G = 1.5 EBL Spectra &ExtragalacticBackgroundLight 1 ES 1101 G = 2.9±0.2 H 2356 (x 0.1) G = 3.1±0.2 Preliminary

  10. Spectra &ExtragalacticBackgroundLight Source spectrum  Upper limit on EBL too much EBL 1 ES 1101 G = 2.9±0.2 H 2356 (x 0.1) G = 3.1±0.2 Preliminary

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

  12. Reference shape HESS limits Spectra &ExtragalacticBackgroundLight • EBL resolved • Universe more • transparent X measure- ments upper limits X lower limits from galaxy counts

  13. 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

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

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