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H.E.S.S. observations of AGN “probing extreme environments with
extreme radiation”
Anthony BrownNational Astronomy Meeting, 2006
H.E.S.S. = High Energy Stereoscopic System
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Stereoscopic Method
~ 10 kmAir shower
~ 1o
Che
renk
ov li
ght
~ 120 m
→ Angular res better than 0.1o event-by-event
+
Field of View
→ source location better than 20”
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Active Galactic Nuclei
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Table of known AGN at VHESource Redshift TypeM87 0.004 FR IMkn 421 0.031 BL LacMkn 501 0.034 BL Lac1ES 2344+514 0.044 BL Lac1ES 1959+650 0.047 BL LacPKS 2005-489 0.071 BL LacPKS 2155-304 0.117 BL LacH1426+428 0.129 BL LacH2356-309 0.165 BL Lac1ES 1218+304 0.182 BL Lac1ES 1101-232 0.186 BL LacPG 1553+113 0.25 < z < 0.74 BL Lac
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Instrinsic properties
X-ray emission in low state: sourcein quiescent state
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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
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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
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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
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Spectra & ExtragalacticBackgroundLight
1 ES 1101 = 2.9±0.2
H 2356 (x 0.1) = 3.1±0.2
EBL
Source spectrum = 1.5
Preliminary
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Spectra & ExtragalacticBackgroundLight
1 ES 1101 = 2.9±0.2
H 2356 (x 0.1) = 3.1±0.2
too muchEBL
Source spectrum
Upper limit on EBL
Preliminary
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Spectra & ExtragalacticBackgroundLight
1 ES 1101 = 2.9±0.2
H 2356 (x 0.1) = 3.1±0.2
too muchEBL
Not really a solution:
add huge amountof UV photonsto EBL
problems withsource energetics,X-ray/gamma-raySED ratio
UVEBL
Preliminary
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Spectra & ExtragalacticBackgroundLight
lower limitsfrom galaxy
counts
measure-ments upper
limits
Reference shape
HESS limits
X
XEBL resolvedUniverse more
transparent
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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
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Poster Advert
• Optical spectroscopy of PKS 2155-304
• Looking for emission lines → found evidence for H alpha
• Found evidence of unusual variability