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

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”

Active Galactic Nuclei

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

Instrinsic properties

X-ray emission in low state: sourcein quiescent state

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

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

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

Spectra & ExtragalacticBackgroundLight

1 ES 1101 = 2.9±0.2

H 2356 (x 0.1) = 3.1±0.2

EBL

Source spectrum = 1.5

Preliminary

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

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

Spectra & ExtragalacticBackgroundLight

lower limitsfrom galaxy

counts

measure-ments upper

limits

Reference shape

HESS limits

X

XEBL resolvedUniverse more

transparent

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

Poster Advert

• Optical spectroscopy of PKS 2155-304

• Looking for emission lines → found evidence for H alpha

• Found evidence of unusual variability