active galactic nuclei at tev energies

Active Galactic Nuclei at TeV energies

Hélène SolLUTH, Observatoire de Paris MG12, UNESCO, Paris, July 2009

Outline

• Introduction• The sample of AGN detected at TeV• Similarities and discrepancies : some

specific cases of blazars• An emerging family of TeV AGN : the

radiogalaxies M87 and Cen A• Remarks on SSC scenarios

Ground-based TeV gamma-ray astronomy :Atmospheric Cherenkov Telescopes

HESS

CANGAROO

VERITAS MAGIC

Typical example of Typical example of VHE spectrum and SEDVHE spectrum and SED

VHE power-law, two-peaked SED,variability

E2 dN

/dE

energy E

0decay

IC

e- (TeV) Synchrotron (eV-keV)

(TeV) Inverse Compton

SSC or EC (eV)

B

leptonic acceleration

-

0

+

(TeV)

p+ (>>TeV)

matter(or photons)

hadronic acceleration

Basic scenarios for SED modeling

(adapted from De Lotto, 2009)

Strong relativistic boosting (~ factor δ4) favours detection of blazars/BL Lac

Radio galaxiesFR I , FR II BL Lac

(HBL, LBL) and FSRQ

Radioquasars

TeV emitting zone(s) : in a jet or outflow with relativistic bulk motion

Relativistic jet

emitting plasma V ~ c

from J.P. Lenain

Blazars : HBL, IBL, LBL, FSRQ …

The blazar sequence

Average SED for a sample of 126 blazars binned according to Lradio

(Fossati et al, 1998) :A continuous sequence from the most powerful FSRQ, through LBL and IBL,to the weaker HBL

A proposed unifying scheme based on leptonic models with SSC and external Compton (EC) emission, the importance of EC decreasing from FSRQ to HBL (Ghisellini et al, 1998)

However : growing evidences that not all objects fit the trend (2003-2009; Padovani et al) need of a revisited sequence …

FSRQ

HBL

Outline




from J. Hinton

Now28 AGN

Cen A 0.00183 FR I 0.008 2.7 -- Aharonian et al, 2009

M 87 0.004233 FR I 0.014-0.033

2.2 day Aharonian et al, 2004

3C 66B ? 0.0215 FR I 0.027 ? 3.1 ? year ? Aliu et al, 2009

Mkn 421 0.0300 HBL 0.3-10 2.1 mins Punch et al, 1992

Mkn 501 0.03364 HBL 0.06-11 2.7-1.9 mins Quinn et al, 1996

1ES 2344+514 0.044 HBL 0.08-0.5 2.8-2.5 day Catanese et al, 1998

Mkn 180 0.046 HBL 0.11 3.3 -- Albert et al, 2006

1ES 1959+650 0.048 HBL 0.05-2.2 3.2-2.5 week Nishiyama, 1999

BL Lac 0.0688 LBL 0.03 3.6 -- Albert et al, 2007

PKS 0548-322 0.068998 HBL 0.014 2.8 -- Aharonian et al, 2007

PKS 2005-489 0.071022 HBL 0.025 4.0 year Aharonian et al, 2005

RGB J0152+017 0.080 HBL 0.02 2.95 month Aharonian et al, 2008

W Comae 0.1020 IBL 0.09 3.8 day Acciari et al, 2008

PKS 2155-304 0.117 HBL 0.1-15 3.2-3.9 mins Chadwick et al, 1999

source redshift type flux in Crab unit

photon index Γ

shortest variability

discovery paper

RGB J0710+591 0.125 HBL 0.02 2.8 -- Ong, 2009

H 1426+428 0.129 HBL 0.19 3.5 days Horan et al, 2002

1ES 0806+524 0.138 HBL 0.018 3.6 -- Acciari et al, 2009

1ES 0229+200 0.1396 HBL 0.018 2.5 -- Aharonian et al, 2007

PKS 1424+240 0.16 ? IBL 4.6 Ong, 2009

H 2356-309 0.1671 HBL 0.023 3.09 month Aharonian et al, 2006

1ES 1218+304 0.182 HBL 0.08 3 -- Albert et al, 2006

1ES 1101-232 0.186 HBL 0.023 2.94 year Aharonian et al, 2006

1ES 0347-121 0.1880 HBL 0.02 3.1 year Aharonian et al, 2007

1ES 1011+496 0.212 HBL 0.07 4 -- Albert et al, 2007

S5 0716+714 0.31 L/ IBL 3.5 months Teshima et al, 2008

3C 66A 0.444 ? IBL 0.06 4.1 day Acciari et al, 2009

3C 279 0.5362 FSRQ 4.11 day Albert et al, 2008

PG 1553+113 >0.25 HBL 0.034 4.5 year Aharonian et al, 2006

source redshift type flux in Crab unit

photon index Γ

shortest variability

discovery paper

The VHE sample (July 2009)

• 25 blazars : - 19 HBL (High-frequency peaked BL Lac) - 4 IBL and 1 LBL (Intermediate and Low-frequency peaked BL Lac) - 1 FSRQ (Flat Spectrum Radio Quasar)

• 2 (or 3) radio galaxies (+ 1 G.C. ?)

Number of TeV sources per type : highly peculiar !

Redshifts : from 0.00183 to 0.536 (+ 3 uncertain)

TeV variability : already seen in 18 sources (despite poor temporal coverage) « Shortest observed time scales » minutes : 3 sources (flares) day : 6 sources week : 1 source month : 3 sources year : 5 sources

flux-z

0

2000

4000

6000

8000

10000

12000

0 10 20 30 40 50

flux-z

0

100

200

300

400

500

600

0 10 20 30 40 50

0.2 0.4 z

0.2 0.4 z

8

4

0.4

0.2

Fluxes in Crab units versus redshift(high and low states)

Distribution versus redshiftat lower fluxes

No clear trend with distance

A highly biaised sample• Current dynamical range of ACT ~ 5000

• Active states of TeV AGN : x 10, x 200• Doppler boosting : x 104, x 106

Entirely biaised towards strong boosting : 100% of boosted sources (except possibly FR I ?)

Biaised towards active states : due to sensitivity limits and to strategy of observation (VHE and multi-lambda alerts)

Possibly biaised towards low redshifts : due to strategy of observation, to optimize detection probability despite EBL absorption effect.

Absorption of VHE gamma-rays by the EBL, Extragalactic Background Light

γVHE γEBL e+ e- , dominant absorption,

where σ is maximal for ε ~ (500 GeV / E) eVi.e. opt/IR photons for VHE gamma-rays

~ E-2

Photon spectrum :Φobs (E,z) = e - τ

γ (E,z) Φem

τγ(E,z) = optical depth Гobs(z) ~ Гem + τγ(E,z)

Observed spectral index Γ versus redshift

Γobs

z

from De Angelis et al, 2009

Domain expected for Гem ~ 2.4 andEBL model from Stecker et al, 92

Domain expectedfor a photon-axionoscillation scenario

Observed spectral index Γ versus redshift (July 2009)

Γobs

zNo apparent increase of Г with z above z > 0.2Does not follow the expected standard trend.Even below « Axion-Like-Particle scenario » …?

Observed spectral index Γ versus redshift (July 2009)

Hardening of emitted spectral with redshift : possible if VHE spectrum hardens with increasing fluxes (as observed in Mkn 421, Mkn 501, and PKS2155-304), and if high states are more often detected at high z but IC bump enters more into the Klein-Nishina regime and Гem remains basically unchanged

Stronger relativistic shocks at high z would imply smaller index Гem but why stronger shocks at high z ?

Evolution of moderate photon absorption inside the source change of Гem

but why absorption evolves with z ?

Entirely different emission scenarios for high-z « bright » sources compare to low-z « fainter » sources ? but it is Doppler boosting and activity level which dominate the apparent fluxes, not the z !

Open question, requires further analyses

Ecut

power-law

log(E)

Flux

Ebreak

power-law1

2

log(E)

Flux

In several cases, spectra are known to deviate from a powerlaw in their high energy part.

This is better seen during active states : Mkn 421, Mkn 501,PKS 2155-304

Origin not yet identifed (γmax , KN, EBL …)

Such effects come to limit the validity of any pure power-law studies. Biais also due to observed spectral ranges ?

Curvature in VHE spectra

Curvature also seen in the Galactic Center VHE source, possibly the nearest weak « AGN ».

PKS2155Big FlareHESS

The VHE AGN « non-sample »

Targeted sources : Several tens of upper limits already obtained by present ACT. Sometimes constraining the SED (ex: 3C454.3).

Untargeted sources : in the field of view of observed sources and galactic plane survey

694 nearby AGN < 100Mpc

Example with HESS (2004-2008) :

Upper limits of 1-10% of theCrab flux for 61 untargeted AGN

Study by Herr, Hofmann, HESS paper in preparationobserved area

Outline




The HBL PKS 2155-304Quiescent state

Various hadronic and leptonic models can often fit present available spectra of HBL in stationary state

Monitoring an extraordinary active state of PKS 2155-304 in 2006, detected by HESS + multi-lambda campaign.

Variability down to minute time scale Emitting zone smaller than Rg or very high bulk Lorentz factor

1st big flare2nd flare

The HBL PKS 2155-304Extremely active state

(from Katarzynski et al, 2008)

The HBL PKS 2155-304Extremely active state : 1st flare

Example of modeling light curves and SED by time dependent SSC scenario, with 5 compact components in jet with slightly different parameters C2-6

+ a more extended slowly evolving component C1

from A. Zech, ICRC, 2009

The HBL PKS 2155-304Extremely active state : 2nd flare

Fit of the 2nd flare by SSC time dependent modeling :

Reproduces light curves and spectra in X and gamma rays

(from J.P. Lenain)

The HBL PKS 2155-304Extremely active state : 2nd flare

Variation of VHE spectral indexduring the 2nd flare highly correlatedwith VHE flux variation :- Harder spectra for higher flux

Variation of VHE spectral indexover years highly correlatedwith VHE flux variation but :- Harder spectra for higher flux during active state- Steeper spectra for higher flux during quiescent state.

The blazars W Comae (IBL) and BL Lac (LBL)

High variability and broad band spectra Stringent necessity of coordinated HE and multi-lambda monitoring to constrain SED and evolution.

BL Lac

Flat spectrum radio-quasar at z=0.536

Brightest EGRET source. Highly variable, fast variability (~6 hours)

MAGIC observed it in 2006 for 9.7 hours in 10 nights : clear detection

First FSRQ in TeV gamma-rays and highest z

The FSRQ 3C279

Strategy of multi-lambda triggersStrategy of multi-lambda triggers

Mkn 180z = 0.045

Optical ToO trigger

ToO trigger 1ES 1011+496 z = 0.212

S5 0716+714MAGIC PRELIMINARY

Significance 6.8 σ

S5 0716+714 z = 0.31

KVA optical telescope at la Palma

MAGIC

The region of 3C66 A and B A : an IBL B : a radiogalaxy

MAGIC VERITAS

2 sources separated by only 0.12 ° :- MAGIC favours a detection of 3C66B in 2007, and excludes 3C66A- VERITAS favours a detection of 3C66A in 2008, and excludes 3C66B

3C66A : Detection of a flare by VERITAS and Fermi

Simultaneous observations of the VHE flare with Fermi/LAT(Reyes et al, ICRC, July 2009)

Firmly establishes 3C 66A as VHE sourceSED is better fitted by a SSC+EC scenario

Either it was 3C66A all along, or both sources A and B varied in opposite way between 2007 and 2008

Outline




The radiogalaxy M87

M87 : TeV day variability 3 possible emitting zones :- The peculiar knot HST-1 at ~ 65 pc from the nucleus- The inner VLBI jet- The central core and the black hole environment

TeV radio

HST

X

Monitoring of the core ofM87 by VLBA at 43 GHzevery 5 days

Explore sub-mas scale(0.21mas x 0.43mas)to probe the jet collimationregion

Significant rise at in core at the time of VHE activityand enhanced emission along inner jet (Wagner et al, ICRC, 2009;Science, 2009)

0.5 mas = 0.04 pc = 140 Rs

X-ray light curve of HST-1 obtained by Chandra in 2008 does not follow the TeV one

Correlated core emission (radio, X and VHE) favours scenarios with TeV emission from inner jet or central core.

Inner jet : Spine-layer SSC+EC scenario

(Tavecchio, Ghisellini, 2008)Multi-blob SSC scenario at 100Rs

(Lenain et al, 2008)Core : Acceleration in Black Hole

magnetosphere (Neronov, Aharonian, 2007; Rieger,

Aharonian, 2008; Istomin, Sol, 2009)

• Recent discovery of VHE emission from Cen A with HESS (Aharonian et al, 2009)

• Together with M87, establishes radio galaxies as a new class of VHE emitters

• Three different types of AGN now detected at VHE (blazars, radiogalaxies, and weak AGN as Galactic Center)

is VHE emission a general feature of AGN (and SMBH) ?

Richness of the extragalactic space at VHE, to further explore with MAGIC II, HESS II and later on with CTA / AGIS

The radiogalaxy Cen A

Cen A

5σ

Origin of the VHE emission ? Compatible with radio core and inner kpc jets of Cen A

Ex : SSC emission from jet formation zone(Lenain et al, 2008)

Possible VHE zones ?- BH magnetosphere- base of jets- jets and inner lobes- pair halo in host galaxy

Link to UHECR ?

Starburst galaxies and ULIRG : TeV detection still difficult but flux upper limits approaching theoretical predictions

Arp 220 (ULIRG) NGC 253

HST

Chandra

Collision,star formation,superwind …

Recent detection of M82 by VERITAS and of NGC 253 by HESS (ICRC, July 11th, 2009)

AGN-starburst connection ?

Outline




Size constraints from variability

• TeV variability of M87, PKS2155-304, Mrk501 … requires very small emitting zone, of the order of a few rg or even less (even for high δ) under causality argument.

• Challenge to efficiently accelerate particles in such small zones (core around BH, or very inner jet).

• Fermi processes in shocks and turbulence widely invoked

• Alternatives : magnetic reconnection, direct electric forces, centrifugal force

M87

PKS2155-304

Mrk 501

Applying simple SSC scenarios to SED and light curves

Assuming given a particle distribution with broken power-law :

Ne(γ) = K1 γ -n1 for γmin < γ < γbreak and Ne(γ) = K2 γ -n2 for γbreak < γ < γmax

1) High energy SED of stationary states of HBL can be well reproduced by one-zone scenario with typical parameters : B ~ 0.1 – 0.2 G Size of emitting zone R ~ 1014 – 1016 cm Doppler factor ~ 20 – 50 Density factor K1 ~ 103 – 104 cm-3

Index n1 ~ 2 (1.5 – 2.5) Index n2 ~ 2.7 – 4.7

2) Active variable states of HBL can be reproduced by SSC time dependent models with at least two types of SSC emitting zones (compact components + an extended slowly varying one)

3) IBL, LBL and FSRQ : SSC can work, but EC contribution helps.

4) Radiogalaxies : need SSC multi zone models. Seems to possibly work but still several options. Open issue. Different situation for M87 and Cen A.

Domain of parameters ?Index of particle distribution can significantly differ from canonicalvalues in relativistic shocks

(Stecker et al, 2007)(Baring, 2004)

Spectral index sensitive to compression ratio r, upstream flow, obliquity of the shock, nature of scattering, strength of turbulence and anisotropy of the diffusion.

Parallel ultrarelativistic shocks, index 2.25 : corresponds to r ~ 3 and small scatteringOK for n1. For n2 : oblique relativistic shocks ? more inefficient. (needs tacc < tcool)

Further observing AGN at VHE

• TeV observations disentangle non-thermal effects from thermal ones possibly present at others wavelengths provide a simplified view of the physics at the highest energies.

• Explore variability at the shortest time scales jet physics, particle acceleration and radiation processes physics of supermassive Black Hole environnement; accretion physics build a sample of sources at different redshifts (evolution, validity of

Lorentz invariance, analysis of the EBL).

• Gather samples of different AGN types to allow statistical studies for unification schemes (are HBL highly beamed LBL ?). Check the ‘blazar sequence’, probe the quiescent states

• Look for VHE emission from « dormant » BH or « dead » quasars. Studies of AGN and SMBH evolution, AGN feedback and co-evolution with host-galaxies.

• Importance of multi-messenger and multi-lambda analyses.

----- Complements -----A limited number of VHE γ-rays emission mechanisms : 2 (+1)

• Leptonic scenarios : synchrotron and Inverse-Compton (IC) radiation of relativistic electrons (positrons)

e + B e + B + γ, in magnetic field B (also X-rays) e + γ0 e + γ, with hν ~ min [γe

2hν0, γemec2], IC on synchrotron emission (SSC) or on external photon field (EC)

• Hadronic scenarios : Interaction of energetic protons (CR) with local gas and radiation backgrounds

p + p N + N + n1(π+ + π-) + n2 π0 ( N = p or n) p + γ p + π0, n + π+, others (for γphν > mπc2); or p + e+ + e- (for γphν > 2mec2)

Then decay π0 2 γ produce VHE photons with Eγ ~ Eπ /2 ~ 10% Ep,i

+ Decay pions muons secondary electrons and neutrinos (also X-rays) Alternatives : curvature and synchrotron radiation of VHE protons.

• (Annihilation of Dark Matter particles : predictions of supersymmetric theories, Kaluza-Klein scenarios open questions to explore. No detection yet. A great challenge, but not yet granted !)

EBL : optical depth

3C279 and the gamma-ray horizon3C279 and the gamma-ray horizon

Recent inner jet scenarios(adapted from standard TeV models for HBL)

• Spine-layer scenario (Tavecchio, Ghisellini, 2008)

Assumes a structured jet with two components

- a fast core emitting radio to X-ray/GeV photons, surrounded by

- a slower sheet emitting TeV photons, which can dominate at large viewing angles

SSC + EC (spine/layer) emissionTurbulent acceleration in layer ?2 sets of free parameters, but it reproduces

SED of LBL at small angles

B ~1G (0.3G) in spine (layer)Г ~ 12 (4) in spine (layer)

GLASTlimit 1yr

TeV high state in 2005

‘Low state’B ~ 10 mG at 100 Rg

Г ~10θ ~ 15°

M87 : Differential Doppler boosting in emitting zoneSSC emission. Multi-blob scenario (Lenain et al)Electron distribution described by a broken power law

Fitting low and high TeV states with inner jet parameters reasonably well consistent with values from GRMHD simulations of McKinney, 2006 (used as initial constraints).

SNR SgrA East(90 cm)

van Eldik et al., ICRC (2007)

Point-like source: intrisic size < 1.2’ (≈ 2.9 pc) at the 99% C.L.

position:

l=359°56’41.1’’± 6.4’’± 6’’ b=-0°2’39.2’’ ± 5.9’’ ±6’’centroid emission located at 7’’ ± 12’’ from Sgr A*Sgr A East excluded at the 7C.L.G359.95-0.04 still inside error bars (8.7’’ from Sgr A*)

from M. Vivier

Galactic Center

MWL source in the central parsecs of our Galaxy: emitting from radio to TeV -raysFrom radio to X-rays: highly variable, originates from the SMBH Sgr A*Origin of the hard X-rays/Tev -rays?

Radio

IR

X-rays

TeV

Sgr A* ?

active galactic nuclei at tev energies

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