Active Galactic Nuclei at TeV energies

1. Active Galactic Nuclei at TeV energies Hélène Sol LUTH, Observatoire de Paris MG12, UNESCO, Paris, July 2009

2. 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

3. 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

4. VERITAS MAGIC Ground-based TeV gamma-ray astronomy : Atmospheric Cherenkov Telescopes CANGAROO HESS

5. Typical example of VHE spectrum and SED VHE power-law, two-peaked SED, variability

6. leptonic acceleration Synchrotron  (eV-keV) e- (TeV) B  (TeV) Inverse Compton SSC or EC  (eV) hadronic acceleration p+ (>>TeV) IC  (TeV) 0 matter (or photons) E2 dN/dE - 0decay + energy E Basic scenarios for SED modeling (adapted from De Lotto, 2009)

7. TeV emitting zone(s) : in a jet or outflow with relativistic bulk motion Radio galaxies FR I , FR II Radio quasars BL Lac (HBL, LBL) andFSRQ Relativistic jet Strong relativistic boosting (~ factor δ4) favours detection of blazars/BL Lac

8. emitting plasma V ~ c Blazars : HBL, IBL, LBL, FSRQ … from J.P. Lenain

9. 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 FSRQ 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 …

10. 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

11. Now 28 AGN from J. Hinton

14. 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

15. Fluxes in Crab units versus redshift (high and low states) 8 4 0.2 0.4 z 0.4 0.2 Distribution versus redshift at lower fluxes  No clear trend with distance 0.2 0.4 z

16. 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.

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

18. γVHEγEBL e+ e- , dominant absorption, where σ is maximal for ε ~ (500 GeV / E) eV i.e. opt/IR photons for VHE gamma-rays Photon spectrum : Φobs (E,z) = e - τγ (E,z)Φem τγ(E,z) = optical depth Гobs(z) ~ Гem + τγ(E,z) ~ E-2

19. Observed spectral index Γ versus redshift Domain expected for Гem ~ 2.4 and EBL model from Stecker et al, 92 Γobs Domain expected for a photon-axion oscillation scenario z from De Angelis et al, 2009

20. Observed spectral index Γ versus redshift(July 2009) Γobs z No apparent increase of Гwith z above z > 0.2 Does not follow the expected standard trend. Even below « Axion-Like-Particle scenario » …?

21. 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

22. Flux power-law Ecut log(E) Flux power-law G1 G2 Ebreak log(E) Curvature in VHE spectra In several cases, spectra are known to deviate from a power law 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 ? PKS2155 Big Flare HESS Curvature also seen in the Galactic Center VHE source, possibly the nearest weak « AGN ».

23. 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 the Crab flux for 61 untargeted AGN Study by Herr, Hofmann, HESS paper in preparation observed area

24. 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

25. The HBL PKS 2155-304Quiescent state Various hadronic and leptonic models can often fit present available spectra of HBLin stationary state

26. The HBL PKS 2155-304Extremely active state 2nd flare 1st big flare 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

27. 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 Katarzynski et al, 2008)

28. The HBL PKS 2155-304Extremely active state : 2nd flare from A. Zech, ICRC, 2009

29. 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)

30. Variation of VHE spectral index during the 2nd flare highly correlated with VHE flux variation : - Harder spectra for higher flux Variation of VHE spectral index over years highly correlated with VHE flux variation but : - Harder spectra for higher flux during active state - Steeper spectra for higher flux during quiescent state.

31. The blazars W Comae (IBL) and BL Lac (LBL) BL Lac • High variability and • broad band spectra • Stringent necessity of coordinated HE and multi-lambda monitoring to constrain SED and evolution.

32. The FSRQ 3C279 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

33. Strategy of multi-lambda triggers Optical ToO trigger ToO trigger KVA optical telescope at la Palma Mkn 180 z = 0.045 MAGIC 1ES 1011+496 z = 0.212 S5 0716+714 z = 0.31 S5 0716+714MAGIC PRELIMINARY Significance 6.8 σ

34. 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

35. 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 source SED 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

36. 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

37. The radiogalaxy M87 TeV radio • 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 HST X

38. Monitoring of the core of M87 by VLBA at 43 GHz every 5 days Explore sub-mas scale (0.21mas x 0.43mas) to probe the jet collimation region Significant rise at in core at the time of VHE activity and enhanced emission along inner jet (Wagner et al, ICRC, 2009; Science, 2009) 0.5 mas = 0.04 pc = 140 Rs

39. 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)

40. The radiogalaxy Cen A • 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

41. Cen A 5σ

42. Origin of the VHE emission ? Compatible with radio core and inner kpc jets of Cen A • Possible VHE zones ? • BH magnetosphere • base of jets • jets and inner lobes • pair halo in host galaxy • Link to UHECR ? Ex : SSC emission from jet formation zone (Lenain et al, 2008)

43. Starburst galaxies and ULIRG : TeV detection still difficult but flux upper limits approaching theoretical predictions • Recent detection of M82 by VERITAS and of NGC 253 by HESS (ICRC, July 11th, 2009) AGN-starburst connection ? Chandra HST Collision, star formation, superwind … Arp 220 (ULIRG) NGC 253

44. 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

45. Size constraints from variability M87 • 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. • Challengeto 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 PKS2155-304 Mrk 501

46. Applying simple SSC scenarios to SED and light curves Assuming given a particle distribution with broken power-law : Ne(γ) = K1γ-n1 for γmin < γ<γbreak andNe(γ) = K2γ-n2 forγbreak <γ <γmax 1) High energy SED of stationary states of HBL can be well reproducedby 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 statesof 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.

47. Domain of parameters ? Index of particle distribution can significantly differ from canonical values 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 scattering OK for n1. For n2 : oblique relativistic shocks ? more inefficient. (needs tacc < tcool)

48. 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.

50. ----- 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 [γe2hν0, 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-) + n2π0( 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 !)