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The Origin of X-Ray Emission in the Nuclei of Radio Galaxies

Dan Evans (Harvard) -with- Diana Worrall (U. Bristol), Martin Hardcastle (U. Herts), Ralph Kraft (SAO), Mark Birkinshaw (U. Bristol), Judith Croston (U. Herts). The Origin of X-Ray Emission in the Nuclei of Radio Galaxies. Contents. Brief review of AGN physics & phenomenology

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The Origin of X-Ray Emission in the Nuclei of Radio Galaxies

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  1. Dan Evans (Harvard) -with- Diana Worrall (U. Bristol), Martin Hardcastle (U. Herts), Ralph Kraft (SAO), Mark Birkinshaw (U. Bristol), Judith Croston (U. Herts) The Origin of X-Ray Emission in the Nuclei of Radio Galaxies

  2. Contents • Brief review of AGN physics & phenomenology • Nuclear X-ray emission: accretion flow or jet? • The 3CRR sample of radio galaxies • Is an obscuring torus ubiquitous? • A nuclear Fanaroff-Riley dichotomy? • Implications for accretion-flow structure

  3. Accretion Processes In AGN Animation of accretion onto a supermassive black hole

  4. Accretion Processes In AGN • Accretion flow surrounded by dusty torus • BB radiation from disk  ‘big blue bump’ • B-field loops  optically thin corona • Isotropic X-rays from Comptonization of disk photons in hot corona • Power law X-ray spectrum

  5. Fe Ka Production • Fe Ka lines are the most commonly used accretion diagnostic • Width and centroid of Fe Ka line give location of fluorescing material w.r.t. black hole Fe Kα George & Fabian (1991)

  6. Radiatively Inefficient vs. Efficient Accretion Flows • Viscous energy heats ions • Radiative losses mainly from electrons • Energy advected onto BH • X,Edd (=LX/LEdd) small (~10-5) Inefficient flow (e.g. ADAF) Efficient flow m=M/M • High accretion rates • Geometrically thin, optically thick accretion disk (e.g. Shakura-Sunyaev 1973) forms • X,Eddhigher (up to ~10%)

  7. 30 arcsec = 500 pc Astrophysical Jets in Radio-Loud AGN Centaurus A: Chandra / VLA Hardcastle et al. (2003)

  8. Hotspot Jet Core Lobe Hotspot High-power (FRII) Low-power (FRI)

  9. The Fanaroff-Riley Dichotomy Is the dichotomy • Environmental? • Interaction of the jet with ambient medium either causes the jet to decelerate (FRI) or propagate supersonically to large distances (FRII) • Intrinsic? • Properties of the central engine govern large-scale morphology (FRI/FRII)

  10. Astrophysical Jets in Radio-Loud AGN Chandra commonly resolves kpc-scale X-ray jet emission in nearby RL AGN: • FRIs  kpc X-ray emission synchrotron in nature (e.g., Worrall et al. 2001) • FRIIs  X-ray emission tends to be inverse-Compton (e.g., Sambruna et al. 2004) • What about (unresolved) parsec-scale X-ray jets?

  11. Parsec-scale X-ray Jets • Anisotropic emission, power law X-ray spectrum • Relativistic Doppler beaming, dependent on bulk speed (Γ), angle to line of sight

  12. Radio-Galaxy Nuclei – Two Competing Models • Is the nuclear X-ray emission dominated by: • The parsec-scale jet? -or- • The accretion flow? Radio Galaxy

  13. Evidence for jet-dominated nuclear X-ray emission • Correlations between the ROSAT soft X-ray and VLA radio core fluxes and luminosities in the B2 (Canosa et al. 1999) and 3CRR (Hardcastle & Worrall 1999) samples • Parsec-scale radio emission is jet-generated and strongly affected by beaming • Tight correlations suggest X-ray emission affected by beaming in same manner as radio • Soft X-ray emission originates in a jet • Double-peaked SED (modeled with syn+SSC) NGC 6251 - 5 GHz VLBI 10 pc Jones et al. (1986)

  14. Evidence for accretion-dominated nuclear X-ray emission • Often accompanied by high intrinsic absorption (NLRGs) • Short (~ks) timescale variability in broad-line FRII 3C 390.3 (Gliozzi et al. 2005) • Broadened Fe K line emission in some radio galaxies (e.g., Gliozzi et al. 2004) • Implies Fe K origin in inner regions of accretion flow Gliozzi et al. (2004)

  15. Radio Galaxy Summary of Introduction • X-ray continuum emission in the nuclei of RL AGN consists of: • “Radio-quiet” accretion-related component • “Radio-loud” jet-related component ROSAT Chandra/XMM • Which dominates the X-ray emission?

  16. The 3CRR Sample • Criteria: • 178-MHz flux density > 10.9 Jy • Declination > 10o • |b| > 10o • Advantages: • No orientation bias • Spectroscopic identification • High-resolution radio observations • Select sources with z<0.1 • Unambiguously spatially separate unresolved nuclear emission from contaminating emission • Rich variety (FRI/FRII, broad/narrow lines, large luminosity range) • 19/35 X-ray observations of low-z 3CRRs, 16 of them with Chandra • Complete X-ray spectral analysis of each 5 GHz VLA (Leahy, Bridle, & Strom)

  17. Radio Galaxy The 3CRR Sample: Aims Unified AGN scheme: • Dominant X-ray emission mechanism: • Accretion Flow: Fe K, variability -or- • Jet: Radio-X-ray luminosity correlations, SED • Nature of accretion flow: thin disk? RIAF? • Is the torus ubiquitous? • FRI-FRII dichotomy?

  18. FRI FRII 3C 390.3 Distribution of intrinsic absorption • Measure intrinsic absorption associated with dominant component of X-ray emission • Note that some sources have two continuum components (one absorbed; one unabsorbed) • Bimodal distribution • FRIs have low (or no) intrinsic absorption • FRIIs have NH > 1023 cm-2 • NB BLRG FRII 3C 390.3

  19. Origin of intrinsic absorption • FRI sources with the highest intrinsic absorption are associated with host galaxies with circumnuclear disks at high inclinations • Heavily absorbed emission in FRIIs likely in gas associated with dusty torus (c.f. Seyfert 2s)

  20. Luminosity-Luminosity Correlations • Consider LX and LR • Considerable scatter

  21. Luminosity-Luminosity Correlations • Consider LX and LR • Considerable scatter • Strong correlation between components with NH ≤ 5 x 1022 (significant at 99.99%) • Apparent in flux-flux too • Suggests X-ray emission affected by beaming in same manner as radio • Origin of X-ray emission in pc-scale jet (outside any torus) Jet

  22. Jet Luminosity-Luminosity Correlations • FRIs are dominated by these soft components only • FRIIs, although heavily absorbed, also contain these components • Cannot distinguish between X-ray jet components in FRIs and FRIIs Jet

  23. Luminosity-Luminosity Correlations • Components with NH ~ 1023 lie above trendline • As does 3C 390.3, unobscured BLRG • All have Fe K lines • Accretion-dominated and surrounded by a torus • FRIIs are dominated by these components Jet NH≤ 5 x 1022

  24. Where is the torus in FRIs? • X-ray emission is jet dominated and exists on scales larger than any torus • Cannot determine presence or absence of torus directly • Assume there exists a ‘hidden’ accretion component obscured by a torus of intrinsic absorption 1023 atoms cm-2 • Find upper limits to luminosity of accretion-related emission • Data don’t exclude luminosities of LX,acc ~ 1039-1041 ergs s-1 (X,Edd ~ 10-7-10-5) • Substantially lower than FRIIs, LX,acc ~ 1043-1044 ergs s-1 (X,Edd ~ 10-3-10-2) ‘Hidden’ Accretion e.g. 3C 274 (M87)

  25. Intermediate Summary e.g. 3C 264 (FRI) • X-ray emission of FRI radio-galaxy nuclei is dominated by a parsec-scale jet, with little or no intrinsic absorption • X-ray emission of FRII radio-galaxy nuclei is dominated by an accretion flow and is heavily absorbed (except BLRG 3C 390.3) • Each FRII also has an unabsorbed component of X-ray emission  jet origin • Cannot rule out a torus in FRIs • ‘Hidden’ accretion flows in FRIs are substantially sub-Eddington (c.f. FRIIs) e.g. 3C 403 (FRII)

  26. Nuclear F-R Dichotomy? “HYMORS” sources (Gopal-Krishna & Wiita 2000) How can a dichotomy in the subparsec-scale accretion-flow mode influence the kpc-scale deceleration of jets into FRI and FRII structures (e.g., Bicknell 1995)?

  27. Nuclear F-R Dichotomy? Low-excitation – High-excitation Dichotomy?

  28. BLRG NLRG LERG Low-Excitation Radio Galaxies • ‘Low-excitation’ vs ‘High-excitation’ (e.g., Hine & Longair 1979). Based on strength of high-excitation lines like [OIII] • FRIs are almost entirely low-excitation • Significant population of low-excitation FRIIs at 0.1<z<0.5 • Extend sample to these redshifts

  29. X-ray Spectra NLRG FRIIs FRIs / LERG FRIIs

  30. Sample results • All of the NLRG FRIIs show evidence for an absorbed nuclear component with NH ~ 1023cm-2  accretion flow dominates • Almost all the FRIs and LERG FRIIs show no evidence for an absorbed component (exceptions are debatably classed as LERG)  jet dominates • Again, can place upper limits on the accretion-flow luminosity in the presence of a torus

  31. Luminosities • For a given 178-MHz radio power, FRIs and LERG FRIIs produce significantly less radiative accretion luminosity • Pointing to a dichotomy in the excitation properties of AGN? White = LERG Red = NLRG Green = BLRG; Blue = quasar Circle => FRI Line goes through FRII NLRG Limits assume NH = 1023 cm-2

  32. Understanding the Dichotomy • Do LERGs represent a class of radio galaxies that don’t participate in unified AGN schemes? • What implications does the observed dichotomy have on the structure of the central engine in AGN? Several possible interpretations…

  33. Model 1: FRI/LERG FRII tori are Compton thick • Luminous accretion flow surrounded by thicker torus than NLRG FRIIs • Would give rise to reduced observed flux from accretion flow • Assume NH = 1024 cm-2X,Edd ~ 10-6-10-4, still lower than NLRG FRIIs • For FRI/LERG FRII and NLRG FRII accretion-flow efficiencies to match, need extreme NH (> 1025 cm-2) • Ruled out from infrared data (Muller et al. 2004, Haas et al. 2004, Birkinshaw et al., in prep.) Absorbed X-ray lum. against mid-IR (Ogle et al 2006 data, except for Cen A). Line is the line of equality, not a fit.

  34. Model 2: An intrinsic nuclear dichotomy • Fundamentally different accretion mode in FRI/LERG FRIIs and NLRG FRIIs • Accretion-flow luminosities and radiative efficiencies of LERGs systematically lower than NLRGs • Widely discussed model: hybrid ADAF/thin disk + jet • Critical mass-accretion rate mcrit (=Mcrit/M) Esin et al. (1997)

  35. Low state High state Model 2: An intrinsic nuclear dichotomy Low-exc Hi-exc LX • Analogous to XRBs • Step change in accretion luminosity at mcrit • At low accretion rates, jet still strong and comes to dominate Accretion flow Jet mcrit mEdd m Esin et al. (1997) Körding, Falcke, & Markoff (2002)

  36. Kraft et al. (2006) Model 2: An intrinsic nuclear dichotomy • In this model, both FRI and FRII kpc-scale structures can be produced by nuclei with low accretion luminosity • FRI/FRII dichotomy entirely due to environment and jet power • Excitation dichotomy controlled by accretion mode 3C 388 (LERG FRII)

  37. Summary • The excitation of an AGN is a vital parameter in unification schemes • X-ray emission of FRI and LERG FRII radio-galaxy nuclei is unabsorbed and dominated by a parsec-scale jet • X-ray emission of NLRG FRII radio-galaxy nuclei is heavily absorbed and accretion-related • Data do not exclude the presence of a heavily obscured, accretion-related emission in LERG-type sources • An intrinsic nuclear dichotomy (analogous to XRBs)?

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