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Fe K  Line in AGN

Fe K  Line in AGN. Shane Bussmann AGN Class 4/16/07. Importance of Fe K . High energy astrophysics  study accretion disks around BHs Emission feature arises close to BH  probe strong gravity effects, compare to predictions from GR  determine BH properties. The Standard Model.

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Fe K  Line in AGN

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  1. Fe K Line in AGN Shane Bussmann AGN Class 4/16/07

  2. Importance of Fe K • High energy astrophysics  study accretion disks around BHs • Emission feature arises close to BH  probe strong gravity effects, compare to predictions from GR  determine BH properties

  3. The Standard Model Accretion system: thin disk + corona Wilms et al. 2004 Haardt et al. 1997

  4. Production of Fe K Power law input Fe K • Comptonized photons irradiate accretion disk with power law spectrum • Compton reflection hump • 30 keV peak • absorption/flourescent line emission  Fe K ~ 6.4 keV Lightman & White, 1988

  5. Fe K: Relativistic Effects • Doppler shift: symmetric, double-peaked profile • Relativistic beaming: enhance blue peak relative to red peak • Gravitational redshift: smearing blue emission into red Fabian 2006

  6. Fe K: Ionization Effects • Higher ionization parameter attenuates flourescence emission • Low ionization parameter allows forest of lines; relativistic effects then smear these lines together Fabian 2006

  7. Light Bending Model • X-ray source located at height hs above accretion disk (e.g. the base of a spin-driven magnetic jet) • Variation in hs with time leads to variation in flux • Low hs = region I • High hs = region III • Intermediate hs = region II • Low hs allows gravity to bend light onto accretion disk, reducing continuum flux while enhancing reflection features Miniutti & Fabian 2004

  8. MCG-6-30-15: Poster Child Tanaka et al. 1995 • S0 Seyfert 1 • D = 37 Mpc • MBH ~ 1-20 x 106 Msun • ASCA: first detection of relativitistically- broadened Fe K • Complex variability! Relativistic broadening Fe K Energy (keV)

  9. Fe K Analysis Issues • Continuum subtraction (Fabian et al. 1995) • Alternative emission mechanisms • Comptonization: expect break in continuum at 20 keV (not seen, Zdziarski et al. 1995) • Jets/outflows: no blue shifted emission; radio quiet; OVII absorber  vflow,abs<< vOF • Photoelectric/resonance absorption of blue wing: blue emission falls off too quickly • Spallation converts Fe to lower Z metals: ASCA should have resolved these lines

  10. MCG-6-30-15: ASCA Results • Line profile consistent with • Emission from 3Rs < r < 10Rs • Disk inclination ~ 30o • Flux profile ~ r-3 • Significant variability

  11. MCG-6-30-15: ASCA Variability 1997 1994 • 1994: large flaring event w/ narrow line close to E0  large radii • 1997: large flaring event w/ most emission redshifted  small radii • 1994 Deep minimum (DM) state: continuum drops, very broad, red line: R < 3Rs constrain rotation! Time-avg DM Peculiar Reynolds et al. 2003

  12. Measurement of BH Spin • Assuming some distribution of flux within a disk truncated at rms, rms < 3Rs implies a > 0.94 • Problem: if emission is allowed to originate within rms (the plunging region), redshifts can grow arbitrarily large  MUST understand astrophysics of inner accretion disk Use line profile to differentiate between Schwarzchild and Kerr BH Fabian 2006

  13. Schwarzschild vs. Kerr • Geometrically thick outer disk corona • Irradiates surface of plunging region, producing X-ray reflection signatures • Accretion flow within plunging region not dissipationless • Inner corona could produce X-ray reflection signature  ASCA data consistent with both Schwarzschild and Kerr BHs (Reynolds & Begelman 1997)

  14. MCG-6-30-15: XMM-Epic Part 1 100 ks, 2000 June • Observations in DM state agree w/ ASCA • Improved sensitivity: Schwarzschild case requires all flourescence to originate within Rs < r < 1.5Rs very unlikely • Successive 10 ks frames show iron line flux proportional to 2-10 keV continuum flux Wilms et al. 2001

  15. MCG-6-30-15: XMM-Epic Part 2 • Observations in normal, higher continuum state • Variability in 2-10 keV band continuum flux • Iron line flux does NOT change with continuum flux 325 ks, 2001 July 31–2001 August 5 Fabian et al. 2002

  16. Line vs. Continuum Variability • Difference spectrum = high flux – low flux, normalized by power law continuum • No iron line feature: reflection component relatively constant • Reflection component saturates at high continuum fluxes Difference Spectrum Larsson et al. 2007

  17. Physical Significance • Models suggest a ~ 1 • rapidly spinning BHs can experience a magnetic torque by the fields threading the accretion disk at rms • steepest dissipation profiles obtained when magnetic torque applied completely at rms • Steep emissivity index of ionized disk (~r-6) consistent with magnetic torquing  Accretion disk might be extracting BH spin energy!

  18. Results from Suzaku • Consistent with XMM data • variable power-law continuum • harder constant component with broad iron line and reflection hump Miniutti et al. 2006 8 3 E (keV)

  19. Need for High Spectral Resolution • Broad iron lines typically observed in spectra with signatures of absorption by circumnuclear plasma (warm absorber) • Fe K line might just be leftover continuum • XMM data can’t rule this out (Kinkhabwala 2003) • Prediction: K-shell absorption features between 6.4-6.6 keV Reynolds 2007

  20. Chandra/HETG Data Deep absorption feature at 6.5 keV Reynolds 2007 • Left: Power-law continuum + broad iron line + narrow fluorescent line of FeI + resonant absorption lines of FeXXV and FeXXVI • Right: Power-law continuum + warm absorber

  21. Comparison to Light Bending Model • Low flux = regime I, normal flux = regime II, high flux = regime III • Variability timescale consistent • Regime II: variable continuum + constant reflection component • Disk emissivity in the form of broken power law (steeper in inner disk) • Iron line EW and continuum anti-correlated in normal state • Low flux states have broader line that correlates with continuum • Reflection component dominates more as flux decreases • Iron line in high flux states narrower than low flux states

  22. Fe K in other Seyferts • ACSA-era state of the art: composite spectrum from 18 sources (top) • Excluding MCG-6-30-15 and NGC 4151 does not alter fit (bottom) • Several day long integration necessary for high S/N Nandra et al. 1997

  23. Two More Seyferts • NGC 3516 • red wing tracks continuum flux • blue wing variability uncorrelated with continuum • Absorption line at 5.9 keV could result from infall of material onto BH • NGC 4151 • Iron line profile more variable than continuum • 5 years later, opposite true

  24. NGC 5548 • Very narrow iron line in ASCA data • Chandra data show narrow core of line originates a substantial distance from BH • Removing this component produces significantly smaller inner radius • Affects inclination of disk Reynolds & Nowak 2003 • XMM data show non-detection  transitory broad Fe lines?

  25. NGC 5548 Variability Reynolds & Nowak 2003 • Simultaneous ASCA & RXTE observations • Iron line flux (ASCA) constant while continuum source varies • Continuum reflection (RXTE) increases with continuum flux • Counter-intuitive: different facets of same phenomenon should be correlated Fe EW Reflection normalization  Flux-correlated changes in ionization state of disk?

  26. Seyferts: Summary • Fe K from relativistic accretion disk is generic feature of Seyfert I objects • Understanding line variability very important • Nandra et al. (2006): XMM observations of 30 Seyfert 1’s broadly consistent with results from ASCA

  27. Fe K in other AGN • Low luminosity AGN example: NGC 4258 • ASCA: Narrow iron line  r > 50 Rs • XMM: non-detection  variable on year-long timescale, iron line originates in accretion disk • Typical LLAGN do not show broadened iron line (but S/N is low)

  28. Fe K in HLAGN • Fe K EW decreases for Lx > 1044-45 erg s-1 • Highly ionized disks possible explanation Nandra et al. 1997

  29. Fe K and Radio-loud AGN • Fe K ideal way to study central engines of radio-loud and radio-quiet AGN • Result: broad iron lines are generally weak or absent in radio-loud sources • Beamed jet swamps Seyfert-like X-ray spectrum • Hot, radiatively inefficient, optically thin inner disk • Radiatively efficient and optically thick inner disk, but highly ionized

  30. Fe K From Galactic BHCs • Inner accretion disk similar in AGN and GBHC (GBHC disk more highly ionized) • Characteristic timescales very different • AGN tvisc ~ tens of years • GBHC tvisc ~ days to weeks • Can study changes with accretion rate by observing GBHC

  31. Remaining Issues • Narrow Fe K lines ubiquitous, clear broad lines not: requires iron overabundance? EW depends on Eddington ratio? • What is the nature of the illuminating X-ray source? How does it change height? • Interpretation of complex, time-varying broad iron lines in context of BH spin

  32. Future Prospects • Next generation missions with larger collecting area and higher spectral res. will obtain significantly larger sample of broad iron line sources • Transient relativistic iron line features  dynamical effects near BH • Con-X and XEUS will do these both locally and at high redshift • Cosmic history of SMBHs • Reverberation mapping of X-ray flares: test GR in strong field limit

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