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

Haardt et al. 1997Wilms et al. 2004

Accretion system: thin disk + corona

Production of 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 keVLightman & White, 1988

Fe K Power law input

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

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

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 featuresMiniutti & Fabian 2004

MCG-6-30-15: Poster Child

• S0 Seyfert 1• D = 37 Mpc

• MBH ~ 1-20 x 106 Msun

• ASCA: first detection of relativitistically- broadened Fe K

• Complex variability!

Energy (keV)

Fe K

Relativistic broadening

Tanaka et al. 1995

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

MCG-6-30-15: ASCA Results

• Line profile consistent with– Emission from 3Rs < r < 10Rs

– Disk inclination ~ 30o

– Flux profile ~ r-3

• Significant variability

MCG-6-30-15: ASCA Variability

• 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!Reynolds et al. 2003

1994 1997

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DM

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

Fabian 2006

Use line profile to differentiate between Schwarzchild and Kerr BH

Schwarzschild vs. Kerr

1. Geometrically thick outer disk corona• Irradiates surface of plunging region, producing X-

ray reflection signatures

2. 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)

MCG-6-30-15: XMM-Epic Part 1

• 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

100 ks, 2000 June

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

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 Larsson et al. 2007

Difference Spectrum

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!

Results from Suzaku

• Consistent with XMM data– variable power-law continuum– harder constant component

with broad iron line and reflection hump

E (keV)3 8

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2006

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

Chandra/HETG Data

• 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

Rey

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Deep absorption feature at 6.5 keV

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

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

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

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?

NGC 5548 Variability

• 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

Reynolds & Nowak 2003

Flux-correlated changes in ionization state of disk?

Fe

EW

Reflection normalization

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

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)

Fe K in HLAGN

• Fe K EW decreases for Lx > 1044-45 erg s-1

• Highly ionized disks possible explanation

Nandra et al. 1997

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

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

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

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