Transcript Slide 1

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
Accretion system:
thin disk + corona
Wilms et al. 2004
Haardt et al. 1997
Production of Fe K
• Comptonized photons
irradiate accretion
disk with power law
spectrum
• Compton reflection
hump
Fe K
Power law input
– 30 keV peak
– absorption/flourescent
line emission
 Fe K ~ 6.4 keV
Lightman & White, 1988
Fe K: Relativistic Effects
• Doppler shift:
symmetric, doublepeaked 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 features
Miniutti & Fabian 2004
MCG-6-30-15: Poster Child
Tanaka et al. 1995
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Relativistic broadening
Fe K
Energy (keV)
S0 Seyfert 1
D = 37 Mpc
MBH ~ 1-20 x 106 Msun
ASCA: first detection
of relativitisticallybroadened Fe K
• Complex variability!
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
1997
DM
Peculiar
• 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
1994
Reynolds et al. 2003
Measurement of BH Spin
Use line profile to differentiate
between Schwarzchild and Kerr BH
Fabian 2006
• 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
Schwarzschild vs. Kerr
1. Geometrically thick outer disk corona
•
Irradiates surface of plunging region, producing Xray 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
100 ks, 2000 June
Wilms et al. 2001
• 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
MCG-6-30-15: XMM-Epic Part 2
325 ks, 2001 July 31–2001 August 5
Fabian et al. 2002
• Observations in
normal, higher
continuum state
• Variability in 2-10 keV
band continuum flux
• Iron line flux does
NOT change with
continuum flux
Line vs. Continuum Variability
Difference Spectrum
Larsson et al. 2007
• 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
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!
Miniutti et al. 2006
Results from Suzaku
• Consistent with XMM data
– variable power-law continuum
– harder constant component
with broad iron line and
reflection hump
3
E (keV)
8
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
Reynolds 2007
Deep absorption feature at 6.5 keV
• 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
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
• XMM data show non-detection
 transitory broad Fe lines?
Reynolds & Nowak 2003
NGC 5548 Variability
Reynolds & Nowak 2003
– 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
• Simultaneous ASCA &
RXTE observations
Reflection normalization
 Flux-correlated changes in ionization state of disk?
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