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