X-ray Spectral Signatures of
Accretion onto Supermassive
Black Holes
Laura Brenneman (CfA)
Accretion Processes in X-rays
July 13, 2010
Outline
• The innermost AGN in X-rays: evidence for accretion
• Broad iron lines as inner disk diagnostics
• The prevalence of broad iron lines in AGN
• Constraints on black hole spin in AGN
• Implications for AGN evolution, feedback
• Conclusions and open questions
Evidence for AGN Accretion in X-rays
• Combination of high
luminosity, large X-ray
contribution, small spatial
size, fast variability can
only be accounted for by
accretion onto SMBH.
• Relativistically broadened
inner disk signatures such
as Fe Kα imply a Comptonthick disk down to ~ISCO.
• Persistence of corona, size
constraints link it to inner
disk.
• Relativistic outflows (e.g.,
jets) require a power
source.
X-ray Signatures in AGN
•
•
•
•
•
•
•
Power-law continuum (IC from corona).
Compton hump/Fe K edge, Compton shoulder,
narrow Fe Kα and Kβ lines (reprocessing of
continuum by outer disk, torus).
Warm absorber (partly ionized, can range in NH
and ξ).
Broad Fe Kα, soft excess (reprocessing of
continuum by inner disk).
Jet emission in radio-loud objects (IC, SSC).
Study of these spectral features  insight into
AGN structure, accretion flow/geometry, physical
conditions, properties of spacetime closest to black
holes.
Superposition of these features in spectra makes
separating them a challenge.
Iron Line Fluorescence
• Looking at inner accretion
flow = looking at broad Fe Kα
line.
• Fluorescent lines are
produced when a “cold,”
optically thick disk is
irradiated by X-ray continuum
photons, exciting a series of
fluorescent emission lines.
• The high energy, abundance
and fluorescent yield of iron
enable visibility above the
power-law continuum, making
it a better diagnostic feature
than lines of other elements.
Reynolds & Nowak (2003)
KERRDISK model (Brenneman & Reynolds 2006)
Probing the Inner Disk with
Broad Iron Lines
Plunging region
inside ISCO
BH
3-D MHD simulation of a geometrically-thin accretion
disk
Clearly shows transition at the innermost stable
circular orbit which will lead to truncation in iron line
emission
Reynolds & Fabian (2008)
Iron Line Spectra as a Probe of
Inner Disk Radius and Spin
NON-SPINNING BLACK HOLE
Non-spinning
RAPIDLY-SPINNING BLACK HOLE
Rapidly-spinning
Ionization of the “Reflionx” Disk
ionized
neutral
Soft excess
Compton hump
Ross & Fabian (2005)
First Observation of Broad Fe Kα
ASCA/SIS
MCG—6-30-15: Tanaka+ (1995)
Broad Fe K Lines in AGN…
XMM-Newton: Nandra+ (2007)
Broad Lines with XMM, Suzaku
• Relativistic Fe K lines in ~50% of 30 observed Sy 1 AGN, after
including absorption and narrow lines (XMM; Nandra+ 2007).
• 9 Sy 1 AGN with rin < 20 rg (XMM; Nandra+ 2007).
• Subset of 5 of these confirmed by Suzaku, also including 3C 120
(Reeves+ 2006).
• J. Miller’s 2007 review identifies 3 “tiers” of broad lines in AGN
according to strength and robustness of detection over multiple
epochs: 30 total sources, 9 in tier I (strong & robust), 6 in tier II
(less strong & robust), 15 in tier III (detected, but requiring further
confirmation).
• Detection depends STRONGLY on photon statistics: detection
rate of BLs increases to over 50% with > 200,000 photons (Guainazzi+
2006). As such, a flux-limited survey of BLs is not possible at this
time.
• True flux-limited survey will be necessary to determine true
fraction of broad iron line AGN; recent theory suggests average EW
~ 100 eV (Ballantyne 2010)  Astro-H, IXO.
MCG—6-30-15: the Most Extreme
Broad Line
~385 ks Suzaku (Miniutti+ 2007)
Can count how many iron line photons are expected
given size of hard X-ray bump… require extreme
broadening to fit those photons in the spectrum!.
How to Constrain a Broad Fe Kα Line
1) Proper Continuum Modeling (e.g., power-law, cutoff?)
2) Include cold, distant reflection (e.g., GAUSS Fe K
lines + PEXRAV; MYTORUS [Murphy & Yaqoob 2009,
Yaqoob+ 2010]).
1) Account for absorption: Galactic, intrinsic… both
neutral (e.g., PHABS, PCFABS) and ionized (e.g.,
ZXIPCF, ABSORI, WARMABS, XSTAR grids). Timing
analysis!!
2) Add in any thermal or photoionized soft emission (e.g.,
MEKAL, APEC; PHOTEMIS).
3) What remains is the contribution from relativistic inner
disk reflection: manifests in broad Fe Kα, Compton
hump, soft excess (e.g., KERRCONV*REFLIONX)).
SMBH Spin Constraints from Broad
Iron Lines
AGN
EW
(eV)
MCG—6-30-15
Rin
(rg)
a
6.0±0.
3
2.0±0.
5
104±1
~130
3.6±0.
4
0.65±0.05
44±
1
5
0.8±0.
2
4±1
~220
3.8±0.
8
0.6±0.2
46±
4
5.3±1.
7
1.5±0.
3
40±35
~1200
<1.3
>0.98
59±
1
6.6±1.
9
>7
50±40
---
3.4±0.
4
0.7±0.1
24±
1
4.4±1.
2
1.2
119±6
6
~85
<2.2
>0.92
24±
3
4
1.5±0.
4
<58
(Gallo+ 2010, submitted)
NGC 3783**
(Brenneman+ 2010, in prep.)
cm/s)
30±
1
(Fabian+ 2009; Zoghbi+ 2010)
Mrk 79*
ξ (ergs
>0.98
(Miniutti+ 2009)
1H0707
Fe/solar
<1.6
(Schmoll+ 2009)
SWIFT J2127
q1
~400
(Brenneman & Reynolds
2006; Miniutti+ 2007)
Fairall 9
i (°)
Suzaku Key Project (AO4-AO6)
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•
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Purpose: study inner accretion flow, constrain black
hole spin in AGN.
PI: Chris Reynolds, lead Co-I: Laura Brenneman
AGN sample pre-selected to display broad iron lines.
Target selection:
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Broad iron line targets drawn from XMM/Suzaku literature
(Nandra+ 2007, Miller+ 2007); broad line must also show up
in our own analysis of existing Suzaku data for inclusion in
the sample.
Final list; NGC 3516, NGC 3783, Fairall 9, Mrk 766,
Mrk 841, 3C 120.
Spans range of luminosities, host galaxy types, radioloudness.
Requested observing time calculated to measure spin at the
level of a=0.1 (based on XSPEC simulations)… recent
accretion vs. merger history (e.g., Volonteri+ 2008).
Broad-band capabilities of Suzaku crucial to robustness
of our results.
Broad Iron Lines in the preAO4 Suzaku Archival Data
NGC 3783: Our First Target
• Ratio of XIS and
PIN spectra to a
power-law.
• Absorption
dominates below 3
keV, also influences
Fe K band.
• Soft excess below
~210 ks Suzaku XIS+PIN
0.7 keV… reflection?
Photoionized emission?
• Distant reflection
Brenneman+ (2010)
seen in prominent
narrow iron lines.
• Fe K edge seen at ~7.1 keV shaping modest Compton reflection hump.
NGC 3783 Light Curve
0.3-1 keV
See significant short-timescale
variability; XIS color mostly
tracks flux very well.
Count Rate vs. Softness Ratio
2-10 keV
10-60 keV
Conclude : spectral variability is
mostly broad band, not emergence
of distinct absorption/emission
components only affecting one
band.
Brenneman+ (2010)
Difference spectra fit with
a simple power-law.
Difference spectra fit with
power-law + non-variable
warm absorption… note poor
PIN fit.
Reis+ (2010)
Detailed Structure in
Time-averaged Fe K Band
• Strong 6.4 keV Fe Kα
line.
• Asymmetry suggests
strong Compton shoulder
 confirms origin in
Compton thick matter.
Also a broad red wing?
Fe XXV
abs
V
• Recombination line of
H-like iron (blended).
• Highly ionized
absorption line ~6.7
keV… Fe XXV?
Brenneman+ (2010)
Model independent approach…
convolve HETG data to XIS
resolution and drop on top of XIS
data (refit only for normalization).
Only significant residuals are
<0.9keV and 5.5-6.5keV.
Courtesy of Mike Nowak
Modeling approach: refit
3-zone WARMABS model
to XIS+PIN spectrum.
Need to include distant
reflection and relativistic
disk.
Fe Kα EW (narrow) ~100 eV
From distant, neutral
reflection
PEXRAV + ZGAUSS +
ZGAUSS
Fe Kα EW (broad) ~85 eV
From ionized inner disk
KERRCONV*REFLIONX
Brenneman+ (2010)
Evidence for inner disk ρ
and/or ξ changing during the
observation… change in
accretion flow??
Other components: power-law (Γ~1.83)
blackbody (kT~0.10 keV)
photoionized emission (ξ~537 ergs cm/s; PHOTEMIS)
Brenneman+ (2010)
a = 0.92-0.93
3σ
Brenneman+ (2010)
i = 18-24°
3σ
Brenneman+ (2010)
The Light-Bending Model
Distance of
hard X-ray
source from
disk (Rs)
varying in rg.
Miniutti & Fabian (2004)
• Regime I: RDC and PLC linearly correlated
• Regime II: RDC ~constant, PLC varying
• Regime III: RDC and PLC anticorrelated
• Can explain spectral/timing properties of many AGN this way, e.g.,
MCG-6, NGC 4051, NGC 3783 (?).
Black Hole Spin and AGN Evolution
Coalescence only
Coalescence + chaotic
accretion
Coalescence +
prolonged accretion
•
Constraining spin can help us determine the role of accretion
vs. mergers in AGN growth over time (Berti & Volonteri 2008).
• Need 105-6 photons (2-10 keV) on hundreds of AGN, also
quality simultaneous data > 10 keV.
• Suzaku can begin to tackle this, but to get enough sources,
detail need more collecting area, improved spectral resolution.
• Astro-H, IXO, also GEMS.
In Search of a Disk/Jet Connection
• Minority of AGN have observed relativistic radio jets.
• Jets, X-ray emission from the inner accretion disk originate
very close to black hole, so the connection between the jet origin
and inner accretion flow (and, by proxy, black hole spin) can be
studied by comparing X-ray properties of radio-quiet vs. radio
loud AGN.
• Evidence suggests inner accretion flow experiences a disruption
event ~coincident with radio jet ejections: X-ray light curve
shows dips as VLBI shows radio “blobs” emitted (e.g., Marscher+
2000).
• RLAGN tend to have weaker reflection features, narrower Fe K
lines than RQAGN (Eracleous+ 2000).
• Highly ionized inner disk? Reflection drowned out by jet
emission? ADAF??
Is Black Hole Spin the Key?
that combination of
magnetic flux-trapping,
Blandford-Znajek and
Blandford-Payne models
 retrograde-spin black
holes have the most
powerful jets.
• Evolution through
prograde accretion
eventually weakens and
turns off jets.
• So generally, RLAGN
have a<<0, RQAGN have
a>>0.
Jet Power
• Garofalo+ (2010) argue
Black Hole Spin (a)
• Mergers more capable of creating retrograde disk-BH system,
consistent with most RLAGN appearing in giant elliptical hosts.
Conclusions & Future Work
• Broad Fe Kα lines and other inner disk reflection signatures are
promising inner accretion flow diagnostics: can help us constrain
Fe/solar, ξ, incl, rin, q1/q2/rbr, a.
• Broad-band spectral coverage is crucial!
Timing analysis also very
important for determining absorber properties, whether timeaveraged spectrum can be used for increased s/n.
• Photon intensive process: must have at least 200,000 photons in 210 keV band. Sources must be bright, nearby.
• XMM-Newton and Suzaku are making the first precision
measurements of broad iron lines now, but many more sources
needed before broad line demographics can truly be addressed in
AGN: evolution, radio-loudness, etc.
• Future instruments such as Astro-H, NuSTAR & IXO will further
our ability to analyze these spectra >10 keV, examine many more
AGN with greater s/n.
• Open questions: coronal location, geometry, relation to inner
disk? State of disk during jet production? Distribution of
SMBH spins, dM/dt, etc.?