Measuring Black Hole Spin in AGN
Laura Brenneman
CfA Postdoc Symposium
October 25, 2013
Adviser: Aneta Siemiginowska
Measuring Black Hole Spin in AGN
Collaborators: C. Reynolds, M. Elvis, A. Fabian, G. Risaliti, G.
Matt, A. Marinucci, F. Fuerst, D. Walton, G. Madejski, R. Reis,
M. Nowak, J. Miller, F. Harrison, D. Stern, J. García, J.C. Lee
The Importance of Black Hole Spin
• Provides rare means of probing strongfield gravity regime.
• Indicator of recent gas accretion vs.
merger history of supermassive BHs.
• Thought to play a role in jet production
and outflows in all BHs, seeding the
ISM/IGM with matter and energy.
How Can We Measure BH Spin?
• Thermal Continuum Fitting
- X-ray Spectra (XRBs, some AGN attempts)
• Inner Disk Reflection Modeling
- X-ray Spectra (both XRBs and AGN)
• Quasi-periodic Oscillations**
- X-ray Timing (both XRBs and AGN, only one seen in AGN so far)
• Fe K Reverberation Lags, Orbiting Disk Hot Spots**
- X-ray Timing and Spectra (easier in AGN)
• Polarization Degree & Angle vs. Energy**
- X-ray Spectra, polarimetry (easier for XRBs)
• Imaging the Inner Disk and Event Horizon**
- ≤mm-VLBI Imaging (AGN only: must be large, e.g., Sgr A*, M87)
How Can We Measure BH Spin?
• Thermal Continuum Fitting
- X-ray Spectra (XRBs, some AGN attempts)
• Inner Disk Reflection Modeling
- X-ray Spectra (both XRBs and AGN)
• Quasi-periodic Oscillations**
- X-ray Timing (both XRBs and AGN, only one seen in AGN so far)
• Fe K Reverberation Lags, Orbiting Disk Hot Spots**
- X-ray Timing and Spectra (easier in AGN)
• Polarization Degree & Angle vs. Energy**
- X-ray Spectra, polarimetry (easier for XRBs)
• Imaging the Inner Disk and Event Horizon**
- ≤mm-VLBI Imaging (AGN only: must be large, e.g., Sgr A*, M87)
/
: the Reflection Spectrum
Modeling
Static disk spectrum
REFLIONX XILLVER
• Relativistic
electrons
in corona
KERRCONV
/RELCONV
:
Compton Effects
scatter of
thermal
photons
spacetime
(UV) from the accretion disk,
twisting
producingwarping,
power-law
continuum
spectrum in X-rays.
• Some X-ray continuum photons are
scattered back down onto the inner
disk (“reflected”).
• 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.
Fe Kα & Compton Hump
Ross & Fabian (2005), García+ (2013)
Brenneman & Reynolds (2006)
Dauser+ (2010, 2013)
Reynolds & Nowak (2003)
The Innermost Stable Circular Orbit
© Sky & Telescope, May 2011
• Maximally-spinning
• Maximally-spinning
• Non-spinning BH.
prograde BH
(spinning in same
direction as disk).
• Accretion disk still
rotates!
retrograde BH
(spinning in opposite
direction as disk).
• ISCO at 6 GM/c2.
• ISCO at 9 GM/c2.
• No frame-dragging:
orbits cease to spiral
in and instead plunge
toward BH inside
ISCO.
• Frame-dragging acts
in opposition to disk
angular momentum,
causing orbits to
plunge farther out.
• ISCO at 1
GM/c2.
• Frame-dragging
rotationally supports
orbits closer to BH
before plunging.
Effect of Spin on Reflection Features
Spectral Complexity
Soft excess
Warm absorption
Fe Kα
Compton hump
MCG—6-30-15
Brenneman & Reynolds (2006),
Miniutti+ (2007),
Chiang & Fabian (2011)
Spectral components with continuum power-law modeled out
Separating Reflection from Absorption
• Multi-epoch & time-resolved spectral analysis assess variability of
three spectral components: continuum, reflection, absorption.
• A physically consistent model should be able to explain ALL the
data: spin, disk inclination, abundances shouldn’t change.
• NuSTAR (launched in June 2012) has high enough collecting area,
spectral resolution and low enough background >10 keV to
differentiate between reflection and absorption (e.g., MCG—6:
Miller, Turner & Reeves 2008 vs. Brenneman & Reynolds 2006).
• When used simultaneously with XMM and/or Suzaku, will achieve
best-ever constraints on BH spin (precision increased by factor ~10,
more confident in measurement accuracy).
reflector
absorbe
r
χ2/ν=1.48
reflector
absorber
Suzaku/PIN
χ2/ν=3.55
NuSTAR
Case Study: NGC 1365
NGC 1365:
Suzaku 2008
Brenneman+ (2013)
NGC 1365:
NuSTAR/XMM 2013
Risaliti+ (2013)
• Suzaku and NuSTAR finally give us access to data >10 keV to
properly measure continuum, Compton hump.
• Allows reflection, continuum, absorption to be separated and
breaks modeling degeneracies, enabling more accurate and precise
spin measurements.
The Distribution of SMBH spins (so far)
Brenneman (SpringerBrief 2013); Reynolds (2013); Walton+ (2013)
Where Do We Go From Here?
• Astro-H (2015): higher E.A., better spectral resolution than
Suzaku
- separate absorption from emission in Fe K band
- break degeneracy between truncated disk and lower spin(?)
- hard X-ray detector also more robust
• ASTROSAT (??): Simultaneous UV & X-ray spectroscopy
- tighter constraints on disk thermal emission, warm
absorption
• ATHENA+/LOFT/other (??): Further large increase in E.A. over
these missions
- probe accretion physics on orbital timescales
- increase sample size of spin measurements by ~few-10x
- trace individual hotspots in the disk, Fe K reverberation
measurements much more robust
Summary
• Narrow and broad Fe K lines seen in many AGN, implying the
existence of distant reflection (most) and inner disk reflection
(⅓ – ½) in these sources.
• Reflection modeling gives SMBH spin constraints now,
though care must be taken in model fitting, assumptions.
• Wide range of measured spins for AGN, but so far all RQAGN
are consistent with a ≥ 0, with average a = 0.6-0.7.
• Larger sample size of AGN spins (esp. RLAGN) must be
obtained with combination of broad-band X-ray time-resolved
spectroscopy, multi-epoch spectroscopy and timing analysis
with various instruments to begin understanding spin
demographics, AGN structure, relation to jets.
• NuSTAR, Astro-H and a next-generation large area X-ray
telescope (ATHENA+, LOFT, other) moving this science toward
the future.
EXTRAS
Fe Kα emission line from different disk annuli
a=0, i=30°, q=3 (disk emits as rq).
400 rg
100 rg
30 rg
RISCO
6 rg
10 rg
Inclination
angle
KERRDISK or RELLINE model (Brenneman & Reynolds 2006; Dauser+ 2010)
SMBH Spins in AGN
• Current sample Size: ~30-40 SMBHs in bright AGN with broad
Fe Kα lines (Miller+ 2007, Nandra+ 2007, de La Calle Pérez+
2010).
- Out of 1011-12 estimated SMBHs in the accessible universe.
- Must have high line EW, high X-ray s/n (≥200,000 photons
from 2-10 keV), and line must be relativistically broad with
rin ≤ 9 rg. Not all type 1 AGN have such features.
• Technique used: Inner Disk Reflection (broad Fe Kα):
KERRCONV, RELCONV or KYCONV × REFLIONX or XILLVER
Brenneman & Reynolds (2006)
Dauser+ (2013)
Dovčiak+ (2004)
García+ (2013)
Ross & Fabian (2005)
CAVEATS:
disk truncation radius
disk ionization, density, Fe abundance
disk irradiation profile
complex absorption, soft excess
The Importance of Time-resolved
Spectroscopy
• AGN are variable on
timescales ranging from hours
to years; gets reflected in
temporal and spectral
variability in X-rays.
• Behavior is commonly seen
in AGN, can be more
pronounced over shorter time
in those with smaller masses.
Marinucci+ (in prep.)
The Importance of Time-resolved
Spectroscopy
• Changes in flux and hardness ratio
can imply significant changes in
spectral shape  roles of different
model components in shaping
spectrum.
• Using only time-averaged
spectra may wash out
important variability
information, e.g., absorbers,
power-law flux changes.
What about the Soft Excess (e.g., NGC 3783)?
• Present in majority of AGN that are not totally absorbed
<2 keV.
• 0.5-2 keV range accounts for most of s/n in AGN
observations due to higher collecting area at these low
energies, so parameterization of this region can highly
influence spectral fitting!
• Physical origin of this emission is still a mystery, may differ
source-to-source (e.g., Crummy+ 2006, Lohfink+ 2013a):
• Scattered emission?
• Comptonization?
• Photoionized lines?
• Blurred relativistic reflection?
• Combination? Something else??
What about the Soft Excess (e.g., NGC 3783)?
Brenneman+ (2011)
Patrick+ (2011)
Χ2ν = 1.27
Χ2ν = 1.26
scattered emission + relativistic reflection
a > 0.98
Comptonization
a < 0.31
Other Factors: Iron Abundance in
NGC 3783
• Fit drives a > 0.88 (99%
conf.), Fe/solar = 2-4 (MCMC).
• Strict assumption of
Fe/solar = 1 worsens fit
significantly, allows for low
spin.
• Supersolar Fe consistent
with measurements from BLR
in other AGN (e.g., Warner+
2004, Nagao+ 2006).
• Caveat: Fe abundance and
spin clearly correlated!
Fe/solar = free, tied
Fe/solar = untied, inner free
Fe/solar = 1 (both)
• More Fe  stronger
reflection more blurring
required to fit data  higher
spin values.
• Illustrates importance of
exploring wide range of
modeling assumptions.
Reynolds+ (2012)
Assumption of ISCO Truncation
Plunging region inside ISCO
BH
3D MHD simulation of a geometrically-thin
accretion disk.
Clearly shows transition at the ISCO which
will lead to truncation in iron line emission.
Rapid drop in τ, rise in ξ within ISCO.
Reynolds & Fabian (2008)
Systematic Error from Emission ≤ISCO
h/r = 0.01
h/r = 0.25
h/r = 0.5
h/r = 1.0
Reynolds & Fabian (2008)
The Distribution of SMBH spins (so far)
Brenneman (SpringerBrief, 2013)
BR06 = Brenneman &
Reynolds (2006)
B11 = Brenneman+ (2011)
P12 = Patrick+ (2012)
L12 = Lohfink+ (2012)
W13 = Walton+ (2013)
Black Hole Spin and Galaxy Evolution
Mergers only
Berti & Volonteri (2008)
Mergers + chaotic
accretion
Mergers + prolonged
accretion
• Mergers of galaxies (and, eventually, their supermassive BHs) result in a
wide spread of spins of the resulting BHs.
• Mergers and chaotic accretion (i.e., random angles) result in low BH
spins.
• Mergers and prolonged, prograde accretion result in high BH spins.
Black Hole Spin and Jet Production
• Blandford & Znajek (1977):
rotating black hole + magnetic
field from accretion disk =
energetic jets of particles along
the BH spin axis.
• Magnetic field lines thread
disk, get twisted by differential
rotation and frame-dragging.
• Results in a powerful outflow,
though many specifics are still
unknown, including how/why
jets launch, dependence on spin,
magnetic field, accretion rate.
• Some observational indication
of spin correlation with jet
power in microquasars… can we
extend to AGN?
Narayan & McClintock (2012), Steiner+ (2012)
Accretion Disk Tomography
• X-ray eclipses of the inner disk by BLR clouds cited in NGC 1365 (e.g.,
Risaliti+ 2011, Brenneman+ 2013) can also differentiate between the
reflection and absorption-only spectral modeling interpretations.
• Can verify the existence of relativistic emission features from the inner
accretion disk by examining change in morphology of putative Fe K line as
the eclipse progresses.
• This type of accretion disk tomography possible for high-contrast
eclipses: e.g., factor ~10 increase in column density during high flux
state.
uneclipsed
red side only
blue side only
Risaliti+ (2011)
Fe Kα Reverberation Mapping
1H0707-495
NGC 4151
Zoghbi+ (2012)
Kara+ (2013)
• Time lags in frequency space  time-lag spectrum over energy in a given source,
probes the location of the emitting regions for relativistically broadened Fe Kα.
• NuSTAR will allow Fe Kα, Compton hump lags to be measured simultaneously!
• Next generation X-ray telescopes (e.g., LOFT) will further improve upon this science.