Advection-dominated Accretion:

1
Advection-dominated Accretion From Sgr A to
Other Low-Luminosity AGNs
Feng Yuan Shanghai
Astronomical Observatory
2
Outline

• Sgr A as a unique laboratory for extremely low
  luminosity accretion
• ADAF models for other low-luminosity AGNs
• Complexity

3
Sgr A a Unique Laboratory for Low-Luminosity
Accretion

• Best evidence for a BH (stellar orbits)
• M ? 4x106 M?
• Largest BH on the sky (horizon ? 8 µ"),
• thus most detailed constraints on
• ambient conditions around BH
• Direct observational determination
• to the accretion rate
• Outer boundary conditions
• Abundant observational data
• Detailed SED
• polarization
• X-ray IR flares probe gas at Rs
• Accretion physics at extreme low luminosity (L
  10-9 LEDD)
• Useful laboratory for other BH systems

4
Fuel Supply
IR (VLT) image of central pc
Chandra image of central 3 pc
Baganoff et al.
Genzel et al.
Young cluster of massive stars in the central
pc loses 10-3 M? yr-1 ( ? 2-10" from BH)
Hot x-ray emitting gas (T 1-2 keV n 100
cm-3) produced via shocked stellar winds
5
Outer Boundary Conditions at Bondi Radius

• Bondi radius
• Mass accretion rate estimation
• this is roughly consistent with the numerical
  simulation of Cuadra et al. (2006, MNRAS)
• Temperature 2keV Density 130cm-3
• Angular momentum quite large, the
  circularization radius 104 Rs, not a spherical
  accretion (Cuadra et al.)

6
Observational Results for Sgr A (I) Spectrum

• flat radio spectrum
• submm-bump
• two X-ray states
• quiescent photon indx2.2
• the source is resolved
• flare phton index1.3
• Total Luminosity 1036 ergs s-1
• 100 L? 10-9 LEDD 10-6 M c2

Flare
VLA BIMA SMA
Keck VLT
Quiescence
7
Observational Results for Sgr A (II)
Variability Polarization

• 1.X-ray flare timescale hour timescale
  (duration) 10 min (shortest)
• 
  ?10Rs
• amplitude can be 45
• 2.IR flare timescale 30-85 min (duration) 5
  min (shortest)
• ?similar to
  X-ray flares
• amplitude 1-5, much smaller than X-ray
• 3. Polarization
• at cm wavelength no LP but strong
  CP
• at submm-bump high LP(7.2 at 230
  GHz lt2 at 112
• GHz) ? a strict
  constraint to density B field
• RM (Faraday rotation
  measure) can not be too large

8
The Standard Thin Disk Ruled Out

• inferred low efficiency
• where is the expected
• blackbody emission?
• observed gas on 1 scales
• is primarily hot spherical,
• not disk-like
• absence of stellar eclipses
• argues against ? gtgt 1 disk
• (Cuadra et al. 2003)

9
Radiation-hydrodynamics Equations for ADAF(RIAF)

Mass accretion rate
The radial and azimuthal Components of the
momentum Equations
The electron energy equation
The ions energy equation
old ADAF s0 dltlt1 new ADAF (RIAF) sgt0 d1
10
Old ADAF Model for Sgr ANarayan et al.,
19951998

• The old ADAF (e.g., Ichimaru 1977 Rees et al.
  1982 Narayan Yi 19941995 Abramowicz et al.
  1995)
• ADAF most of the viscously dissipated energy is
  stored in the thermal energy and advected into
  the hole rather than radiated away.
• Tp1012KTe1091010K ?geometrically thick
• Accretion rate const.
• Efficiencyltlt0.1, because electron heating is
  inefficient
• Success of this ADAF model
• low luminosity of Sgr A
• rough fitting of SED
• Problems of this ADAF model
• predicted LP is too low because RM is too large
• predicted radio flux is too low.

11
Theoretical Developments of ADAF

• Outflow/convection
• Very little mass supplied at large radii
  accretes into the black hole (outflows/convection
  suppress accretion)
• Electron heating mechanism direct viscous
  heating?
• turbulent dissipation magnetic
  reconnection?
• Particle distribution nonthermal?
• (1) e..g., weak shocks magnetic
  reconnection (2) collisionless plasma
• ?nonthermal?

MHD numerical simulation result (however,
collisionless-?kinetic theory?)
(Stone Pringle 2001 Hawley Balbus 2002
Igumenshchev et al. 2003)
12
RIAF Model for the Quiescent State
total emission from both thermal and power-law
electrons
synchrotron emission from power-law electrons
synchrotron, bremsstrahlung and their
Comptonization from thermal electrons
bremsstrahlung from the transition region around
the Bondi radius
Yuan, Quataert Narayan 2003
13
RIAF Model for Sgr A Interpreting the
Polarization Result
Yuan, Quataert Narayan 2003
14
Understanding the IR X-ray flares of Sgr A
Basic Scenario

• At the time of flares, at the innermost region of
  accretion flow, 10Rs, some transient events,
  such as magnetic reconnection (solar flares!),
  occur.
• These processes will heat/accelerate some
  fraction of thermal electrons in accretion flow
  to very high energies.
• The synchrotron its inverse Compton emissions
  from these high-energy electrons can explain the
  IR X-ray flares detected in Sgr A

15
Understanding the IR X-ray flares of Sgr A
Basic Scenario
Machida Matsumoto, 2003, ApJ
16
Synchrotron SSC models for IR X-ray flares
Power-law electrons With p1.1, R2.5Rs
630.
Yuan, Quataert, Narayan 2003, ApJ
17
The Size Measurements of Sgr A
Bower et al. 2004, Science Shen et al. 2005,
Nature

• An independent test to accretion models
• Observed size of Sgr A(FWHM)
• 7mm 0.712 mas (Bower et al.) or 0.724 mas (Shen
  et al. )
• 3.5mm 0.21 mas (Shen et al.)
• Intrinsic size of Sgr A(by subtracting the
  scattering size)
• 7mm 0.237 mas (Bower et al. ) or 0.268 mas (Shen
  et al.)
• 3.5mm 0.126 mas (Shen et al.)
• Note the results require the intrinsic intensity
  profile must be well characterized by a Gaussian
  profile. However, this may not be true

18
Testing the RIAF Model with the Size Measurements
Yuan, Shen Huang 2006, ApJ

• Calculating the intrinsic intensity profile from
  RIAFs---not Gaussian
• Assumptions Schwarzschild BH face-on RIAF
• Taking into account the relativistic effects
  (gravitational redshift light bending Doppler
  boosting ray-tracing calculation) again not
  Gaussian
• We therefore simulate the observed size by taking
  into account the scattering broadening and
  compare it with observations
• Results
• 7mm 0.729 mas (observation 0.712 0.724 mas)
• 3.5 mm 0.248 mas (observation 0.21 mas)
• Slightly larger a rapidly rotating BH in Sgr A??

19
Input intensity profile
Simulation result
Gaussian fit
7mm(up) 3.5mm(lower) simulation results
Yuan, Shen, Huang 2006, ApJ
20
Summary the efficiency of RIAF in Sgr A

• Mdot 10-6 Msun/yr, L 1036erg/s, so efficiency
  10-6
• In the old ADAF(no outflow), this low
  efficiency is due to the inefficient electron
  heating (or ion energy advection)
• In the new ADAF (with outflow and ),
• MdotBH 10-8Msun/yr, so outflow contributes
  a factor of 0.01
• The other factor of 10-4 is due to electron
  energy advection the energy heating electrons is
  stored as their thermal energy rather than
  radiated away (electron energy advection)

21
When the luminosity/accretion rate increases...
22
Low-luminosity AGNs Observations

• LLAGNs are very common, over 40 of nearby
  galaxies contain LLAGNs (Ho et al. 1997)
• Lbol / LEdd 10-5 -- 10-3
• Given the available accretion rates, the
  efficiency should be 1-4 orders of magnitude
  lower than 0.1 (Ho 2005)
• Unusual SED no BBB
• No broad iron K line
• Double-peaked H line ? Rin (100-1000)Rs

23
Average SED of Low-luminosity AGNs
Radio-loud AGNs
low-luminosity AGNs, no BBB!
L
Radio-quiet AGNs
Ho (1999)
24
Current Accretion Scenario for Low-luminosity
AGNs
Jet radio
Transition radius
ADAF X-ray
Truncated standard thin disk T106K?opticalUV
25
The Transition Radius

• Two mechanisms for the transition
• Evaporation
• (e.g.,Meyer  Meyer-Hofmeister, 1994 Liu,
  Meyer  Meyer-Hofmeister, 1995 Liu et al. 1999
  Rózanska Czerny 2000)
• Turbulent energy transportation
• (e.g., Honma 1996 Manmoto Kato 2000)

Transition radius vs. luminosity from Yuan
Narayan 2004
26
M 81
Quataert et al. 1999
Rtr 100 Rs
27
NGC 1097 the best example?
Nemmen et al. 2006
From a truncated thin disk, with Rtr 225 Rs
Double peaked Balmer line? Rtr225Rs, consistent
with spectral fitting result!
28
Hard state of black hole X-ray binary XTE
J1118-480

• Hard state of black hole X-ray binary is
  generally assumed to be the analogy of LLAGNs or
  Seyfert galaxies.
• The value of the transition radius is well
  determined by the EUV data, Rtr 300 Rs
• A QPO of frequency 0.07---0.15 Hz is detected
• If we explain the QPO as the p-mode oscillation
  of the ADAF, this QPO frequency also suggests
  that the transition radius to be 300 Rs

Yuan, Cui Narayan 2005
Radiation from the truncated thin disk, with Rtr
300 Rs
29
Other examples include

• Ellipticals Fabian Rees 1995
• FRI Reynolds et al 1996 Begelman Celloti 2004
• XBONGs Yuan Narayan 2004
• Seyfert 1 galaxies Chiang Blaes 2003
• Blazar Maraschi Tavecchio 2003

However
Although ADAF works well for Sgr A and some
LLAGNs, many details of ADAF need to be
investigated (e.g., the dynamical role of
magnetic field the 2-D solution-outflow the
transition mechanism between SSD and ADAF jet
formation), modeling to more sources is
required to check deepen our understanding to
the accretion process.
30
One example of complexity the role of jet in
LLAGNs

• It is almost certain the radio emission comes
  from jets but it is possible that for some
  sources jets also dominate the emission at other
  wavebands.
• One possible example NGC4258
• The IR spectrum and the mass accretion rate seem
  to argue against an ADAF for the emission
• A jet can interpret the spectrum if 1) a
  significant fraction of accretion flow is
  transferred into the jet and 2) the underlying
  accretion flow is described by an ADAF.

Yuan, Markoff, Falcke Biermann 2002
31
Thank you!