Advection-dominated Accretion: From Sgr A* to Other Low-Luminosity AGNs Feng Yuan Shanghai Astronomical Observatory

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

Sgr A*: a Unique Laboratory for Low- Luminosity Accretion Best evidence for a BH (stellar orbits) –M  4x10 6 M  Largest BH on the sky (horizon  8 μ" ), thus most detailed constraints on thus most detailed constraints on ambient conditions around BH ambient conditions around BH –Direct observational determination to the accretion rate to the accretion rate –Outer boundary conditions Abundant observational data: –Detailed SED –polarization –X-ray & IR flares probe gas at ~ R s Accretion physics at extreme low luminosity (L ~ L EDD ) Useful laboratory for other BH systems

Fuel Supply IR (VLT) image of central ~ pc Chandra image of central ~ 3 pc Genzel et al. Baganoff et al. Hot x-ray emitting gas (T = 1-2 keV; n = 100 cm -3 ) produced via shocked stellar winds Young cluster of massive stars in the central ~ pc loses ~ M  yr -1 (  2-10 " from BH)

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): 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 ~10^4 Rs, not a spherical accretion (Cuadra et al.)

Observational Results for Sgr A* (I): Spectrum flat radio spectrum submm-bump two X-ray states –quiescent: photon indx=2.2 the source is resolved the source is resolved –flare: phton index=1.3 Total Luminosity ~ ergs s-1 ~ 100 L  ~ L EDD ~ M c 2 Flare Quiescence Keck VLT VLA BIMA SMA

Observational Results for Sgr A* (II): Variability & Polarization 1.X-ray flare: timescale: ~hour timescale (duration) ~10 min (shortest)  10Rs;  10Rs; amplitude: can be ~45 amplitude: can be ~45 : timescale: ~30-85 min (duration); ~5 min (shortest) 2.IR flare: timescale: ~30-85 min (duration); ~5 min (shortest)  similar to X-ray flares;  similar to X-ray flares; amplitude: 1-5, much smaller than X-ray amplitude: 1-5, much smaller than X-ray 3. Polarization: at cm wavelength: no LP but strong CP; at cm wavelength: no LP but strong CP; at submm-bump: high LP(7.2% at 230 GHz; <2% at 112 at submm-bump: high LP(7.2% at 230 GHz; <2% at 112 GHz)  a strict constraint to density & B field: GHz)  a strict constraint to density & B field: RM (Faraday rotation measure) can not be too large: RM (Faraday rotation measure) can not be too large:

The Standard Thin Disk Ruled Out 1.inferred low efficiency 2.where is the expected blackbody emission? 3.observed gas on ~ 1” scales is primarily hot & spherical, not disk-like 4.absence of stellar eclipses argues against  >> 1 disk (Cuadra et al. 2003)

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: s=0; δ <<1 “new” ADAF (RIAF): s>0; δ≤1

“Old” ADAF Model for Sgr A* Narayan et al., 1995;1998 The “old” ADAF The “old” ADAF (e.g., Ichimaru 1977; Rees et al. 1982; Narayan & Yi 1994;1995; 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. –T p =10 12 K;T e =10 9 —10 10 K;  geometrically thick –Accretion rate = const. –Efficiency<<0.1, 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.

Theoretical Developments of ADAF Outflow/convection Very little mass supplied at large radii accretes into the black hole (outflows/convection suppress accretion) 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  turbulent dissipation & magnetic reconnection  Particle distribution: nonthermal? (1) e..g., weak shocks & magnetic reconnection (2) collisionless plasma (1) e..g., weak shocks & magnetic reconnection (2) collisionless plasma  nonthermal?  nonthermal? (Stone & Pringle 2001; Hawley & Balbus 2002; Igumenshchev et al. 2003) MHD numerical simulation result: (however, collisionless-  kinetic theory?)

RIAF Model for the Quiescent State synchrotron emission from power-law electrons synchrotron, bremsstrahlung and their Comptonization from thermal electrons bremsstrahlung from the transition region around the Bondi radius total emission from both thermal and power-law electrons Yuan, Quataert & Narayan 2003

RIAF Model for Sgr A*: Interpreting the Polarization Result Yuan, Quataert & Narayan 2003

Understanding the IR & X-ray flares of Sgr A*: Basic Scenario At the time of flares, at the innermost region of accretion flow, ≤10R s, 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*

Understanding the IR & X-ray flares of Sgr A*: Basic Scenario Machida & Matsumoto, 2003, ApJ

Synchrotron & SSC models for IR & X-ray flares Yuan, Quataert, Narayan 2003, ApJ Power-law electrons With p=1.1, R=2.5Rs =630.

The Size Measurements of Sgr A* An independent test to accretion models Observed size of Sgr A*(FWHM): –7mm: mas (Bower et al.) or mas (Shen et al. ) –3.5mm: 0.21 mas (Shen et al.) Intrinsic size of Sgr A *(by subtracting the scattering size) –7mm: mas (Bower et al. ) or mas (Shen et al.) –3.5mm: 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… Bower et al. 2004, Science; Shen et al. 2005, Nature;

Testing the RIAF Model with the Size Measurements 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: mas (observation: & mas) –3.5 mm: mas (observation: 0.21 mas) –Slightly larger: a rapidly rotating BH in Sgr A*?? Yuan, Shen & Huang 2006, ApJ

Yuan, Shen, & Huang 2006, ApJ 7mm(up) & 3.5mm(lower) simulation results Input intensity profileSimulation resultGaussian fit

Summary: the efficiency of RIAF in Sgr A* Mdot ~ M sun /yr, L ~ erg/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 ), Mdot BH ~ M sun /yr, so outflow contributes a factor of 0.01 Mdot BH ~ M sun /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)

When the luminosity/accretion rate increases…...

Low-luminosity AGNs: Observations LLAGNs are very common, over 40% of nearby galaxies contain LLAGNs (Ho et al. 1997) L bol / L Edd ~ 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  R in ~ ( )R s

Average SED of Low-luminosity AGNs Ho (1999) Radio-loud AGNs Radio-quiet AGNs low-luminosity AGNs, no BBB! L

Current Accretion Scenario for Low-luminosity AGNs Jet: radio Truncated standard thin disk: T~10 6 K  optical&UV ADAF: X-ray Transition radius

The Transition Radius Two mechanisms for the transition: 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) (e.g., Honma 1996; Manmoto & Kato 2000) Transition radius vs. luminosity; from Yuan & Narayan 2004

M 81 Quataert et al R tr ~ 100 R s

NGC 1097: the best example? Double peaked Balmer line  R tr =225R s, consistent with spectral fitting result! From a truncated thin disk, with R tr = 225 R s Nemmen et al. 2006

Hard state of black hole X-ray binary: XTE J 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, R tr ~ 300 R s A QPO of frequency 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 R s Yuan, Cui & Narayan 2005 Radiation from the truncated thin disk, with R tr = 300 R s

Ellipticals: Fabian & Rees 1995 FR I : Reynolds et al 1996; Begelman & Celloti 2004 XBONGs: Yuan & Narayan 2004 Seyfert 1 galaxies: Chiang & Blaes 2003 Blazar: Maraschi & Tavecchio 2003 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. Other examples include: However:

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

Thank you!