1. Advection-dominated Accretion: From Sgr A* to Other Low-Luminosity AGNs Feng Yuan (Shanghai Astronomical Observatory) Collaborators: Ramesh Narayan; Eliot Quataert; Rodrigo Nemmen; Wei Cui; Zhiqiang Shen; Sera Markoff; Heino Falcke…

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  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 ~ 10 -9 L EDD ) Useful laboratory for other BH systems

4. 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 ~ 10 -3 M  yr -1 (  2-10 " from BH)

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): this is roughly consistent with the numerical simulation of Cuadra et al. (2006): Temperature: 2keV; Density: 130cm^-3 Angular momentum: quite large, the circularization radius ~10^4 Rs, not a spherical accretion (Cuadra’s talk)

6. 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 ~ 10 36 ergs s-1 ~ 100 L  ~ 10 -9 L EDD ~ 10 -6 M c 2 Flare Quiescence Keck VLT VLA BIMA SMA

7. Observational Results for Sgr A* (II): Variability & Polarization 1.X-ray flare (Baganoff et al. 2001): timescale: ~hour timescale (duration) ~10 min (shortest)  10Rs; timescale: ~hour timescale (duration) ~10 min (shortest)  10Rs; amplitude: can be ~45 amplitude: can be ~45 : timescale (Genzel et al 2003): 2.IR flare: timescale (Genzel et al 2003): ~30-85 min (duration); ~5 min (shortest)  similar to X-ray flare amplitude: 1-5, much smaller than X-ray ~30-85 min (duration); ~5 min (shortest)  similar to X-ray flare 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 (Aitken et al. 1999; Bower et al. 2003; Marrone et al. 2007; Zhao’s talk): (Aitken et al. 1999; Bower et al. 2003; Marrone et al. 2007; Zhao’s talk):

8. 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)

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

10. “Old” ADAF Model for Sgr A* Narayan et al., 1995;1998; Manmoto et al. 1997 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 (adiabatic) 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: Narayan & Yi 1994; Blandford & Begelman 1999; Narayan, Igumenshchev & Abramowicz 2000; Quataert & Gruzinov 2000 ) Very little mass supplied at large radii accretes into the black hole (outflows/convection suppress accretion: Narayan & Yi 1994; Blandford & Begelman 1999; Narayan, Igumenshchev & Abramowicz 2000; Quataert & Gruzinov 2000 ) Electron heating mechanism: direct viscous heating? turbulent dissipation & magnetic reconnection  turbulent dissipation & magnetic reconnection  Particle distribution: nonthermal? –weak shocks & magnetic reconnection –collisionless plasma  nonthermal? (Stone & Pringle 2001; Hawley & Balbus 2002; Igumenshchev et al. 2003) MHD numerical simulation result: (however, collisionless  kinetic theory?)

12. 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

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

14. Summary: the efficiency of RIAF in Sgr A* Mdot ~ 10 -6 M sun /yr, L ~ 10 36 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 ~ 10 -8 M sun /yr, so outflow contributes a factor of 0.01 Mdot BH ~ 10 -8 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)

15. 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. During this process, some fraction of thermal electrons will be heated & accelerated (reconnection current sheet? shock?) The synchrotron & its inverse Compton emissions from these high-energy electrons can explain the IR & X-ray flares detected in Sgr A* Yuan, Quataert & Narayan 2003; 2004; Yuan et al. 2007

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

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

18. The Size Measurements of Sgr A* 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… Bower et al. 2004, Science; Shen et al. 2005, Nature;

19. Testing the RIAF Model with the Size Measurements 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.) 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 Simulating the observed size by taking into account the scattering broadening 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*?? Yuan, Shen & Huang 2006, ApJ

20. Yuan, Shen, & Huang 2006, ApJ 7mm(up) & 3.5mm(lower) simulation results Input intensity profileSimulation resultGaussian fit 7mm 3.5mm Testing the RIAF Model with the Size Measurements

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) L bol / L Edd ~ 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  R in ~ (100-1000)R s

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

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

25. 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

26. M 81 Quataert et al. 1999 R tr ~ 100 R s

27. 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

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, R tr ~ 300 R s 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 R s Yuan, Cui & Narayan 2005 Radiation from the truncated thin disk, with R tr = 300 R s

29. Ellipticals: Fabian & Rees 1995 FR I : Reynolds et al 1996; Begelman & Celloti 2004; Wu, Yuan & Cao 2007 XBONGs: Yuan & Narayan 2004 Seyfert 1 galaxies: Chiang & Blaes 2003 Blazar: Maraschi & Tavecchio 2003 Many questions remain unsolved: The X-ray emission from luminous sources such as quasar and the very high state of X-ray binaries The broad iron line detected in the hard state (if it is true) How to form a jet …… Other examples include: However:

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 example: NGC 4258 –The IR spectrum and the mass accretion rate seem to be hard to explain by an ADAF –A jet can interpret the spectrum if a significant fraction of accretion flow is transferred into the jet; and the underlying accretion flow is described by an ADAF. Yuan, Markoff, Falcke & Biermann 2002

31. Thank you!