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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… 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 LowLuminosity 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 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 Outer Boundary Conditions at Bondi Radius Bondi radius: GM R A 2 1" 10 5 Rs cs Mass accretion rate estimation M captured 4R A2 c s | R RA 10 5 M yr 1 this is roughly consistent with the numerical simulation of Cuadra et al. . (2006): M 3 10 6 M yr 1 Temperature: 2keV; Density: 130cm^-3 Angular momentum: quite large, the circularization radius ~10^4 Rs, not a spherical accretion (Cuadra’s talk) 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 – flare: phton index=1.3 Total Luminosity ~ 1036 ergs s-1 ~ 100 L ~ 10-9 LEDD ~ 10-6 M c2 Flare VLA BIMA SMA Keck VLT Quiescence Observational Results for Sgr A* (II): Variability & Polarization 1.X-ray flare (Baganoff et al. 2001): timescale: ~hour timescale (duration) ~10 min (shortest)10Rs; amplitude: can be ~45 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 3. Polarization: at cm wavelength: no LP but strong CP; at submm-bump: high LP(7.2% at 230 GHz; <2% at 112 GHz) a strict constraint to density & B field: RM (Faraday rotation measure) can not be too large (Aitken et al. 1999; Bower et al. 2003; Marrone et al. 2007; Zhao’s talk): RM 8.110 ne B r dr (5.6 0.7) 105 rad m2 5 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) R M 4RH v M out Rout dv 1 dp 2 v k 2 r dr dr p 2 v(r j ) r . Mass accretion rate: The radial and azimuthal Components of the momentum Equations: . s The electron energy equation: The ions energy equation: d e p e d q qie q 2 dr dr d p d (1 )q qie v i 2i dr dr v “old” ADAF: s=0; δ<<1 “new” ADAF (RIAF): s>0; δ≤1 “Old” ADAF Model for Sgr A* Narayan et al., 1995;1998; Manmoto et al. 1997 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. – Tp=1012K;Te=109—1010K; 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. 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 & MHD numerical simulation result: (however, collisionlesskinetic theory?) Begelman 1999; Narayan, Igumenshchev & Abramowicz 2000; Quataert & Gruzinov 2000) Electron heating mechanism: direct viscous heating? turbulent dissipation & magnetic reconnection ~ 0.5 Particle distribution: nonthermal? (Stone & Pringle 2001; Hawley & Balbus 2002; Igumenshchev et al. 2003) – weak shocks & magnetic reconnection – collisionless plasmanonthermal? RIAF Model for the Quiescent State Yuan, Quataert & Narayan 2003 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 RIAF Model for Sgr A*: Interpreting the Polarization Result Yuan, Quataert & Narayan 2003 Summary: the efficiency of RIAF in Sgr A* -6 Mdot ~ 10 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 ~ 0.5), 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) Understanding the IR & X-ray flares of Sgr A*: Basic Scenario Yuan, Quataert & Narayan 2003; 2004; Yuan et al. 2007 At the time of flares, at the innermost region of accretion flow, ≤10Rs, 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* Testing the RIAF Model with the Size Measurements Yuan, Shen, & Huang 2006, ApJ Input intensity profile Simulation result 7mm 3.5mm 7mm(up) & 3.5mm(lower) simulation results Gaussian fit 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) 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 Hline Rin ~ (100-1000)Rs Average SED of Low-luminosity AGNs Radio-loud AGNs low-luminosity AGNs, no BBB! L Radio-quiet AGNs Ho (1999) Current Accretion Scenario for Low-luminosity AGNs Jet: radio Transition radius ADAF: X-ray Truncated standard thin disk: T~106Koptical&UV The Transition Radius Two mechanisms for the transition: Evaporation (e.g.,Meyer & MeyerHofmeister, 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 M 81 Quataert et al. 1999 Rtr ~ 100 Rs NGC 1097: the best example? Nemmen et al. 2006 From a truncated thin disk, with Rtr = 225 Rs Double peaked Balmer line Rtr=225Rs, consistent with spectral fitting result! 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 Other examples include: Ellipticals: Fabian & Rees 1995 FRI: 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 However: 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 …… 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 Thank you!