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Accretion Model of Sgr A* in Quiescence

Accretion Model of Sgr A* in Quiescence. Ramesh Narayan. Ultra-Dim Galactic Nuclei. Sgr A* exemplifies an old and famous problem: Why are SMBHs in the nuclei of normal (non-AGN) galaxies so dim? True, the gas supply is less

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Accretion Model of Sgr A* in Quiescence

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  1. Accretion Model of Sgr A* in Quiescence Ramesh Narayan

  2. Ultra-Dim Galactic Nuclei • Sgr A* exemplifies an old and famous problem: Why are SMBHs in the nuclei of normal (non-AGN) galaxies so dim? • True, the gas supply is less • But the gas supply is less by only a few orders of magnitude, not by 8 orders of magnitude as Sgr A*’s luminosity suggests • Apart from the ultra-low luminosity, the spectrum also suggests something other than the usual thin accretion disk found in QSOs and bright AGN • What is the mode of accretion, and what determines the luminosity? • If we could figure this out in Sgr A* it would help us to understand a large class of galactic nuclei, and also to figure out quasar evolution

  3. Luminosity of Sgr A* • MBH ~ 4x106 M(Schodel et al. 2003; Ghez et al. 2003) • Sgr A* is extremely dim: (Baganoff et al. 2001ab; Genzel et al. 2003; Ghez et al. 2003) Sgr A*

  4. Bondi Accretion Rate in Sgr A* • Thermal gas with kT~1 keV seen near Galactic Center. Gas with kT~ 4 keV spatially resolved at ~1 arcsec105 RS around Sgr A* • Capture radius for Bondi accrn is ~105 RS , so Bondi accrn rate can be estimated accurately: • The accretion is highly radiatively inefficient: • Even more true if some emission is from a jet, or if there are other sources of gas Baganoff et al. (2001)

  5. Thin Accretion Disk (Shakura & Sunyaev 1973; Novikov & Thorne 1973;…) Radiatively efficient Advection-Dominated Accretion Flow, ADAF (Ichimaru 1977; Rees et al. 1982; Narayan & Yi 1994, 1995; Abramowicz et al. 1995) Radiatively inefficient Two Kinds of Accretion

  6. Why Is the Flow Advection-dominated? • Radiation comes primarily from electrons • At low , ion-electron (Coulomb) coupling is weak • Plasma becomes two-temperature --- heat energy is locked up in the ions and advected to the center • Radiative efficiency of electrons is also low, so electrons also advect their energy • Very hot, optically thin gas. Quasi-spherical. Non-blackbody spectrum (Shapiro, Lightman & Eardley 1976; Ichimaru 1977; Bisnovatyi–Kogan & Lovelace 1997; Quataert 1998; Gruzinov 1998; Quataert & Gruzinov 1998 ; Blackman 1998; Medvedev 2000)

  7. ADAF Models of Sgr A* • Melia (1992, 1994,…) (Bondi model, no rotn, 1-T) • Narayan, Yi & Mahadevan (1995) • Fabian & Rees (1995) • Manmoto, Mineshige & Kusunose (1997) • Narayan et al. (1998) • Mahadevan (1998) • Quataert & Narayan (1999) • Manmoto (2000) • Ozel, Psaltis & Narayan (2000) • Quataert (2002) • Yuan, Quataert & Narayan (2003)

  8. Not All the Available Gas Accretes • ADAFs are likely to have strong outflows (Narayan & Yi 1994, 1995; Blandford & Begelman 1999; Stone et al. 1999; Igumenshchev et al. 1999, 2000; Hawley & Balbus 2002) and also to be strongly convective (Narayan & Yi 1994; Narayan, Igumenshchev & Abramowicz 2000; Quataert & Gruzinov 2000) • For both reasons, accretion onto the BH is significantly reduced (true also for Bondi accretion, cf. Igumenshchev & N 2001): • Radio polarization data (Aitken et al. 2000;Bower et al. 2003) constrain gas density at small radii (Quataert & Gruzinov 2000; Agol 2000) and help determine s

  9. Quiescent Model of Sgr A* • Chandra gives density at capture radius because 50-100% of X-rays is resolved (likely bremsstrahlung --- Baganoff et al. 2001) • Polarization data constrain density near the BH s ~ 0.3 • Set viscosity parameter =0.1 and magnetic field strength plasma=10 (from MHD simulations), but results are insensitive • Need to choose , the fraction of viscous heat that goes into electrons:  ~ 0.5 is natural and works fine • Assume thermal distribution for the bulk of the electrons, but allow a fraction of the electrons to be nonthermal(Mahadevan 1998; Ozel et al. 2000; Yuan, Quataert & Narayan 2003)

  10. ADAF Model of Sgr A* with Only Thermal Electrons • Low luminosity is explained!! • Sub-mmpeak in the spectrum explained naturally as synchrotron emission from thermal electrons near BH • X-ray emission is mostly bremsstrahlung from outer electrons • Disagreement in the radio can be explained with a small fraction of nonthermal electrons (Mahadevan 1998; Ozel et al. 2000) • This would also help fit the new quiescent IR data

  11. Model with Both Thermal and Nonthermal Electrons • Assume that a fraction  of the electrons have a power-law distribution with index p: n() ~ -p • Obtain a reasonable fit to the spectrum with  = 0.015 and p = 3 • Takes care of both the radio and IR data Yuan, Quataert & Narayan (2003)

  12. The Model Satisfies the Polarization Constraints Bower et al. (2003); Yuan et al. (2003)

  13. What Have We Learned from Sgr A*? • Accretion mode is different from a thin disk --- radiatively inefficient hot two-temperature accretion flow (ADAF). Dim galactic nuclei are not just dim versions of AGN. Different physics!! • Everything conspires to make Sgr A* ultra-dim: • Less gas  (bright AGN ~MdotEdd) • Choked off accretion  • Radiatively inefficient  • Mostly thermal electrons, but also an important fraction of nonthermal electrons

  14. Accretion versus Jet • Not much difference between the inner regions of the accretion flow and the base of the jet • Some radiation may come from accretion flow and some from jet • If a large fraction of luminosity is from jet, then accretion flow is even dimmer! • All models are basically ADAFs: Bondi, RIAF, ADIOS, ADAF-jet,… Horizon swallows energy

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