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 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

Bright (High-z) AGN Plenty of gas available Accretion at nearly the Eddington rate Shakura-Sunyaev thin disk Big Blue Bump identified with blackbody radiation from the disk X-rays produced by a corona Infrared from reprocessing by dust Krolik (1999)

Luminosity of Sgr A * M BH ~ 4x10 6 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*

Bondi Accretion Mass M in a uniform medium of density  and sound speed c s Capture radius R c =GM/c s 2 Gas free-falls from R c onto M If gas has angular momentum, it should form a viscous accretion disk  c s RcRc M

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  10 5 R S around Sgr A * Capture radius for Bondi accrn is ~10 5 R S, 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)

Dim Nuclei in Giant Ellipticals Fabian & Canizares (1988) first pointed out a famous problem: The nuclei of nearby giant ellipticals are much too dim compared to the Bondi accretion rate estimated from their X-ray cooling flows: Chandra has brought the problem into sharp focus. Because the spatial resolution is of order the capture radius R c for several galaxies, Chandra gives an accurate estimate of Mdot Bondi for these systems NGC 6166, M87 (Di Matteo et al. 2001, 2003), NGC 1399, 4472 (Loewenstein et al. 2001)

Spectrum  Not a Thin Disk Shape of the spectrum is completely unlike thin disk Sgr A * cannot have a thin disk because of low IR flux Not even a dead disk with Mdot =0 is allowed, because it would produce too much IR when the infalling gas strikes the surface (Falcke & Melia 1997; Coker et al. 1999) Thin disk model with Mdot even as low as M  /yr is ruled out by the infrared data

Two Kinds of Accretion 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

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)

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)

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

Quiescent Model of Sgr A* Chandra gives density at capture radius because % 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)

ADAF Model of Sgr A* with Only Thermal Electrons Low luminosity is explained!! Sub-mm peak 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

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  = and p = 3 Takes care of both the radio and IR data Yuan, Quataert & Narayan (2003)

Model Radial Profiles Yuan et al. (2003)

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

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

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