sgra

Slide1: 

Accretion onto the Massive Black Hole in the Galactic Center Eliot Quataert (UC Berkeley)

Why focus on the Galactic Center?: 

Why focus on the Galactic Center? Best evidence for a BH (stellar orbits) M  4x106 M Largest BH on the sky (horizon  8 μ") VLBI imaging of horizon X-ray & IR variability probes gas at ~ Rs Extreme low luminosity (L ~ 10-9 LEDD) illuminates accretion physics Most detailed constraints on ambient conditions around BH Feeding the (rather weak, and actually not that impressive) “monster” Stellar dynamics & star formation in Galactic Nuclei Binary BHs Useful laboratory for other BH systems GR! Genzel et al.;also Ghez et al.

Outline: 

Outline How does the gas get from the surrounding medium to the BH? What determines the accretion rate, radiative efficiency, and observed emission from the BH? ?? ??

Fuel Supply: 

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) 1" = 0.04 pc  105 RS @ GC

1D Simulation of Gas Flow in Central Parsec“Cluster Wind” + Accretion onto BH: 

1D Simulation of Gas Flow in Central Parsec “Cluster Wind” + Accretion onto BH ~ 1% of gas flows in towards BH ( 10-5 M yr-1) ~ 99% of gas driven away in a ‘cluster wind’ ( 10-3 M yr-1) Radial Velocity Bondi Accretion Radius BHs ‘sphere of influence’ observed  & T 

Slide6: 

Extended X-ray source coincident w/ the BH is a signature of gas being gravitationally captured from the surrounding star cluster (ala Bondi) Predicted X-ray Surface Brightness Compared to Observations

Slide7: 

Total Luminosity ~ 1036 ergs s-1 ~ 100 L ~ 10-9 LEDD ~ 10-6 M c2 Extensive Linear & Circular Polarization Data In Radio  Chandra Xmm Keck VLT VLA BIMA Scuba SMA … X-ray Flares Inferred efficiency <<<<< ~ 10% efficiency in luminous BHs

Arguments Against Accretion at smaller radii proceeding via an Optically Thick, Geometrically Thin Disk, as in Luminous AGN: 

Arguments Against Accretion at smaller radii proceeding via an Optically Thick, Geometrically Thin Disk, as in Luminous AGN inferred low efficiency where is the expected blackbody emission? observed gas on ~ 1” scales is primarily hot & spherical, not disk-like (w/ tcool >> tflow) absence of stellar eclipses argues against  >> 1 disk (Cuadra et al. 2003)

Radiatively Inefficient Accretion Flow: 

Radiatively Inefficient Accretion Flow Accretion with angular momentum in which Hot optically thin collisionless plasma near BH Tp ~ 1012 K Te ~ 1010-1011 K (particles likely nonthermal) Rotating w/  ~ K but geometrically “thick” (e.g., Ichimaru 1977; Rees et al. 1984; Narayan & Yi 1994) grav. pot. energy stored as thermal energy instead of being radiated

Slide10: 

Initial Models (ADAFs) had Very little mass supplied at large radii accretes into the black hole (outflows/convection suppress accretion) (e.g., Narayan & Yi 1994) Low efficiency because electron heating is assumed to be very inefficient (electrons radiate, not protons) (e.g., Igumenschev & Abramowicz 1999, 2000; Stone et al. 1999; Blandford & Begelman 1999; Narayan et al. 2000; Quataert & Gruzinov 2000; Stone & Pringle 2001; Hawley & Balbus 2002; Igumenschev et al. 2003; Pen et al. 2003) very little radiation because very little gas makes it to the BH Efficiency ~ 10-6

Numerical Simulations: 

Numerical Simulations Hydrodynamic MHD (Igumenshchev & Abramowicz 1999, 2000; Stone et al. 1999) (Stone & Pringle 2001; Hawley & Balbus 2002; Igumenshchev et al. 2003) Theoretical Aside: If magnetic field is “weak” (β > ~ 10), convection dominates flow dynamics If magnetic field is stronger (β ~ 1), MHD turbulence dominates (Narayan, Quataert, Igumenshchev, & Abramowicz 2002)

Are the Simulations Relevant toan Intrinsically Collisionless System?: 

Are the Simulations Relevant to an Intrinsically Collisionless System? Perhaps, but … Physics of angular momentum transport is different in collisionless plasmas Kinetic simulations in progress kinetic MHD Kinetic theory MHD Growth Rate Wavevector Quataert, Dorland, & Hammett 2002 Magnetorotational instability

Preliminary Nonlinear Kinetic Sims: 

Preliminary Nonlinear Kinetic Sims MHD Kinetic Sharma, Hammett, Quataert, & Stone Magnetic Energy Time (Orbital Periods) Kinetic sims initially saturate at much lower field strength (due to anisotropic pressure tensor) Further nonlinear evolution unclear (work in progress …)

Slide14: 

very little mass available at large radii accretes into the BH low accretion rate confirmed by detection of ~ 10% linear polarization in the radio emission from the Galactic Center Faraday Rotation (< 106 rad/m2) constrains the plasma density near the BH (QG 2000; Agol 2000; Bower et al. 2003) Overall Energetics

Slide15: 

X-ray Emission: Quiescent + Flares Several times a day X-ray flux increases by a factor of ~ few-50 for ~ an hour timescale  emission arises close to BH ~ 10 RS Orbital period at 3RS = 28 min

Variable IR Emission(Genzel et al. 2003; Ghez et al. 2003): 

Variable IR Emission (Genzel et al. 2003; Ghez et al. 2003) Time (min) Light crossing time of Horizon: 0.5 min Orbital period at 3RS (last stable orbit for a = 0): 28 min Genzel et al. 2003

Slide17: 

Accretion flow is highly time-dependent, with fluctuations in density, temperature, dissipation of magnetic & kinetic energy, etc. suggests observed variability due to turbulent plasma very close to horizon Hawley

Analogy: Solar Corona: 

Analogy: Solar Corona SOHO Movie of Active Regions (UV) (Solar & Heliospheric Observatory)

Synchrotron Emission from MHD Simulations: 

Synchrotron Emission from MHD Simulations 1mm/300 GHz (thermal; optically thin) Goldston, Quataert, & Igumenshchev 2004

A Day in the Life of Sgr A*: 

A Day in the Life of Sgr A* Factors of ~ 2-5 variability over several hours

Final Ingredient: Particle Acceleration: 

Final Ingredient: Particle Acceleration assume that close to BH ~ 10% of electron thermal energy transiently dumped into a power law tail IR: synchrotron from  ~ 103 e- X-rays: synch. from  ~ 105 e- Prominence of nonthermal emission unsurprising because of collisionless magnetized two-temperature turbulent plasma (in GR!) Quiescent Flares

Why our Galactic Center?: 

Why our Galactic Center? Key is L <<<<< LEDD: analogous ‘flares’ harder to detect in more luminous systems because they are swamped by emission from the bulk (~ thermal) electrons (next best bet is probably M32) GC

Inward Bound: 

Inward Bound GC horizon: RS  1012 cm  4x10-13 rad  8 -arcsec GC is largest BH on the sky! can plausibly be directly imaged with VLBI at mm λ’s in the next ~ decade Size of Sgr A* Bower et al. 2004 Simple extrapolation Size  Horizon as λ  1mm Observed Size (RS) Wavelength (cm)

Inward Bound: 

Inward Bound 30 RS M87 at 7 mm (RS 2 x smaller on sky) Biretta et al. 1999 Shep Doeleman & collaborators have achieved 34” at 1.3 mm on 3C279 (~ 4RS for Sgr A*)

Toy Models Predict a True “Black Hole”(light bending, grav. redshift, photons captured by BH, …  suppression in observed flux from near the BH): 

Toy Models Predict a True “Black Hole” (light bending, grav. redshift, photons captured by BH, …  suppression in observed flux from near the BH) Falcke et al. 2000; based on Bardeen 1973  10 RS

Work in Progress: “Realistic”Images from Simulations: 

Work in Progress: “Realistic” Images from Simulations Newtonian: No GR Transport Yet Encouraging: emission strongly peaked near BH where GR effects important Emission from very small radii also implied by rapid variability  10 RS

A ‘Concordance’ Model of Sgr A*: 

A ‘Concordance’ Model of Sgr A* Stars supply ~ 10-3 M yr-1 to the central pc of the GC ~ 10-5 M yr-1 captured by the BH supported by extended X-ray source coincident w/ BH ~ 10-8 M yr-1 (or perhaps less) accretes onto the BH via a hot radiatively inefficient accretion flow (efficiency > 10-3) most mass driven away rather than accreting onto BH supported by detection of polarization in mm emission Variable IR & X-ray Emission nonthermal synchrotron radiation from accelerated electrons unique probe of gas dynamics and particle accel. very close to BH encouraging for project of imaging horizon of BH Initial Radiatively Inefficient Accretion Models (ADAFs) had