Mystery and Predictions for Accretion onto Sgr A* Ue-Li Pen 彭威禮 CITA, Univ. of Toronto With: B. Pang, C. Matzner (Toronto), S. Green (Chicago), M. Liebendorfer (Basel)

Baganoff et al (Chandra) 1” Resolution=0.04 pc 21 pc Genzel et al

Baganoff et al (Chandra) 1” Resolution=0.04 pc 21 pc Genzel et al

Outline 1. Theoretical difficulties with Sgr A*’s luminosity 2. A few proposals for overcoming them 3. A classification scheme for models 4. Our simulations: “magnetic frustration” 5. Some implications and caveats

The Problem: Energetics. Inside R B BH force dominates Bondi Estimated radiative efficiency

Chandra 1” Resolution = 0.04 pc R B = 0.04 pc Observed: a low-contrast X-ray source, L X ~ erg/s Predicted: a brilliant source, L X ~ erg/s An Immense Discrepancy

More Energetics Cannot form disk even at Bondi radius (Nayashkin). Serious challenge for all proposed solutions (ADIOS, CDAF)

1. Theoretical difficulties with Sgr A*’s luminosity 2. A few proposals for overcoming them 3. A classification scheme for models 4. Our simulations: “magnetic frustration” 5. Some implications and caveats

Proposed Solution: Advection-Dominated Accretion Flows (ADAFs: Narayan & Yi) -Inflow rate close to Bondi’s rate -Rotation-supported gas spirals in -If only Coulomb collisions heat electrons, radiation is very inefficient

Yes No 1.Some nearby Low-Luminosity AGN (e.g., M87) appear to have 1.ADAFs can roughly match the broadband spectrum 1.Observed submillimeter polar- ization from inner accretion flow implies [Bower] 2.X-ray background indicates AGN are bright when they acquire their mass 3.A stunning suppression of electron temperature is required to keep  so low 4.ADAFs are not stable and contain a positive Bernoulli constant

Proposed Revision: Advection-Dominated Inflow-Outflow Solution (ADIOS: Blandford & Begelman) -A constant fraction of inflow gets returned in every decade of radius. Energetics?

Dynamical problems with ADAFs, #2: They’re unstable and could become convective. Proposed Revision: Convection-Dominated Accretion Flow (CDAF: Quataert & Gruzinov) -Original CDAF: Rotation-supported – problem at pole? -Supersonic convection: outflow at Bondi rate. Energetic conversion near horizon. Mass flow in, energy flow out. Flattened profile

1. Theoretical difficulties with Sgr A*’s luminosity 2. A few proposals for overcoming them 3. A classification scheme for models 4. Our simulations: “magnetic frustration” 5. Some implications and caveats

Transport Thermal Energy: in/out Mass: in/out Angular momentum: in/out Magnetic Flux:

Free-free luminosity predominantly from ~R B Central luminosity spike log  log r r -n n 3/21/215/4 CDAF Bondi ADAF Hot inflows: density index

3 n 3/2 1/21 log  log r r -n 5/3 32 4/3 flatsteep softstiff Bondi solutions Hydrostatic profiles Entropy can only increase (2d law of thermodynamics) All hydrostatic atmospheres must be unstable

n 3/2 1/21 3 5/3 32 4/3 flatsteep softstiff Bondi solutions Hydrostatic profiles

Why n=1/2? Saturation: n=1/2 CDAF uses rotation

1. Theoretical difficulties with Sgr A*’s luminosity 2. A few proposals for overcoming them 3. A classification scheme for models 4. Our simulations: “magnetic frustration” 5. Some implications and caveats

[Show Simulation Animation]

Support is hydrostatic

Magnetic stress balances buoyancy exactly

Rotation unimportant Convective velocity not in equipartition with buoyancy

Rotation unimportant Convective velocity not in equipartition with buoyancy Magnetic stress balances buoyancy exactly Magnetically Frustrated Convection

This talk 1. Theoretical difficulties with Sgr A*’s luminosity 2. A few proposals for overcoming them 3. A classification scheme for models 4. Our simulations: “magnetic frustration” 5. Some implications and caveats

Comparison ModelMLcLc nAng MomChallenge ADAFin 3/2outη<<1 CDAF QG 001/2inpole CDAF Iinout1/20Bondi L ADIOSinout??Phy model BDAFin 1outBC

Predictions Faraday Rotation Measure

RM Time scale - electrons relativistic at r<100 r_S - RM arises in non-relativistic electorns - BDAF predicts coherence over years, not days

Observational Prospects - multi-frequency RM (-500,000 rad/m 2 ) has only been measured once (Marrone 2007), GHz - potentially rules out ADAF/CDAF - multi-year monitoring needed - lower frequencies (40 GHz): EVLA, ATCA, VLBI

Conclusions - Magnetically-frustrated accretion is intermediate between ADAF & CDAF - self-consistent, BC consistent numerical solution - makes testable predictions for RM

Caveats Unresolved inner physics dominates global energetics Direction of convective flux depends on central boundary condition Mass & energy input from stars ignored Inflow slower than slow cooling at R B How strong must B be? Proga and Begelman BC?

The End