mystery and predictions for accretion onto sgr a* ue-li pen 彭威禮 cita, univ. of toronto with:...

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. 2001(Chandra)

1” Resolution=0.04 pc

21 pc

sun6104.2 MM

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 RB BH force dominates

Bondi 1952

sBB

sB

cRM

c

MR

02

2

4

pc04.0G

+ Estimated radiative efficiency

?%10

2

cML B

Chandra 1” Resolution = 0.04 pcRB = 0.04 pc

Observed: a low-contrastX-ray source, LX ~ 1033 erg/s

Predicted: a brilliant source, LX ~ 1039 erg/s

An Immense Discrepancy

!10%10

6

BM

M

More Energetics

Cannot form disk even at Bondi radius (Nayashkin).

Serious challenge for all proposed solutions (ADIOS, CDAF)

BsB LcM 2

Proposed Solution: Advection-Dominated Accretion Flows (ADAFs: Narayan & Yi)

710~ -Inflow rate close to Bondi’s rate-Rotation-supported gas spirals in-If only Coulomb collisions heat electrons, radiation is very inefficient

?BMM

Yes No

1. Some nearby Low-Luminosity AGN (e.g., M87) appear to have

1. ADAFs can roughly match the broadband spectrum

2jet 1.0~ cML B

)50/(~ HubbletMM B

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

BMM

Proposed Revision: Advection-Dominated Inflow-Outflow Solution (ADIOS: Blandford & Begelman)

-A constant fraction of inflow gets returned in every decade of radius. Energetics? BMM

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.

2/1~ rFlattened profile

BMM

Transport

Thermal Energy: in/out

Mass: in/out

Angular momentum: in/out

Magnetic Flux:

Free-free luminositypredominantly from ~RB

Centralluminosity spike

log

log r

r -n

n3/21/2 1 5/4

BMM CDAF Bondi

ADAF

Hot inflows:density index

3n

3/21/2 1

log

log r

r -n

5/33 2 4/3

flat steep

softstiff

Bondisolutions

Hydrostatic profiles

Entropy can only increase (2d law of thermodynamics) effAll hydrostatic atmospheres must be unstable

efftmtmPneff

),(),(where1

1

n3/21/2 1 3

5/33 2 4/3

flat steep

softstiff

Bondisolutions

Hydrostatic profiles

Why n=1/2?

KepcHydrostati v sc

2/1conv

conv2/1

conv22

conv

const

4

n

s

s

n

s

rc

cr

crL

v

v

v

Saturation: n=1/2

CDAF uses rotation

[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

MagneticallyFrustratedConvection

This talk






2/1conv

conv2/1

conv22

conv

const

4

n

s

s

n

s

rc

cr

crL

v

v

v

ComparisonModel M Lc

n Ang Mom Challenge

ADAF in in 3/2 out η<<1

CDAF QG

0 0 1/2 in pole

CDAF I in out 1/2 0 Bondi L

ADIOS in out ? ? Phy model

BDAF in in 1 out BC

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/m2) has only been measured once (Marrone 2007), 200-300 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 RB

•How strong must B be? •Proga and Begelman BC?

The End

mystery and predictions for accretion onto sgr a* ue-li pen 彭威禮 cita, univ. of toronto with:...

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