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Simulating the Extreme Environment Near Luminous Black Hole Sources. Omer Blaes University of California, Santa Barbara. Collaborators. Spectral calculations: Shane Davis, Ivan Hubeny, Julian Krolik Simulations: Shigenobu Hirose , Julian Krolik,

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slide1

Simulating the Extreme Environment

Near Luminous Black Hole Sources

Omer Blaes

University of California, Santa Barbara

slide2

Collaborators

Spectral calculations: Shane Davis, Ivan Hubeny,

Julian Krolik

Simulations: Shigenobu Hirose, Julian Krolik,

Jim Stone, Neal Turner

Observers: Chris Done

slide3

Outline

  • Observational Context - Black Hole X-ray Binaries
  • Physical Ingredient 1: Magnetorotational Turbulence
  • Physical Ingredient 2: Radiative Diffusion
  • The Most Recent Thermodynamically Consistent
  • Stratified Shearing Box Simulation
  • Implications and Future Work
slide5

-Charles &

Coe (2003)

slide6

¤

-2´v

v

slide7

ISCO

Black hole accretion is a POWERFUL source of energy!

slide8

-no jet whatsoever

- jet always present

-Remillard (2005)

slide9

Thermal State

Hard State?

Steep Power Law State???

slide10

Implies that

there is a fixed

emitting area,

because of the

ISCO???

Luminosity vs. Temperature in the Thermal Dominant State

Luminosity

Maximum Temperature

-Gierlinski & Done (2004)

slide11

LMC X-3 in the thermal dominant state

BeppoSAX

RXTE

-Davis, Done, & Blaes (2006)

Such detailed spectral fits can potentially constrain the spin of

the black hole, thereby completely determining the spacetime.

But there are uncertainties…

slide12

Accretion power is fundamentally the release of gravitational

binding energy, which can only take place in a disk if

fluid elements can give up their angular momentum:

slide13

Accretion Disk Theory is Undergoing a (Slow) Revolution

Mantra in the 70’s and 80’s: the biggest uncertainty is

the cause of the anomalous stress (“viscosity”) responsible

for outward angular momentum transport.

Shakura & Sunyaev (1973)

3075 citations

and counting…

slide14

Magnetorotational Instability (MRI)

-Balbus & Hawley

1991, 1998

slide15

Magnetorotational Instability (MRI)

B

B

W

W

rotates faster

rotates slower

Magnetic fields in a conducting, rotating plasma behave

EXACTLY like springs!

slide16

Snapshot of

angular momentum

per unit mass in

MRI turbulence.

-Hawley & Balbus (1992)

slide18

There Are MAJOR Uncertainties in the Inner, Most Luminous

Regions, Which are Dominated by Radiation Pressure

  • Chief among these is the prediction of standard (Shakura
  • & Sunyaev) models that the disk is thermally unstable when
  • radiation pressure dominates gas pressure.
  • IF rPrad, then dissipation is proportional to T8, while
  • cooling is proportional to T4, implying a thermal instability.
  • But does the turbulent stress really work this way? People
  • have tried all sorts of choices when building models:
  • rPradrPgasrPgasPrad)1/2
  • How does MRI turbulence behave in this regime?
slide20

Radiation Pressure Dominated Plasma

Is Fragile

Subsonic fluid motions

are generally incompressible:

if fluid is slowly squeezed in

one direction, pressure has

time to force it to expand in

another direction, so density

remains approximately

constant.

slide21

Suppose now that we squeeze

the fluid slowly enough that

photons can diffuse out of the

region faster than the squeezing

is taking place. Then radiation

pressure will NOT build up.

If motions are subsonic, but

supersonic with respect to the

much smaller gas sound speed,

then considerable compression

can occur. Radiation pressure can’t

build up because of diffusion, and

gas pressure does not have time

to act.

g

slide23

“Photon Bubble Instability”

F

g

-Turner et al. (2005)

slide24

z (vertical)

y (azimuthal)

x (radial)

The Stratified Shearing Box

Cartesian box corotating with fluid at

center of box. Boundary conditions are

periodic in y,

shearing periodic in x,

outflow in z.

slide27

Three thermodynamically consistent, radiation MHD simulations

of MRI turbulence in vertically stratified shearing boxes have

been done:

Turner (2004): prad>>pgas

Hirose et al. (2006): prad<<pgas

Krolik et al. (2007), Blaes et al. (2007): prad~pgas

slide29

Radiation

Magnetic

times 10

Gas

slide30

Expect strong (but marginally stable) thermal fluctuations at

low energy and stable (damped) fluctuations at high energy.

slide31

Photosphere

Thermalization

Photosphere

Complex Structure of Surface Layers

slide32

Dynamical Support Against Gravity

Radiation pressure,

Gas pressure,

Magnetic forces,

Gravity

slide33

Magnetic Pressure vs. Magnetic Tension

Upward

pressure

Downward

tension

slide35

Red=fluid velocity

Black=magnetic field

slide36

Heavy regions

associated with

upward tension.

Light regions

associated with

downward tension.

slide37

3D visualization of

tension/density

fluctuation

correlation.

slide38

Strong Density Fluctuations - NOT Because of Radiative

Diffusion, but Because of Strong Magnetic Forces

slide39

Spectral Consequences

  • Magnetically supported upper layers decrease density at
  • effective photosphere, resulting in increased ionization and
  • a hardening of the spectrum.
  • Strong (up to factor 100) irregular density inhomogeneities
  • exist well beneath photosphere of horizontally averaged
  • structure. They will soften the spectrum.
  • Actual photosphere is therefore complex and irregular,
  • which will reduce intrinsic polarization of emerging photons
  • (Coleman & Shields 1990). Magnetic fields may also
  • Faraday depolarize the photons (Gnedin & Silant’ev 1978):
slide40

Photosphere

Parker Unstable

Regions

MRI - the source of

accretion power

Parker Unstable

Regions

Photosphere

Overall Vertical Structure of Disk with Prad~Pgas

Pmag>Prad~Pgas

Prad~Pgas>Pmag

Pmag>Prad~Pgas

-Blaes, Hirose, Krolik, & Stone (2007)

slide41

Conclusions

  • Radiation MHD simulations are beginning to handle not only

the dynamics, but the thermodynamics of accretion disks.

Theory can now begin to make contact with observations of

photon spectra.

  • Annulus is thermally stable at this level of radiation pressure.
  • Upper layers are supported by magnetic fields. No photon
  • bubbles seen. Parker instability dominates, and drives
  • strong density fluctuations.
  • Unclear what this means for spectra and black hole spin
  • measurements - magnetic field support will harden spectra,
  • density fluctuations will soften spectra.
slide42

Work in Progress

  • Monte Carlo radiative transfer calculation of emergent
  • spectra from simulation. This will also test flux-limited
  • diffusion used by the code.
  • Linear instability analysis of atmospheres supported by
  • both radiation and magnetic fields - are photon bubbles
  • suppressed somehow?
  • Radiation pressure dominated simulation is running now.
  • Further work also needed on the regime examined in
  • current simulation - unstable Parker wavelengths barely
  • fit inside the box!!!
slide43

Gravity

Total

Magnetic

Radiation

Gas

slide44

CVI K-edge

i=55

-Blaes et al. (2006)

slide45

CVI K-edge

With magnetic

fields

No magnetic

fields

-Blaes et al. (2006)

slide46

Density fluctuations help thermalize the spectrum.

Blackbody

Modified

blackbody

-Davis et al. (2004)

Density scale height may also decrease as flux is able to escape

through low density channels - this will also soften the spectrum.

slide47

Steep power law

Thermal

Hard

-Gierlinski & Done (2003)

slide48

-dF/dm

Hirose et al. 05

Turner 04

slide50

“Photon Bubble Instability”

F

B

g

-Turner et al. (2005)