Simulating the Extreme Environment Near Luminous Black Hole Sources

1. Simulating the Extreme Environment Near Luminous Black Hole Sources Omer Blaes University of California, Santa Barbara

2. Collaborators Spectral calculations: Shane Davis, Ivan Hubeny, Julian Krolik Simulations: Shigenobu Hirose, Julian Krolik, Jim Stone, Neal Turner Observers: Chris Done

3. 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

4. -figure from Orosz

5. -Charles & Coe (2003)

6. ¤  -2´v v

7. ISCO Black hole accretion is a POWERFUL source of energy!

8. -no jet whatsoever - jet always present -Remillard (2005)

9. Thermal State Hard State? Steep Power Law State???

10. 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)

11. 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…

12. 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:

13. 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…

14. Magnetorotational Instability (MRI) -Balbus & Hawley 1991, 1998

15. Magnetorotational Instability (MRI) B B W W rotates faster rotates slower Magnetic fields in a conducting, rotating plasma behave EXACTLY like springs!

16. Snapshot of angular momentum per unit mass in MRI turbulence. -Hawley & Balbus (1992)

17. Structure of (Non-Radiative) Accretion Flows From Simulation -Hawley & Balbus (2002)

18. 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?

19. GRS 1915+105 - Evidence for Thermal Instability? - Belloni et al. (2000)

20. 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.

21. 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

22. -Turner, Stone, & Sano (2003)

23. “Photon Bubble Instability” F g -Turner et al. (2005)

24. 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.

25. Equations of Radiation Magnetohydrodynamics

26. Flux-Limited Diffusion

27. 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

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

29. Radiation Magnetic times 10 Gas

30. Expect strong (but marginally stable) thermal fluctuations at low energy and stable (damped) fluctuations at high energy.

31. Photosphere Thermalization Photosphere Complex Structure of Surface Layers

32. Dynamical Support Against Gravity Radiation pressure, Gas pressure, Magnetic forces, Gravity

33. Magnetic Pressure vs. Magnetic Tension Upward pressure Downward tension

34. Parker Instability g B

35. Red=fluid velocity Black=magnetic field

36. Heavy regions associated with upward tension. Light regions associated with downward tension.

37. 3D visualization of tension/density fluctuation correlation.

38. Strong Density Fluctuations - NOT Because of Radiative Diffusion, but Because of Strong Magnetic Forces

39. 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):

40. 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)

41. 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.

42. 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!!!

43. Gravity Total Magnetic Radiation Gas

44. CVI K-edge i=55 -Blaes et al. (2006)

45. CVI K-edge With magnetic fields No magnetic fields -Blaes et al. (2006)

46. 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.

47. Steep power law Thermal Hard -Gierlinski & Done (2003)

48. -dF/dm Hirose et al. 05  Turner 04

49. CVI K-edge

50. “Photon Bubble Instability” F B g -Turner et al. (2005)