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The Impact of Magnetic Stresses and Inhomogeneities on Accretion Disk Spectra

The Impact of Magnetic Stresses and Inhomogeneities on Accretion Disk Spectra. Shane Davis IAS. Julian Krolik Jim Stone Shigenobu Hirose. Omer Blaes Ivan Hubeny Neal Turner. Questions Addressed in This Talk.

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The Impact of Magnetic Stresses and Inhomogeneities on Accretion Disk Spectra

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  1. The Impact of Magnetic Stresses and Inhomogeneities on Accretion Disk Spectra Shane DavisIAS Julian KrolikJim StoneShigenobu Hirose Omer BlaesIvan HubenyNeal Turner

  2. Questions Addressed in This Talk • How do magnetic fields and associated inhomogeneities affect disk spectra? (concentrate on local effects) • What can we learn from (local) accretion disk simulations? • What impact does this have on spin estimates?

  3. The Multicolor Disk Model • Assumes simple temperature distribution: • Spectrum assumed to be color-corrected blackbody: F  F 

  4. BHSPEC

  5. Model Parameters L/LEdd Inclination Spin Mass

  6. Spectral Formation = 0 z = 0 abs= 1 z = abs abs = / abs  : emissivity; abs = mean free path to absorption

  7. Spectral Formation = 0 z = 0 abs= 1 z = abs abs = / abs  : emissivity; abs = mean free path to absorption

  8. Spectral Formation = 0 z = 0 abs= 1 z = abs F=  abs = / abs = B(T) : emissivity; abs = mean free path to absorption

  9. Spectral Formation = 0 z = 0 es = 1 z = es z = eff eff = 1 abs = 1 z = abs abs = / abs  es = / es  abs >> es : emissivity; abs, es = mean free path to absorption/scattering eff= es abs)1/2;eff = (abses)1/2

  10. Spectral Formation = 0 z = 0 es = 1 z = es z = eff eff = 1 abs = 1 z = abs abs = / abs  es = / es  abs >> es : emissivity; abs, es = mean free path to absorption/scattering eff= es abs)1/2;eff = (abses)1/2

  11. Spectral Formation = 0 z = 0 es = 1 z = es z = eff eff = 1 abs = 1 z = abs F=  eff = abs (es /abs)1/2 = B(abs /es)1/2 : emissivity; abs, es = mean free path to absorption/scattering eff= es abs)1/2;eff = (abses)1/2

  12. Depth of formation*: optical depth where (esabs)1/2~1  > *: absorbed  < *: escape Thomson scattering produces modified blackbody: Due to temperature gradients Compton scattering gives a softer Wien spectrum Very approximately: fcol ~ T*/Teff ~ 1.5-1.8 for BHBs Spectral Formation

  13. Overall Vertical Structure of Disk with Prad ~ Pgas Photosphere Parker Unstable Regions MRI - the source of accretion power Parker Unstable Regions Photosphere Pmag > Prad ~ Pgas Prad ~ Pgas > Pmag z / H Pmag > Prad ~ Pgas r H Hirose et al.

  14. Pmag = B2/8 taken from simulations Extra magnetic pressure support increases scale height: hmag = Pmag/(d Pmag/dz) > hgas This leads to density reduction at*: * ~ es  h so  ~ * / (es h) Lowermeans lower ratio of absorption to scattering: abs/ es * which means larger T*/Teff and larger fcol Magnetic Pressure: Vertical Structure No B field B field Simulation

  15. Lower * and higher T* combine to give a harder spectrum In this case lower * alters statistical equilibrium -- lower recombination rate relative to photoionization rate yield higher ionization and weaker edges Overall effect is an enhancement of fcol by ~10-15% Magnetic Pressure: Spectrum No B field B field

  16. Compare 3D with 1D average: dependence is important: abs-2 es-1 Non-linear dependence of on  leads to enhanced emission Due to weaker dependence on , photons escape predominantly through low regions Inhomogeneities: Vertical Structure

  17. Spectral shape is approximately unchanged, but flux is enhanced by ~40% Increased efficiency allows lower T* to produce equivalent flux Would lead to reduction of fcol by ~15-20% 40% Inhomogeneities: Monte Carlo Spectra 3-D MC 1-D

  18. Conclusions • Magnetic pressure support acts to make disk spectra harder -- larger fcol • Density inhomogeneities will tend to make disk spectra softer -- smaller fcol • These uncertainties should be accounted for in black hole spin estimates -- approximately 20% uncertainty for a/M ~ 0.8 if fcol is off by ~15%

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