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Effects of Magnetic Stresses on Accretion Disk Spectra - Black Hole Spin Estimates

Explore the impact of magnetic fields on accretion disk spectra through local simulations. Discuss how inhomogeneities affect spectral formation, altering fcol by 15-20%. Consider implications for black hole spin estimates, with 20% uncertainty in a/M if fcol deviates by 15%. Address the role of magnetic pressure in enhancing disk spectra hardness and the influence of density variations on spectral softness, providing crucial insights for understanding black hole environments.

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Effects of Magnetic Stresses on Accretion Disk Spectra - Black Hole Spin Estimates

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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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