The BardeenPetterson effect and the occurence of retrograde accretion

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The Bardeen-Petterson effect and the occurence of
retrograde accretion

• Giuseppe Lodato
• IoA - Cambridge

Lodato Pringle, MNRAS, 368, 1196 (2006)
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What are we talking about?

• Alignment between the black hole spin and the
  angular momentum of its accretion disc
• Combined effect
• Lense-Thirring precession
• Warp propagation in the disc
• Significant effects on
• BH spin direction
• ...but also BH spin magnitude

Warp radius ? 100-1000 Rs
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Coupled BH-disc evolution
The disc
The hole
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Counter-alignment?

• Scheuer Feiler (1996) the relative torques
  always lead to alignment, even for initially
  almost anti-parallel spin
• King et al. (2005) simple, geometrical picture
  leads to the conclusion that counter-alignment is
  possible, if

Problems with Scheuer Feiler

• Assume a steady state taligngtgttwarp
• Implicitly assume that JdgtgtJh

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Generalized spreading ring

• Simulate a single accretion event on the BH
• Initially flat ring, inclined w.r.t hole spin
• Allow the hole spin to move in response to the
  disc torques
• Confirm King et al. results counter alignment is
  possible!
• If Jh dominates counter-aligned during the hole
  accretion event
• If Jd dominates, initial counter-alignment
  followed by alignment

Lodato Pringle 2006a
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BH spin - disc alignment
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Effects of counter-alignment
Effects of disc warping

• Enhanced dissipation, accretion rate
• Basic unknown ratio of ?2/?1

• Location of last stable orbit
• Accretion efficiency
• Shape of X-ray iron lines
• Shape of jets (precession and alignment
  timescales comparable ? 106 yrs)
• Accretion history (increased lum., decreased
  efficiency...)
• Spin history of the hole

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Conclusions